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QUESTION: 1

A force which acts for a small time and also varies with time is called:

Solution:

A large amount of force acting on an object for a short interval of time is called impulse or impulsive force. Numerically impulse is the product of force and time. Impulse of an object is equal to the change in momentum of the object.

QUESTION: 2

If two ends of a spring balance are pulled by a force of 10N, then the reading of the spring balance will be

Solution:

Spring balance works on action reaction principle. Hence, according to Newton’s third law the force measured by Spring balance will be 10 N.

QUESTION: 3

Action and reaction do not cancel each other out because

Solution:

The important thing to notice is that action and reaction forces act on different objects in the interaction. Two equal and opposite forces acting on the same object do not make an action-reaction pair! This is why the two opposing forces don't cancel out.

QUESTION: 4

The force exerted by the floor of an elevator on the foot of a person is _______ the weight of the person if the elevator is going up and speeding up.

Solution:

The force exerted by the floor of an elevator on the foot of a person is more than the weight of the person if the elevator is going up and speeding up.

QUESTION: 5

A man weighs 70 kg. He stands on a weighing scale in a lift which is moving upwards with an acceleration of 5ms^{2}.What would be the reading on the scale? (g=10 ms^{2})

Solution:

As the moving elevator is a non inertial frame hence newton's laws can’t be applied directly to it. So to apply Newton's laws we need to add a pseudo force to the man's body equal to mass times the acceleration of lift in the opposite direction to that of acceleration. Thus the balancing normal force is equal to the weight of the man + mass times the acceleration which is,

Reading = Normal force = 700 + 70 x 5

= 700 + 350

= 1050 N

QUESTION: 6

A ball of mass 150 gram moving at velocity of 15 metre per second is brought to rest by a player in 0.1 second calculate the force acting on the hands of player

Solution:

** Answer :- a**

**Solution :- F = ma **

**F = m ( v-u /t )**

**here u = 0 **

**F = m v /t **

**F = 0.150 x 15 / 0.1 **

**F = 2.25 / 0.1 **

**F = 22.5 N**

QUESTION: 7

An average force of 100N acts on a body for 1s. What is the impulse due to the force?

Solution:

We know that Impulse due to force, I = FΔt

As F = 100N

And Δt = 1sec

We get I = 100 Ns

QUESTION: 8

A ball weighing 0.01kg hits a hard surface vertically with a speed of 5m/s and rebounds with the same speed. The ball remains in contact with the surface for 0.01s. The average force exerted by the surface on the ball is

Solution:

Since impulse I = F x Δt

And also Impulse I = Δp (i.e. change in linear momentum)

Change in momentum

= 0.01 x 5 – (-5)

= 0.10 N s

Impulse = 0.10 N s

Therefore 0.10 = F x 0.01

Or F = 10 N

QUESTION: 9

The principle of working of a rocket is based on

Solution:

Rocket works on the principle of conservation of momentum. Rocket ejaculates gases in backward direction which creates momentum of the gases backwards and thus by conservation of momentum, the rocket gets a momentum in the forward direction making it to move forward.

QUESTION: 10

A bus can accelerate on a horizontal road due the force exerted by

Solution:

Engine of the bus exerts force on the wheel is frictional force which opposes the bus to move forward ( acts opposite to the direction of motion of the body). Road also exert equal force on the wheel which pushes the bus forward

QUESTION: 11

The product of forceand the duration of time for which it acts on a body is equal to the

Solution:

** Answer :- c**

**Solution :- Impulse I = Fdt**

**Also force F= mdv/dt**

**So, I = mdv/dt dt = mdv**

**= m(v2 −v1)**

**= mv2 − mv1**

**=p2 − p1 = change in momentum**

QUESTION: 12

A block is placed on the table. What is the angle between the action of the block on the table and reaction of the table on the block?

Solution:

According to Newton's 3 rd law, every action and reaction have the same magnitude and opposite direction. So the angle between them should be 180°.

QUESTION: 13

The SI unit of impulse is

Solution:

Impulse applied to an object produces an equivalent vector change in its linear momentum, also in the same direction. **The SI unit of impulse is the newton second (N⋅s),** and the dimensionally equivalent unit of momentum is the kilogram meter per second (kg⋅m/s).

QUESTION: 14

From the magnitude of impulse, it is possible to find the change in linear momentum because

Solution:

The impulse experienced by the object equals the change in momentum of the object. In equation form, F • t = m • Δ v. In a collision, objects experience an impulse; the impulse causes and is equal to the change in momentum. In a collision, the impulse experienced by an object is always equal to the momentum change.

QUESTION: 15

A boy kicked a stone and got hurt. Which property of stone is responsible?

Solution:

** Answer :- d**

**Solution :- "The property due to which we hurt when a stone is kicked is the Newton's third law of motion.**

**"Newton's third law of motion" states that each and every action, we get an "equal and opposite" reaction.**

QUESTION: 16

A man in the lift weighs more, when the lift

Solution:

In upward direction, acceleration becomes = g+a

N = m(g+a)

QUESTION: 17

A machine gun has a mass of 20 kg fires 35 g bullets at the rate of 400 bullets per second with a speed of 400ms^{-1} .What force must be applied to the gun to keep in the position?

Solution:

To conserve the momentum each time a single bullet is fired,

the reverse speed gained by the gun from one bullet is

V = 400 X .035 / 20

= 0.7 m/s

Thus total speed gained in a second is = 0.7 X 400 = 280 m/s

As total speed is gained in one second only the acceleration produced = 280 m/s^{2}

Thus total force applied on the gun by the bullets = 20 x 280

= 5600 N

QUESTION: 18

Which law says that every force is accompanied by an equal and opposite force?

Solution:

"Every action has an equal and opposite reaction” -Newton's 3rd Law

QUESTION: 19

A bomb of mass 16kg at rest, explodes into two pieces of masses 4kg and 12kg. After explosion, the velocity of the 12kg mass is 4m/s. What is the velocity of the 4kg piece?

Solution:

Simply by conserving the momentum of the system we get that,

0 (initial momentum) = 12 x 4 + 4 x v (final momentum)

Thus we get v = -12 m/s

QUESTION: 20

The dimensional formula for impulse is

Solution:

We know that I = P, where P is momentum

As subtracting initial momentum from the final momentum won't affect its unit, we get unit if I is the same as that of P.

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