Complete answers to the Think It Over, Pause and Ponder, What if, Think as a Scientist, Revise–Reflect–Refine and The Quest Continues questions from Chapter 6.
NCERT Solutions
Think It Over
Q1Why does a canoe move forward when the canoeist pushes water backwards with their paddle and why does it move faster when they push harder?
A canoe moves forward due to Newton’s third law of motion.
When the canoeist pushes water backwards with the paddle, the water also pushes the paddle and canoe forward with an equal and opposite force.
• The paddle applies force on water in the backward direction.
• Water applies force on the paddle in the forward direction.
• This forward force makes the canoe move forward.
When the canoeist pushes harder, the backward force on water increases. Therefore, the water applies a larger forward force on the paddle and canoe. As a result, the canoe moves faster.
Q2Suppose the same canoeist uses the same paddle force in two different canoes, one empty and one carrying another passenger. In which case will the canoe move faster?
The empty canoe will move faster.
According to Newton’s second law of motion:
a = (F)/(m)
For the same force, acceleration is inversely proportional to mass.
• Empty canoe has less mass, so it gets more acceleration.
• Canoe carrying another passenger has more mass, so it gets less acceleration.
Therefore, the empty canoe will move faster when the same paddle force is applied.
NCERT Solutions
Pause and Ponder — Page 97
Q1A weightlifter lifts a barbell (Fig. 6.8). List two forces that are acting on the barbell. Are these forces balanced if the weightlifter keeps the barbell steady?
Two forces acting on the barbell are:
• Gravitational force acting downward
• Muscular force applied by the weightlifter acting upward
Yes, these forces are balanced if the weightlifter keeps the barbell steady.
This is because the barbell is not moving up or down. So, the upward force applied by the weightlifter is equal to the downward gravitational force.
Therefore,
Net force = 0
So, the forces are balanced.
Q2Two players R and S are participating in an arm-wrestling match (Fig. 6.9). At the instant, when the arms tilt to the front direction (out of the page towards you), are the forces exerted by the players balanced? If not, which player exerted the larger force?
No, the forces exerted by the players are not balanced.
If the arms tilt to the front direction, it means that the net force is acting in the front direction.
Therefore, the player who is applying force towards the front direction is exerting the larger force.
In Fig. 6.9, the arms tilt towards the side of player R, so player R exerted the larger force.
NCERT Solutions
What if... — Page 98
What ifWhat if the force of friction disappears in the world? How will the motion of objects be impacted?
If the force of friction disappears, motion in the world will be greatly affected.
The following changes will happen:
• Moving objects will not stop on their own.
• A ball, bicycle or car once moving will continue moving for a very long time unless another force stops it.
• We will not be able to walk properly, because friction between our feet and the ground helps us move forward.
• Vehicles will not be able to move safely because tyres need friction with the road.
• Brakes will not work properly, because brakes depend on friction.
• It will be difficult to hold objects, write with a pencil, or light a matchstick.
• Objects kept on surfaces may slide easily.
Thus, friction is very important. It may oppose motion, but it also helps us in many daily activities.
NCERT Solutions
Pause and Ponder — Page 101
Q3An object is moving with a constant velocity. Is there a net force acting upon it?
No, there is no net force acting upon the object.
According to Newton’s first law of motion, if an object is moving with constant velocity, its motion will not change unless a net force acts on it.
Since the velocity is constant:
• Speed is not changing.
• Direction is not changing.
• Acceleration is zero.
Therefore,
Net force = 0
Q4Suppose, no net force is acting on an object. Which of the following situations are possible? i Object remains at rest if at rest. (ii) Object keeps moving with a constant velocity if already moving. (iii) Object is moving with a constant acceleration.
The possible situations are:
(i) Object remains at rest if at rest.
(ii) Object keeps moving with a constant velocity if already moving.
Situation (iii) is not possible.
Explanation
If no net force is acting on an object, then according to Newton’s first law:
• An object at rest remains at rest.
• A moving object continues to move with constant velocity.
But constant acceleration means velocity is changing. For velocity to change, a net force is required.
Therefore, options (i) and (ii) are correct.
Q5In the real world, it is difficult to find a situation where no forces are acting on an object. But by applying additional forces, a condition can be achieved where the net force on the object is zero. Explain with the help of an example.
In the real world, many forces act on objects, such as gravitational force, frictional force, normal force and applied force.
However, if these forces balance each other, the net force becomes zero.
Example
A book lying on a table has two forces acting on it:
• Gravitational force acts downward.
• Normal force by the table acts upward.
These two forces are equal and opposite, so they balance each other.
Therefore,
Net force = 0
Another example is a box moving with constant velocity. If the force applied forward is equal to the frictional force acting backward, the net force on the box is zero. Hence, the box continues to move with constant velocity.
NCERT Solutions
Think as a Scientist — Page 103
ActivityApart from force, does acceleration depend on any other factor? From everyday experiences, you know that with the same magnitude of force, it is easier to set lighter objects in motion than heavier ones. This leads to a second hypothesis, that for the same force, a smaller mass has a larger acceleration or a larger mass has a smaller acceleration. Now how can you test your second hypothesis?
Yes, acceleration depends on mass also.
To test the hypothesis, we can perform an activity using a cart.
Activity to Test the Hypothesis
• Take a small cart and attach it to a thread passing over a pulley.
• Attach a cup with some weights to the other end of the thread.
• Keep the mass of the cup and weights constant so that the pulling force remains the same.
• First, release the cart and note the time taken by it to cover a fixed distance.
• Now increase the mass of the cart by placing some objects in it.
• Again release the cart and note the time taken to cover the same distance.
Observation
When the mass of the cart is increased, it takes more time to cover the same distance.
This means its acceleration decreases.
Conclusion
For the same force:
• Smaller mass gives larger acceleration.
• Larger mass gives smaller acceleration.
Thus,
a = (F)/(m)
Acceleration is inversely proportional to mass.
NCERT Solutions
Pause and Ponder — Page 106
Q6A toy car of mass 100 g is moving with a constant velocity of 0.5 m s⁻¹. What is the net force acting on the toy car?
The toy car is moving with constant velocity.
When velocity is constant, acceleration is zero.
Using Newton’s second law:
F = ma
Here,
a = 0
So,
F = m × 0 = 0
Therefore, the net force acting on the toy car is:
0 N
Q7Two children of different masses are sitting on identical swings. To impart identical initial acceleration, for which child would you require to apply a larger force? Explain why.
A larger force is required for the child with greater mass.
According to Newton’s second law:
F = ma
If acceleration is the same, then force depends on mass.
• Greater mass requires greater force.
• Smaller mass requires smaller force.
Therefore, to give identical initial acceleration, we need to apply a larger force to the heavier child.
Q8How are glass items packed for transportation using a bubble wrap or hay protected from damage?
Glass items are packed using bubble wrap or hay because these materials increase the time taken to stop the glass items during sudden jerks or collisions.
According to Newton’s second law, when the stopping time increases, the acceleration or deceleration decreases. As a result, the force acting on the glass item becomes smaller.
Bubble wrap or hay also acts as a soft cushion and absorbs shocks.
Thus, glass items are protected from damage during transportation.
NCERT Solutions
Pause and Ponder — Page 110
Q9Why does a fireperson sometimes struggle when holding the pipe issuing water?
A fireperson struggles while holding a pipe issuing water because of Newton’s third law of motion.
When water comes out of the pipe with great force in the forward direction, the water exerts an equal and opposite force on the pipe in the backward direction.
This backward force makes the pipe difficult to hold steady.
Therefore, the fireperson has to apply a large force to control the pipe.
Q10Suppose a spacecraft is moving in a region of space where the gravitational force acting upon it is negligible. Suggest how can it change its velocity.
A spacecraft can change its velocity by firing its engines.
When the spacecraft expels gases in one direction, the gases exert an equal and opposite force on the spacecraft.
This force can change the spacecraft’s:
• Speed
• Direction
• Velocity
For example:
• If gases are expelled backward, the spacecraft moves forward faster.
• If gases are expelled forward, the spacecraft slows down.
• If gases are expelled sideways, the spacecraft changes direction.
Thus, even when gravitational force is negligible, a spacecraft can change its velocity using rocket engines, based on Newton’s third law of motion.
NCERT Solutions
Revise, Reflect, Refine — Answers
Q1Using a horizontal force F, a table is moved across the floor at a constant velocity. How much is the frictional force exerted by the floor on the table?
The table is moving with constant velocity, so its acceleration is zero.
According to Newton’s first law of motion, if an object moves with constant velocity, the net force acting on it is zero.
Therefore, the frictional force exerted by the floor must be equal in magnitude to the applied force F, but opposite in direction.
Frictional force = F
So, the frictional force is F, acting opposite to the direction of motion.
Q2For a ball moving on a smooth frictionless surface, choose the appropriate option that will make the following statements physically correct. (i) If no net force is applied on the ball, the velocity of the ball will remain the same/increase/decrease.
The velocity of the ball will remain the same.
Explanation:
On a frictionless surface, if no net force acts on the ball, there will be no acceleration. So, the ball continues to move with the same velocity.
(ii) If a net force is applied on the ball in the direction of its motion, the magnitude of the velocity of the ball will remain the same/increase/decrease.
The magnitude of the velocity of the ball will increase.
Explanation:
If force acts in the direction of motion, acceleration also acts in the same direction. Therefore, the speed of the ball increases.
(iii) If a net force is applied on the ball in a direction opposite to the direction of its motion, the magnitude of the velocity of the ball will remain the same/increase/decrease.
The magnitude of the velocity of the ball will decrease.
Explanation:
If force acts opposite to the direction of motion, acceleration acts opposite to motion. So, the speed of the ball decreases.
Q3Two blocks P and Q on a smooth horizontal surface are shown in Fig. 6.36a and Fig. 6.36b. Two forces of magnitudes 4 N and 5 N are acting in opposite directions on block P, while block Q is moving with a constant velocity. Which of the following statement is correct? i P experiences a net force and Q does not experience a net force. (ii) P does not experience a net force and Q experiences a net force. (iii) Both P and Q experience a net force. (iv) Neither P nor Q experiences a net force.
The correct option is:
(i) P experiences a net force and Q does not experience a net force.
Explanation:
For block P:
Two forces act in opposite directions:
• 5 N towards one side
• 4 N towards the opposite side
So,
Net force on P = 5 N - 4 N = 1 N
Therefore, block P experiences a net force of 1 N in the direction of the larger force.
For block Q:
Block Q is moving with constant velocity. This means its acceleration is zero.
According to Newton’s first law:
Net force on Q = 0
Therefore, Q does not experience a net force.
Q4While practising for the snake boat race (Vallum kalli in Kerala), 100 oarsmen are rowing a boat together. Out of these, 95 row backwards to propel the boat forward. But by mistake, 5 oarsmen row in the opposite direction. If each oarsman applies a horizontal force of 200 N, what is the net force on the snake boat? Ignore drag forces, air friction, etc.
Force applied by one oarsman:
200 N
Number of oarsmen rowing backwards to move boat forward:
95
Forward force:
95 × 200 = 19000 N
Number of oarsmen rowing in the opposite direction:
5
Opposite force:
5 × 200 = 1000 N
Net force on the boat:
19000 N - 1000 N = 18000 N
Therefore, the net force on the snake boat is:
18000 N
The net force acts in the forward direction.
Q5When a net force acts on an object, we observe that the object accelerates: i opposite to the direction of force, with acceleration proportional to the force acting on the object. (ii) opposite to the direction of force, with acceleration proportional to the mass of the object. (iii) in the direction of force, with acceleration inversely proportional to the force acting on the object. (iv) in the direction of force, with acceleration proportional to the force acting on the object.
The correct option is:
(iv) in the direction of force, with acceleration proportional to the force acting on the object.
Explanation:
According to Newton’s second law of motion:
F = ma
or
a = (F)/(m)
This means:
• Acceleration is in the direction of the net force.
• Acceleration is directly proportional to force.
• Acceleration is inversely proportional to mass.
Therefore, when a net force acts on an object, the object accelerates in the direction of the force, and acceleration is proportional to the force acting on the object.
Q6The position-time graph for four objects A, B, C and D moving along a straight line are given in Fig. 6.37. A net force acts on: i Object A (ii) Object B (iii) Object C (iv) Object D
The correct option is:
(iii) Object C
Explanation:
A net force acts on an object only when its velocity changes, that is, when the object has acceleration.
In a position-time graph:
• A straight line upward shows motion with constant velocity.
• A horizontal line shows the object is at rest.
• A curved line shows that velocity is changing, so the object is accelerating.
• A straight line downward shows motion with constant velocity in the opposite direction.
For the given objects:
• Object A: Straight line upward → constant velocity → no net force
• Object B: Horizontal line → at rest → no net force
• Object C: Curved line upward → changing velocity → net force acts
• Object D: Straight line downward → constant velocity in opposite direction → no net force
Therefore, a net force acts only on Object C.
Q7A sailor jumps out from a small boat to the shore (Fig. 6.38). As the sailor jumps forward, will the boat move? If yes, in which direction and why.
Yes, the boat will move backward, that is, in the direction opposite to the sailor’s jump.
When the sailor jumps forward, he pushes the boat backward with his feet. According to Newton’s third law of motion, the boat also pushes the sailor forward with an equal and opposite force.
As a result:
• The sailor moves forward towards the shore.
• The boat moves backward away from the shore.
Thus, the boat moves backward due to the action-reaction pair of forces.
Q8During a high jump event, a landing mat or sand bed is placed for the athlete to fall upon (Fig. 6.39). Explain the reason behind it.
A landing mat or sand bed is placed to reduce the force of impact on the athlete.
When the athlete falls, their body has a certain velocity. On landing, this velocity becomes zero. If the athlete lands on a hard surface, the body stops in a very short time, producing a large force that may cause injury.
A landing mat or sand bed:
• Increases the time taken to stop the athlete.
• Reduces acceleration or deceleration.
• Reduces the force acting on the athlete’s body.
• Protects the athlete from injury.
According to Newton’s second law of motion:
F = ma
When acceleration or deceleration is reduced, the force also becomes smaller. Therefore, a soft landing mat or sand bed helps in safe landing.
Q9A hand cart loaded with vegetables collides with an identical but empty hand cart. During the collision: i the loaded cart exerts a force of larger magnitude on the empty cart. (ii) the empty cart exerts a force of larger magnitude on the loaded cart. (iii) neither cart exerts a force on the other. (iv) the loaded cart and the empty cart, both exert an equal magnitude of force on each other.
The correct option is:
(iv) the loaded cart and the empty cart, both exert an equal magnitude of force on each other.
Explanation:
According to Newton’s third law of motion, when two objects interact, they exert equal and opposite forces on each other.
So, during the collision:
• The loaded cart exerts a force on the empty cart.
• The empty cart exerts an equal force on the loaded cart in the opposite direction.
Although the forces are equal, their effects may be different because the carts have different masses.
Using Newton’s second law:
a = (F)/(m)
The empty cart has less mass, so it may experience greater acceleration. The loaded cart has more mass, so it may experience smaller acceleration. But the force exerted by both carts on each other is equal in magnitude.
Q10The acceleration-mass graph for the acceleration produced by a force on objects of different masses is plotted in Fig. 6.40. Plot the force-mass graph for this case.
Given points from the acceleration-mass graph are:
So, the force is constant, equal to:
F = 10 N
Therefore, the force-mass graph will be a horizontal straight line parallel to the mass axis at:
F = 10 N
Points for Force-Mass Graph
( 1,10),(2,10),(4,10),(5,10 )
Thus, the graph shows that the same force of 10 N is acting on objects of different masses.
| Mass, m (kg) | Acceleration, a (m s⁻²) | Force, F = ma |
|---|---|---|
| 1 | 10 | 1 × 10 = 10 N |
| 2 | 5 | 2 × 5 = 10 N |
| 4 | 2.5 | 4 × 2.5 = 10 N |
| 5 | 2 | 5 × 2 = 10 N |
Q11The velocity-time graph of an object of mass 10 kg moving along a straight line is shown in Fig. 6.41. Calculate the force acting on the object by using the graph.
Given:
m = 10 kg
From the velocity-time graph:
At t = 0 s,
u = 10 m s⁻¹
At t = 8 s,
v = 30 m s⁻¹
Now,
a = (v - u)/(t)
a = (30 - 10)/(8)
a = (20)/(8)
a = 2.5 m s⁻²
Using Newton’s second law:
F = ma
F = 10 × 2.5
F = 25 N
Therefore, the force acting on the object is:
25 N
Q12A bullet of mass 50 g moving with a speed of 100 m s⁻¹ enters a heavy stationary wooden block and stops after penetrating a distance of 50 cm. Estimate the stopping force acting on the bullet (assume that the bullet undergoes constant acceleration within the block).
Given:
m = 50g = 0.05 kg
u = 100 m s⁻¹
v = 0 m s⁻¹
s = 50 cm = 0.5 m
Using the equation:
v² = u² + 2as
0² = 100² + 2 × a × 0.5
0 = 10000 + a
a = - 10000 m s⁻²
The negative sign shows that acceleration is opposite to the direction of motion.
Now, using:
F = ma
F = 0.05 × ( - 10000)
F = - 500 N
Therefore, the stopping force acting on the bullet is:
500 N
The force acts opposite to the direction of motion of the bullet.
Q13An ace footballer converted a penalty shot by kicking the football with a speed of 108 km h⁻¹. The estimated force they imparted was 800 N. The mass of the football was 0.4 kg. Calculate the time of contact between their foot and the ball.
Given:
F = 800 N
m = 0.4 kg
Initial velocity:
u = 0
Final velocity:
v = 108 km h⁻¹
Convert km h⁻¹ into m s⁻¹:
108 × (5)/(18) = 30 m s⁻¹
So,
v = 30 m s⁻¹
Using Newton’s second law:
F = ma
a = (F)/(m)
a = (800)/(0.4)
a = 2000 m s⁻²
Now, using:
v = u + at
30 = 0 + 2000 × t
t = (30)/(2000)
t = 0.015 s
Therefore, the time of contact between the foot and the ball is:
0.015 s
Q14An object of mass 2 kg moving with a constant velocity of 10 m s⁻¹ encounters a rough patch where the force of friction on the object is 7 N. At the same time, an additional constant force of 3 N opposing the motion is applied on the object. After entering the rough patch, how much distance does the object travel before coming to rest?
Given:
m = 2 kg
u = 10 m s⁻¹
v = 0
Frictional force:
7 N
Additional opposing force:
3 N
Total opposing force:
7 + 3 = 10 N
So,
F = - 10 N
Negative sign shows that force is opposite to motion.
Using Newton’s second law:
F = ma
a = (F)/(m)
a = (- 10)/(2)
a = - 5 m s⁻²
Now, using:
v² = u² + 2as
0² = 10² + 2 × ( - 5) × s
0 = 100 - 10 s
10 s = 100
s = 10 m
Therefore, the object travels:
10 m
before coming to rest.
Q15A tractor pulls a harrow (a ploughing tool) of mass m₁ with a net force F resulting in an acceleration of a₁. The same tractor pulls a trolley of mass m₂ with a force F producing an acceleration of a₂. If the tractor now pulls the trolley with the harrow placed on it (with the same force F), then obtain an expression for the resulting acceleration in terms of a₁ and a₂. Ignore friction.
For the harrow:
F = m₁a₁
So,
m₁ = (F)/(a₁)
For the trolley:
F = m₂a₂
So,
m₂ = (F)/(a₂)
When the harrow is placed on the trolley, total mass becomes:
m₁ + m₂
Resulting acceleration:
a = (F)/(m₁ + m₂)
Substitute the values of m₁ and m₂:
a = (F)/((F)/(a₁) + (F)/(a₂))
Take F common in the denominator:
a = (F)/(F( (1)/(a₁)+(1)/(a₂) ))
a = (1)/((1)/(a₁) + (1)/(a₂))
a = (a₁a₂)/(a₁ + a₂)
Therefore, the resulting acceleration is:
a = (a₁a₂)/(a₁ + a₂)
Q16When the pole of a bar magnet is brought close to a magnetic compass, the bar magnet and the compass needle (which is also a magnet) exert a magnetic force on each other. As per Newton’s third law of motion, both the forces are equal in magnitude and opposite in direction. However, the compass needle moves, whereas the bar magnet does not move (Fig. 6.42). Explain why.
According to Newton’s third law of motion, the bar magnet and the compass needle exert equal and opposite magnetic forces on each other.
However, the compass needle moves while the bar magnet does not appear to move because:
• The compass needle has very small mass.
• It is free to rotate easily.
• The bar magnet has much larger mass.
• It may also be held in hand or placed firmly, so its motion is not easily visible.
Using Newton’s second law:
a = (F)/(m)
For the same force:
• Smaller mass gets larger acceleration.
• Larger mass gets smaller acceleration.
Therefore, the compass needle shows noticeable motion, while the bar magnet does not move noticeably.
NCERT Solutions
The Quest Continues
ExplanationHow do scientists reduce harmful friction in real-world machines, vehicles and transport systems?
Friction in the Real World
Friction is always present when two surfaces are in contact. It acts in the direction opposite to motion and can slow down or stop moving objects.
Although friction is useful in many activities like walking, writing, braking vehicles and holding objects, it can also cause problems.
Problems caused by friction
• It causes wear and tear of machine parts.
• It produces heat.
• It wastes energy.
• It reduces the efficiency of machines.
• It slows down moving objects.
Therefore, scientists and engineers try to reduce friction wherever it is harmful.
Methods Used to Reduce Friction
1. Using Lubricants
Lubricants are substances like oil, grease or graphite that are applied between two surfaces.
They reduce direct contact between surfaces and make movement smoother.
Examples:
• Oil is used in bicycle chains.
• Grease is used in machines.
• Engine oil is used in vehicles.
2. Using Coatings
Special coatings are applied on surfaces to make them smoother and reduce friction.
Examples:
• Non-stick coating on pans
• Smooth coating on machine parts
• Protective coating on tools
3. Texturing of Surfaces
Sometimes, surfaces are given special patterns or textures to reduce friction or control it.
In some cases, texture helps reduce resistance by allowing smoother movement of air or liquid over the surface.
4. Streamlining Shapes
Streamlining means giving objects a smooth shape so that they can move easily through air or water.
This reduces friction due to air or water, also called drag.
Examples:
• Aeroplanes have streamlined bodies.
• Cars are designed with smooth shapes.
• Boats and ships have pointed fronts.
• Fish have streamlined bodies.
5. Magnetic Levitation
In magnetic levitation, an object is lifted slightly above the surface using magnetic force.
Since there is no direct contact with the surface, friction becomes very small.
Example:
• Maglev trains use magnetic levitation to move at very high speeds.
Conclusion
Friction is always present when surfaces touch each other. It is useful in many situations, but in machines and vehicles it can waste energy and cause damage. Therefore, friction is reduced using lubricants, coatings, surface texturing, streamlined shapes and magnetic levitation. These methods help machines work smoothly and efficiently.
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