## Force And Laws Of Motion Class 9 Physics Worksheet Pdf

Class 9 Physics students should refer to the following printable worksheet in Pdf for Force And Laws Of Motion in standard 9. This test paper with questions and answers for Grade 9 Physics will be very useful for exams and help you to score good marks

### Class 9 Physics Worksheet for Force And Laws Of Motion

CBSE Class 9 Physics Worksheet - Force and Laws of Motion (3)

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**Exam Questions NCERT Class 9 Science Chapter 9 Force and Laws of Motion**

**Question. Explain why- An inflated balloon lying on the surface of a floor moves forward when pierced with a pin.****Answer.** The momentum of the inflated balloon is zero before it is pierced with a pin. Air comes out with a speed in the backward direction from balloon after it is pierced with a pin. The balloon moves in the forward direction to conserve the momentum.

**Question. Explain our walking in terms of Newton’s third law of motion.****Answer.** When we walk on ground, our foot pushes the ground backward and in return the ground pushes our foot forward. The forward reaction exerted by the ground on our foot makes us walk forward.**Question. There are three solid balls, made up of aluminium,steel and wood of the same shape and volume. Which of them would have highest inertia? Why?****Answer.** Steel ball because the density of steel is maximum. For the same shape and volume, steel ball will have maximum mass.**Question. State Newton’s three laws of motion.****Answer.** Sir Isssac Newton further studied the idea of Galileo’s on force and motion and presented three laws of motion. These laws are as follows :**(i) First Law :** A body remains in resting position unless it is not introduced with an unbalanced external force.**(ii) Second Law :** The rate of change of momentum of a body is directly proportional to the applied unbalanced force and change takes place in the direction of the force.**(iii) Third law :** Action and reaction are equal and opposite and they act on different bodies.

**Question. What is frictional force?****Answer.** The force that always opposes the motion of object is called force of friction.

**Question. Give example to show the effects of force.****Answer.** (i) Place a ball in the ground. Kick it with your foot. The ball starts moving. The ball moves because of the force applied to it.

(ii) If ball is coming towards you, you can kick it in any direction. The direction of motion of the ball changes because of the force applied to it.

(iii) Place a rubber on the ground. Press it with your foot. It is found that the ball is no longer round but takes the shape of an egg, i.e. it is oblong. The shape of the ball has changed because of the force applied on the ball.**Question. Why we tend to get thrown to one side when a motorcar makes a sharp turn at a high speed?****Answer.** We tend to get thrown to one side when a motorcar makes a sharp turn at a high speed due to law of inertia. When we are sitting in moving car on a straight road, we tend to continue in our straight-line motion. But when an unbalanced force is applied on car to change the direction of motion, we slip to one side of the seat due to the inertia of our body.**Question. Why do fielders pull their hand gradually with the moving ball while holding a catch?****Answer.** While catching a fast moving cricket ball, a fielder on the ground pulls his hands backwards with the moving ball. This is done so that the fielder increases the time during which the high velocity of the moving ball decreases to zero. Thus, the acceleration of the ball is decreased and therefore, the impact of catching the fast moving ball is reduced.**Question. What is inertia?****Answer.**The natural tendency of an object to resist a change in their state of rest or of uniform motion is called inertia.

**Question. It is required to increase the velocity of a scooter of mass 80 kg from 5 to 25 ms–2 in 2 seconds. Calculate the force required.****Answer.**

Given : m = 80 kg,

u = 5 ms^{–2}

v = 25 ms^{–2}

and t = 2 s

Now acceleration a = change in velocity time

= v – u/t

= 25 – 5/2 = 10 ms^{–2}

Force = mass × acceleration of F

= ma

Therefore, F = 80 × 10 = 800 N**Question. State Newton’s first law of motion.****Answer.** An object remains in a state of rest or of uniform motion in a straight line unless acted upon by an external unbalanced force.

**Question. How is force represented graphically?****Answer.** Force is a vector quantity. So, force is represented by a line with an arrow head.

(i) The head of arrow represents the direction of the force.

(ii) The length of the line is proportional to the magnitude of the force.

(iii) This is done by choosing a convenient scale.

(iv) For example : A force of 10 N can be represented by a line with an arrow head.

(v) The length of the line depends upon the scale chosen. So, if a scale of 1 cm = 2 N is chosen, the force of 10 N is represented by a line of 5 cm in length.

**Question. What are the disadvantages of friction?****or****Why friction is considered wasteful?****Answer. **Friction is considered wasteful because :

(1) Friction leads to a loss of energy. Therefore, it reduces the efficiency of machines.

(2) Friction cause wear and tear of machine’s parts.

**Question. The following is the distance-time table of an object in motion:**

**(a) What conclusion can you draw about the acceleration? Is it constant, increasing, decreasing, or zero?(b) What do you infer about the forces acting on the object.**

**Answer.**(a) The distance covered by the object at any time interval is greater than any of the distances covered in previous time intervals. Therefore, the acceleration of the object is increasing.

(b) As per the second law of motion, force = mass × acceleration. Since the mass of the object remains constant, the increasing acceleration implies that the force acting on the object is increasing as well

**Question. Two persons manage to push a motorcar of mass 1200 kg at a uniform velocity along a level road. The same motorcar can be pushed by three persons to produce an acceleration of 0.2 m s ^{-2}. With what force does each person push the motorcar? (Assume that all persons push the motorcar with the same muscular effort)**

**Answer.**Given, mass of the car (m) = 1200kg

When the third person starts pushing the car, the acceleration (a) is 0.2ms

^{-2}. Therefore, the force applied by the third person (F = ma) is given by:

F = 1200kg × 0.2 ms

^{-2}= 240N

The force applied by the third person on the car is 240 N. Since all 3 people push with the same muscular effort, the force applied by each person on the car is 240 N.

**Question. A hammer of mass 500 g, moving at 50 m s ^{-1}, strikes a nail. The nail stops the hammer in a very short time of 0.01 s. What is the force of the nail on the hammer?**

**Answer.**Given, mass of the hammer (m) = 500g = 0.5kg

Initial velocity of the hammer (u) = 50 m/s

Terminal velocity of the hammer (v) = 0 (the hammer is stopped and reaches a position of rest).

Time period (t) = 0.01s

a = -5000ms^{-2}

Therefore, the force exerted by the hammer on the nail (F = ma) can be calculated as:

F = (0.5kg) * (-5000 ms^{-2}) = -2500 N

As per the third law of motion, the nail exerts an equal and opposite force on the hammer. Since the force exerted on the nail by the hammer is -2500 N, the force exerted on the hammer by the nail will be +2500 N.

**Question. A motorcar of mass 1200 kg is moving along a straight line with a uniform velocity of 90 km/h. Its velocity is slowed down to 18 km/h in 4 s by an unbalanced external force. Calculate the acceleration and change in momentum. Also calculate the magnitude of the force required.****Answer.** Given, mass of the car (m) = 1200kg

Initial velocity (u) = 90 km/hour = 25 meters/sec

Terminal velocity (v) = 18 km/hour = 5 meters/sec

Time period (t) = 4 seconds

Therefore, the acceleration of the car is -5 ms^{-2}.

Initial momentum of the car = m × u = (1200kg) × (25m/s) = 30,000 kg.m.s^{-1}

Final momentum of the car = m × v = (1200kg) × (5m/s) = 6,000 kg.m.s^{-1}

Therefore, change in momentum (final momentum – initial momentum) = (6,000 – 30,000) kg.m.s^{-1}

= -24,000 kg.m.s^{-1}

External force applied = mass of car × acceleration = (1200kg) × (-5 ms^{-2}) = -6000N

Therefore, the magnitude of force required to slow down the vehicle to 18 km/hour is 6000 N

**Question. An object of mass 100 kg is accelerated uniformly from a velocity of 5 ms–1 to 8 ms ^{–1} in 6 s. Calculate the initial and final momentum of the object. Also, find the magnitude of the force exerted on the object.**

**Answer.**Given, mass of the object (m) = 100kg

Initial velocity (u) = 5 m/s

Terminal velocity (v) = 8 m/s

Time period (t) = 6s

Now, initial momentum (m × u) = 100kg × 5m/s = 500 kg.m.s^{-1}

Final momentum (m × v) = 100kg × 8m/s = 800 kg.m.s^{-1}

Therefore, the object accelerates at 0.5 ms^{-2}. This implies that the force acting on the object (F = ma) is equal to:

F = (100kg) × (0.5 ms^{-2}) = 50 N

Therefore, a force of 50 N is applied on the 100kg object, which accelerates it by 0.5 ms^{-2}.

**Question. Akhtar, Kiran, and Rahul were riding in a motorcar that was moving with a high velocity on an expressway when an insect hit the windshield and got stuck on the windscreen. Akhtar and Kiran started pondering over the situation. Kiran suggested that the insect suffered a greater change in momentum as compared to the change in momentum of the motorcar (because the change in the velocity of the insect was much more than that of the motorcar). Akhtar said that since the motorcar was moving with a larger velocity, it exerted a larger force on the insect. And as a result the insect died. Rahul while putting an entirely new explanation said that both the motorcar and the insect experienced the same force and a change in their momentum. Comment on these suggestions.****Answer.** Kiran’s suggestion is correct. The mass of the insect is very small when compared to the mass of the car. As per the law of conservation of momentum, the total momentum before the collision between the insect and the car is equal to the total momentum after the collision. Therefore, the change in the momentum of the insect is much greater than the change in momentum of the car (since force is proportional to mass).

Akhtar’s suggestion is also correct. Since the mass of the car is very high, the force exerted on the insect during the collision is also very high.

Rahul’s suggestion is partially correct. As per the third law of motion, the force exerted by the insect on the car is equal and opposite to the force exerted by the car on the insect. However, Rahul’s suggestion that the change in the momentum is the same contradicts the law of conservation of momentum.

**Question. How much momentum will a dumb-bell of mass 10 kg transfer to the floor if it falls from a height of 80 cm? Take its downward acceleration to be 10 ms ^{–2}.**

**Answer.**Given, mass of the dumb-bell (m) = 10kg

Distance covered (s) = 80cm = 0.8m

Initial velocity (u) = 0 (it is dropped from a position of rest)

Acceleration (a) = 10ms

^{-2}

Terminal velocity (v) =?

Momentum of the dumb-bell when it hits the ground = mv

v = 4 m/s

The momentum transferred by the dumb-bell to the floor = (10kg) × (4 m/s) = 40 kg.m.s^{-1}

### More Question

1. A bullet fired from a gun is more dangerous than an air molecule hitting a person, though both bullet and air molecule are moving with same velocity. Explain.

2.Why are road accidents at high speeds very much worse than accidents at low speeds?.

3. What was the misbelief about the theory of motion before Newtonian motion theory?

Why was it overruled?

4. When force acting on a body has an equal and opposite reaction, then why should the body move at all?

5. What can you say about the speed of a moving object if no force is acting on to it.?

6. Is a marble rolling down an inclined plane moving with constant velocity? Explain.

7. Two forces of 5N &22N are acting in a body in the same direction what will be the resultant force& in which direction will it act?

If the two forces in the above example would have been acting in the opposite direction What would be the resultant force& in which direction will it act?

8.If we push the box with a small force, the box does not move, why?

9. What should be the force acting on an object moving with uniform velocity?

10. Give reasons:

(a) Carpet is beaten with a stick to clean it,

(b) Seat belts are provided in the cars to prevent accidents.

(c) Only the carom coin at the bottom of a pile is removed when a fast moving carom coin (or striker) hits it.

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