How Forces Affect Motion Class 9 Science

20 Questions | 52 Marks

When you push a wall, it pushes back on you with an equal and opposite force. Explain why you don't move backwards, but a lighter object being pushed might move. 3marks

A book is lying on a table. List the forces acting on the book and state whether they are balanced or unbalanced. 2marks

A car is moving with a constant velocity. What can you say about the net force acting on the car? Explain your answer with reference to Newton's first law of motion. 3marks

An object of mass 5 kg is acted upon by a net force of 20 N. Calculate the acceleration produced in the object. If the force is doubled, what will be the new acceleration? 3marks

Why is it advisable to use a seat belt in a car? Explain with the help of Newton's first law of motion. 3marks

A cleaner is pushing a heavy box across a floor. The cleaner stops pushing, and the box slows down and stops. Explain this observation using the concept of friction. 2marks

A cricket bat strikes a ball. Explain the forces involved and how they affect the motion of both the ball and the bat, referring to Newton's third law of motion. 3marks

Two teams are playing tug-of-war. Team A pulls with a force of 500 N and Team B pulls with a force of 450 N. What is the net force on the rope, and in which direction will the rope move? 2marks

When you jump forward from a boat, the boat moves backward. Explain this phenomenon using Newton's third law of motion. 3marks

A 1000 kg car is moving at 20 m/s. If the driver applies brakes and the car stops in 10 seconds, calculate the force applied by the brakes. 3marks

A force of 10 N is applied to an object of mass 2 kg. What is the acceleration of the object? If another object of mass 4 kg is acted upon by the same force, which object will have greater acceleration and why? 3marks

A rocket expels gas at a high speed. Explain how this allows the rocket to accelerate upwards, referencing Newton's third law. 3marks

A ball is dropped from a height. Describe the forces acting on it as it falls and what happens to its acceleration if air resistance is negligible. 2marks

Explain why it is harder to push a heavier box than a lighter box, even if you apply the same force. Relate this to Newton's second law of motion. 3marks

A boy throws a ball horizontally from a cliff. What forces act on the ball as it flies through the air (neglecting air resistance)? How does the horizontal component of its velocity change? 2marks

A block of mass 15 kg is pulled by a force of 60 N on a smooth horizontal surface. Calculate the acceleration of the block. 2marks

When a gun is fired, it recoils. Explain why the gun recoils backward while the bullet moves forward. 2marks

A car is moving with a velocity of 30 m/s. The driver applies the brakes and the car stops in 5 seconds. Calculate the acceleration of the car. 2marks

A force of 50 N is applied to an object of mass 20 kg. What is the acceleration of the object? What would be the acceleration if a force of 100 N was applied to the same object? 3marks

While catching a fast-moving cricket ball, a fielder moves their hands backward. Explain why this action reduces the force exerted on the fielder. 3marks

Answers

When you push a wall, the wall exerts an equal and opposite force on you (Newton's third law). Since the wall is massive and stationary, its acceleration is negligible. A lighter object being pushed, however, has a smaller mass, so the same applied force (or a slightly larger net force if there's friction) will cause a much larger acceleration (Newton's second law,

a = F / m

The forces acting on the book are:

Gravitational force (weight) acting downwards.
Normal force exerted by the table acting upwards.
These forces are balanced because the book is at rest, meaning the net force is zero.
3. If a car is moving with a constant velocity, its acceleration is zero. According to Newton's first law of motion, an object in motion continues to move with a constant velocity unless acted upon by a net external force. Therefore, if the velocity is constant, the net force acting on the car must be zero.
4. Given: mass $m = 5$ kg, force $F = 20$ N.
Using Newton's second law, $F = m a$ , we can find acceleration $a$ :
$a = F / m = 20 N / 5 kg = 4 {m/s}^{2}$ .
If the force is doubled to $F^{'} = 2 \times 20 N = 40 N$ , the new acceleration $a^{'}$ will be:
$a^{'} = F^{'} / m = 40 N / 5 kg = 8 {m/s}^{2}$ .
5. According to Newton's first law of motion (the law of inertia), an object in motion tends to stay in motion with the same speed and in the same direction unless acted upon by an unbalanced force. When a car suddenly stops or starts, the passengers inside tend to continue moving with their previous velocity. A seat belt exerts an external force on the passenger, preventing them from continuing in motion and causing them to decelerate along with the car, thus preventing injury.
6. When the cleaner stops pushing, the applied force becomes zero. However, the force of friction between the box and the floor, which opposes motion, is still acting on the box in the backward direction. This unbalanced force of friction causes the box to decelerate and eventually come to rest.
7. When a cricket bat strikes a ball, the bat exerts a force on the ball, changing its direction and/or speed. Simultaneously, according to Newton's third law, the ball exerts an equal and opposite force on the bat. This force on the bat might cause a slight change in the bat's speed or direction, or be felt by the player.
8. The forces are acting in opposite directions.
Net force = Force by Team A - Force by Team B = 500 N - 450 N = 50 N.
Since the net force is positive (towards the direction of Team A's pull), the rope will move in the direction of Team A's pull.
9. When a sailor jumps forward from a boat, they exert a force on the boat to push themselves forward. According to Newton's third law, the boat exerts an equal and opposite force on the sailor, propelling them forward. Simultaneously, the sailor exerts an equal and opposite force on the boat, causing the boat to move backward.
10. Given: mass $m = 1000$ kg, initial velocity $u = 20$ m/s, final velocity $v = 0$ m/s (stops), time $t = 10$ s.
First, calculate the acceleration using $v = u + a t$ :

0 m/s = 20 m/s + (a \times 10 s)

a = - 20 m/s / 10 s = - 2 {m/s}^{2}

.Now, calculate the force using

F = m a

F = 1000 kg \times (- 2 {m/s}^{2}) = - 2000 N

.The force applied by the brakes is 2000 N, acting in the opposite direction of motion.

For the first object: mass

m_{1} = 2

kg, force

F = 10

N.Acceleration

a_{1} = F / m_{1} = 10 N / 2 kg = 5 {m/s}^{2}

.For the second object: mass

m_{2} = 4

kg, force

F = 10

N.Acceleration

a_{2} = F / m_{2} = 10 N / 4 kg = 2.5 {m/s}^{2}

.The object with mass 2 kg will have greater acceleration because acceleration is inversely proportional to mass when the force is constant (

a \propto 1 / m

A rocket expels hot gases downwards at high speed. According to Newton's third law, the expelled gases exert an equal and opposite force on the rocket, pushing it upwards. This upward force (thrust) is the action-reaction pair to the downward force exerted by the rocket engine on the gases. If this upward force is greater than the downward gravitational force on the rocket, the rocket will accelerate upwards.

Neglecting air resistance, the only force acting on the ball as it falls is the gravitational force exerted by the Earth, acting downwards. This force causes the ball to accelerate downwards with acceleration

g

(approximately

9.8 {m/s}^{2}

). The acceleration remains constant throughout its fall.

Pushing a heavier box requires overcoming a larger force of friction and/or inertia. According to Newton's second law,

F = m a

. For the same acceleration

a

, a larger mass

m

requires a larger net force

F

. Therefore, to achieve the same acceleration, you need to apply a larger force to a heavier box compared to a lighter one.

The forces acting on the ball as it flies through the air (neglecting air resistance) is only the gravitational force exerted by the Earth, acting downwards. The horizontal component of its velocity remains constant because there is no horizontal force acting on the ball to change it.

Given: mass

m = 15

kg, force

F = 60

N.Using Newton's second law,

F = m a

:Acceleration

a = F / m = 60 N / 15 kg = 4 {m/s}^{2}

When a gun fires a bullet, the gun exerts a large force on the bullet, propelling it forward (action). Simultaneously, the bullet exerts an equal and opposite force on the gun (reaction), as per Newton's third law. Although the forces are equal in magnitude, the mass of the gun is much larger than the mass of the bullet. Due to this large difference in masses, the acceleration of the gun (recoil) is much smaller than the acceleration of the bullet.

Given: initial velocity

u = 30

m/s, final velocity

v = 0

m/s, time

t = 5

s.Using the kinematic equation

v = u + a t

0 m/s = 30 m/s + (a \times 5 s)

a = - 30 m/s / 5 s = - 6 {m/s}^{2}

.The acceleration of the car is

- 6 {m/s}^{2}

For the first case: mass

m = 2

kg, force

F_{1} = 50

N.Acceleration

a_{1} = F_{1} / m = 50 N / 2 kg = 25 {m/s}^{2}

.For the second case: mass

m = 2

kg, force

F_{2} = 100

N.Acceleration

a_{2} = F_{2} / m = 100 N / 2 kg = 50 {m/s}^{2}

When a fielder catches a fast-moving cricket ball, the ball has a high momentum. To stop the ball, the fielder needs to reduce its momentum to zero. This involves a change in momentum, which is equal to the impulse (

F \times t

). By moving their hands backward, the fielder increases the time duration (

t

) over which the force is applied to stop the ball. Since impulse is conserved (

F \times t = constant

for a given change in momentum), increasing

t

results in a decrease in the magnitude of the force (

F

) exerted on the fielder's hands, thus minimizing the risk of injury.

Download as PDF opens your browser's print dialog: choose Save as PDF as the printer.

How Forces Affect Motion — Important Questions

Question bank

Answers

All Class 9 Science chapters