about the laws and force

F ORCE AND L AWS OF MOTION

1. What is Force?

Ans: Force is defined as that push or pull which produces or changes to produce

destroy or change to destroy motion in a body, increases or decreases the

speed of the body or changes its direction of motion.

2. What is the use of a force?

Ans: A force can be used to

i) change the magnitude of velocity of an object (that is, to make the object

move faster or slower) or

ii) change its direction of motion or

iii) can change the shape and size of objects

3. What are balanced and unbalanced forces?

Ans: If a set of forces acting on a body does not change the state of rest or of

motion of an object such forces are called balanced forces.

If a set of forces acting on a body brings it in motion or change either in its

speed or the direction of its motion, such forces are calledun balanced

forces.

4. What is Friction?

Ans: Whenever a body tends to slide over another’s surface, an opposing force

comes in to play. This force is called the force of friction orsimpl y

Friction.

i.e. i. friction arises between two surfaces in contact;

ii. it acts opposite to the direction of motion (i.e. it opposes the motion of

the body.

iii. it acts in a direction parallel to the contact surfaces.

5. State Newton’s First Law of Motion?

Ans: An object remains in a state of rest or of uniform motion in a straight line

unless compelled to change that state by an external unbalanced force.

6. Why Newton’s First Law of Motion is called the Law of Inertia?

Ans: The tendency of undisturbed objects to stay at rest or to keep moving with

the same velocity is calledinertia. This is why, the first law of motion is also

known as the law of inertia.

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7 How inertia and mass are related?

Ans: There is a resistance offered by an object to change its state of motion. If it

is at rest it tends to remain at rest (inertia of rest); if it is moving it tends to keep moving (inertia of motion). This property of an object is called its inertia. Heavier or more massive objects offer largerinertia.

Quantitatively, the inertia of an object is measured by its mass. We

may thus relate inertia and mass as follows:

Inertia is the natural tendency of an object to resist a change in its state of motion or of rest. The mass of an object is a measure of its inertia. Heavier or more massive objects offer larger inertia.

8. Explain why some of the leaves may get detached from a tree if we

vigorously shake its branch.

Ans: due to inertia of rest. When the branch was vigorously shaked the portion

of the leaves attached to it acquires motion with the branch while the upper

part of the leaves still wanted to remain at rest. Hence get detached.

9. Why do you fall in the forward direction when a moving bus brakes to a

stop and fall backwards when it accelerates from rest?

Ans: We fall in the forward direction when a moving bus brakes to a stop

because the lower part of our body comes to rest with the bus while the

upper part is still in motion.

Similarly, We fall backwards when the bus accelerates from rest because the lower part of our body acquires motion with the bus while the upper part is still at rest..

10. What is momentum? What is the SI unit of momentum?

Ans: The momentum, p of an object is defined as the product of its mass, m and

velocity, v.

That is,

Momentum has both direction and magnitude. Its direction is the same as

that of velocity,v.

The SI unit of momentum is kilogram-metre per second (kg m/s).

A force produces a change of momentum.

11. State Newton’s Second Law of Motion.

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p= mv

Ans: The rate of change of momentum of an object is proportional to the applied

unbalanced force and takes place in the direction of force.

12. What is the Mathematical Formulation of Newton’s Second Law of

Motion?(for details refer book)

Ans: The Mathematical Formulation of Newton’s Second Law of Motion is

i.e.

(Force)= (mass) x (acceleration)

13. What is the SI unit of Force?

Ans: The unit of force is kg m s-2 or Newton, which has the symbol N.

14. How the first law of motion can be mathematically stated from the

mathematical expression for the Second Law?

Ans: From second law of motion, we have

F = ma

or

F = m(v-u)/t

or

Ft = mv – mu

That is, whenF = 0,v = u for whatever time,t is taken.

This means that the object will continue moving with uniform velocity, u throughout the time,t. Ifu is zero thenv will also be zero. That is, the object will remain at rest.

15. What is impulse of a Force?

Ans: Impulse of a Force is the product of force and the time for which the force

acts. According to Newton’s 2nd Law,

F = ma

or

F = m(v-u)/t

or

Ft = mv – mu

i.e. Impulse of a Force=Change in momentum.

Impulse of a Force is defined as the Change in momentum produced by the

force..

16. State Newton’s THIRD Law of Motion. Illustrate with two examples.

Ans:Third law of motion: To every action, there is an equal and opposite

reaction and they act on two different bodies.

ILLUSTRATIONS:

3

F= ma

(a) Recoil of a gun: When a gun is fired, it exerts a forward force on the bullet.

The bullet exerts an equal and opposite reaction force on the gun. This

results in the recoil of the gun.

Since the gun has a much greater mass than the bullet, the acceleration of

the gun is much less than the acceleration of the bullet.

(b)A Rowing boat: when a sailor jumps out of a rowing boat forward, the force

on the boat moves it backwards.

17. State the Law of conservation of momentum and prove it for an isolated

system.

Ans: The sum of momenta of the two objects before collision is equal to the sum

of momenta after the collision provided there is no external unbalanced force acting on them. i.e. In an isolated system, the total momentum remains conserved.

Proof: Suppose two objects (two balls A and B, say) of massesmA andmB are

traveling in the same direction along a straight line.

Let the velocitiesuA anduB, respectively and LetuA >uB.

There are no other external unbalanced forces acting on them.

The two balls collide with each other for a timet,

(For Detail PROOF refer Text )

Then, the force on ball B by ball A(action)= the force on ball A(reaction) by

ball BOR,

FAB=– FBA

This gives,

Total Momentum before Collision= Total Momentum after Collision

18. Find the recoil velocity of a gun.

Ans:Let M and m be the Masses of the gun and the bullet respectively.

Let V and v their respective velocities. Before firing, V=v=0. Momentum before firing=0 & Momentum after firing= MV + mv Therefore,

MV = - mv

Or,

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mA uA+ mB uB =mA vA+ mB vB

V = - mv /M