physics - force and laws of motion

Class 9 - Physics

Force and Laws of Motion

MCQ
Q&A
Question:
A hockey ball of mass 200 g travelling at 10 m/s is struck by a hockey stick so as to return it along its original path with a velocity of 5 m/s.Calculate the change in momentum of the hockey ball by the force applied by the hockey stick.
Answer:

Here.m = 200 g =2001000 kg =15kg
initial velocity,u- 10 ms-1
final velocity.v = -5ms-1,negative sign is used because the ball returns along the original path,change of momentum = final momentum - initial momentum
= mv-mu =15(-5)-15(10)=-3 kg m/s

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Questions and Answers


Force and Laws of Motion - Questions
1.

Identify the action and reaction forces in the following cases:
(a) A television (TV) lying on a table
(b) Pushing a wall with your hand
(c) Firing a bullet from a gun.
(d) Walking of a person on a ground.


Answer
2. Fill in the blanks :
An imaginary co-ordinate system which is either at rest or in uniform and where Newton's laws are valid is called an _______ frame of reference.
Answer
3. Why does a bicycle slow down when the cyclist stop pedalling?
Answer
4. What do you mean by an imapct force?
Answer
5. If the body is found to be accelerated, is the force acting on it balanced or unbalanced?
Answer
6. What did Galileo conclude on the basis of his experiments on the motion of objects?
Answer
7. A body is moving with uniform acceleration. Is its momentum constant?
Answer
8. When a force acting on a body has equal and opposite reaction, then why should the body move at all?
Answer
9.

A hammer of mass 500 g, moving at 50 m/s, 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
10.

It is difficult to balance our body when we accidentally slip on a peel of banana. Explain why?


Answer
11.

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 0.2m s-2. With what force does each person push the motorcar?
(Assume that all persons push the motorcar with the same muscular effort.)
 


Answer
12. Fill in the blanks :
On what factor does the inertia of a body depends? ____________
Answer
13. Can balanced forces change the shape and size of an object?
Answer
14. When a force acts on a body, what are the two essential effects it can produce?
Answer
15. Why do we tend to fall outwards when a car takes a steep turn?
Answer
16. Which has more inertia, a table tennis ball or a rubber ball of the same size?
Answer
17. When you apply a force for a certain amount of time, it creates ________ (fill in the blank).
Answer
18.

Which of the following graph represents unbalanced force.


Answer
19.

Explain, why is it difficult for a fireman to hold a hose, which ejects large amounts of water at a high velocity.


Answer
20.

Suppose a ball of mass m is thrown vertically upward with an initial speed v, its speed decreases continuously till it becomes zero. Thereafter, the ball begins to fall downward and attains the speed v again before striking the ground. It implies that the magnitude of initial and final momentums of the ball are same. Yet, it is not an example of conservation of momentum. Explain why ?


Answer
21.

Two objects of masses 100 g and 200 g are moving along the same line and direction with velocities of 2 m s-1 and 1 m s-1, respectively. They collide and after the collision, the first object moves at a velocity of 1.67 m s-1. Determine the velocity of the second object.
 


Answer
22. Fill in the blanks :
When a car enters a curved path, the person sitting in it is thrown outwards. This is because of the property of ______________.
Answer
23. Fill in the blanks :
When a passenger jumps out of moving bus, he falls down. It is due to _______________.
Answer
24. Fill in the blanks :
An athlete often jumps before taking a long jump. It is due to ________________.
Answer
25. Fill in the blanks :
________ is the tendency of an object to resist a change in motion.
Answer
26. What are balanced forces?
Answer
27. What is the SI unit of impulse.
Answer
28. What is the net momentum of gun and bullet system before firing?
Answer
29. Which principle is involved in the working of a jet plane?
Answer
30.

Name the categories of forces based on interaction.


Answer
31.

A stone of 1 kg is thrown with a velocity of 20 m s-1 across the frozen surface of a lake and comes to rest after travelling a distance of 50 m. What is the force of friction between the stone and the ice? (Calculate time taken and acceleration also).


Answer
32.

An object of mass 5 kg is moving with a velocity 4 m/s. A constant force of 20 N acts on the object. What will be the velocity after 3 s.


Answer
33.

State Newton's third law of motion.


Answer
34.

While driving on a highway, an insect strikes on the car windshield and splatters. Which experiences greater impact (force): an insect or the windshield?


Answer
35.

Using a horizontal force of 200 N, we intend to move a wooden cabinet across a floor at a constant velocity. What is the friction force that will be exerted on the cabinet?


Answer
36. Fill in the blanks :
An umbrella protects us from rain it is based on the property of _____________.
Answer
37. Fill in the blanks :
The property of an object to resist changes in its rotational state of motion is called ________.
Answer
38. Which law of motion gives the measure of force?
Answer
39. When a body moves on flat friction-less surface, will its speed change?
Answer
40. Is force a scalar or vector quantity?
Answer
41. Name the physical quantity that corresponds to the rate of change of momentum.
Answer
42.

What are non-contact forces? Give examples.


Answer
43.

What's the relation between 1N and 1 dyne?


Answer
44.

An automobile vehicle has a mass of 1500 kg. What must be the force between the vehicle and road if the vehicle is to be stopped with a negative acceleration of 1.7 ms-2.
 


Answer
45.

From a rifle of mass 4 kg, a bullet of mass 50 g is fired with an initial velocity of 35 m/s. Calculate the initial recoil velocity of the rifle.


Answer
46.

Two objects, each of mass 1.5 kg, are moving in the same straight line but in opposite directions. The velocity of each object is 2.5 m/s before the collision during which they stick together. What will be the velocity of the combined object after collision?


Answer
47.

A hockey ball of mass 200 g travelling at 10 m s-1 is struck by a hockey stick so as to return it along its original path with a velocity at 5 m s-1. Calculate the change of momentum occurred in the motion of the hockey ball by the force applied by the hockey stick.
 


Answer
48.

A bullet of mass 10 g travelling horizontally with a velocity of 150 m/s strikes a stationary wooden block and comes to rest in 0.03 s. Calculate the distance of penetration of the bullet into the block. Also calculate the magnitude of the force exerted by the wooden block on the bullet.


Answer
49.

Two objects having their masses in ratio 3:5 are acted upon by two forces each on one object. The forces are in the ration of 5:3. Find the ratio in their accelerations.


Answer
50.

The speed-time graph of a car is shown below. The car weighs 1000kg. 
(i) Find the distance covered by the in fits two seconds.
(ii) Find how much force is applied by the car brakes in the fifth second so that the car comes to an halt by sixth second.


Answer
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