# Define Momentum Define Friction

Q.11: Define Momentum. Write down its formula and unit. On which factors momentum depends?

Ans: MOMENTUM:

The quantity of motion contained in a body is called momentum. It is denoted by P. It is a vector quantity.

Formula:

Mathematically, momentum can be defined as the product of mass and velocity. If m is the mass of a body moving with velocity v then the momentum P given by:

P=mV

Unit:

The S.I. unit of momentum is kilogram meter per second (kg-m/s) or Newton second (N-s).

Factors On Which Momentum Depends:

There are two factors on which momentum depends:

The momentum is directly proportional to the mass of the body. It is directly proportional to the velocity of the body.

Q.12: Define law of conservation of momentum?

Ans: Law OF CONSERVATION OF MOMENTUM:

The law of conservation of momentum states that the momentum of an isolated system always remains constant or conserved.

Q.13: Explain the law of conservation of momentum with the help of an example?

Ans: We consider a system of two non-rotating colliding balls A and B having masses m, and m2, moving with velocities u, and u, respectively along a straight line in the same direction.

According to the law of conservation of momentum,

Total momentum of the system before collision

= m1 u1 +m2 u2 ———- (i)

If u1>u2, the balls will collide with each other. Suppose their velocities become v1 and v2 respectively after the collision.

So, the total momentum of the system after collision

=m1 v1 + m2 v2 ——— (ii)

According to the law of conservation of momentum

Total momentum of the system before collision = Total momentum of the system after collision

Therefore,

M1 u1 + m2 u2 = m1 v1 + m2 v2

Q.14: Write down any two application of law of conservation of momentum?

Ans: APPLICATION OF Law OF CONSERVATION OF MOMENTUM:

• If a balloon is blown up and released, it flies round the room. During the flight, air escapes from the balloon in one direction while it moves in the opposite direction. This is due to the law of conservation of momentum of the system.
• The recoil of a gun when a bullet is fired from it is due to the conservation of momentum of the system.

Q.15: Define friction? Write down Its expression and on what factors it depends?

Ans: FRICTION:

It is a force which opposes the motion of a body while in continuous contact with the another body.

Expression For Friction:

Let us consider forces on the wooden block placed on the table. There are two forces acting on it. One is the weight W of the block acting downwards and the other is reaction of the table R acting upwards.

The limiting friction F; is directly proportional to the normal reaction R. Mathematically we can write,

Coefficient of friction is constant for a given pair of surfaces and is different for different pairs of surfaces.

Factors On Which Friction Depends:

There are two factors on which the force of friction depends:

• It depends upon the nature of the surface.
• Itis directly proportional to the normal reaction.

Q.16: Write down the different types of friction?

Ans: STATIC FRICTION:

The resistance force between the two surfaces before the motion starts is called the static friction. The maximum value of the static friction is called limiting friction.

Kinetic Friction:

The friction during motion is called kinetic friction. This friction is slightly less than the limiting friction.

Sliding Friction:

When a body is sliding over a surface, the friction force which exists during the motion is called the sliding friction.

Rolling Friction:

When a spherical body rolls on a surface, the friction force which exists during the motion is called the rolling friction.

Q.17: Define the following terms?

• Limiting Friction

Ans: LIMITING FRICTION:

The maximum value of the resisting force between the two surfaces before the motion starts (static friction) is called as limiting friction.

The force of friction has the ability to increase its value with the increase of applied force till it reaches a maximum valise. Due to this strange nature of frictional force, we call It self-adjusting force.

Q.18: What are the few advantages of friction?

• Friction enables us to walk on the ground.
• The threading on the car tyres is done to provide necessary frictional force between the tyre and the road to enable the car to move smoothly.
• Anail stays in the wood because of friction.
• Nut and bolt can hold a body due to friction.

Q.19: What are the disadvantages of friction?