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**Ans.** Friction can be reduced by:

- Making sliding surface moth.
- Lubricating the sliding surface.

Ans.

F = 20N

a = 2mS-¹

m = ?

Formula F = ma

m = F/a = 20/2

m = 10 kg

**Ans.** The interlocking between ups and downs of the two surfaces need not be ruptured, in case of rolling while in case of sliding these are to be ruptured and results larger force of friction.

Ans.

W = 147N m=?

Formula W = mg

m = W/g

m = 147/10

m = 14.7 Kg

Ans. The inertia of a body is its property due to which it resists any change in its state of rest or motion.

**Ans. **The momentum of a body is the quantity of motion it possesses due to its mass and velocity.

Formula = P = mv

Ans. According to Newton’s third law of motion, there exists a reaction to the centripetal force. The centripetal reaction that pulls the string outward is sometimes called the centrifugal force.

**Ans.**

**Mass:**

- Mass of a body is the quantity of matter possessed by the body.
- Mass is a scalar quantity.

**Weight:**

- The weight of a body is the force of gravity acting on it.
- It is a vector quantity.

**Ans.** The momentum of a body is the quantity of motion it possesses due to its mass and velocity.

**Unit:** Its SI unit is Kgms-¹

Ans.

**Mass:** Mass of a body is the quantity of matter possessed by the body.

**Weight:** Weight of a body is the force of gravity acting on it.

**Ans.** When a net force acts on a body, it produces acceleration in the body in the direction of the net force. The magnitude of this acceleration is directly proportional to the net force acting on the body and inversely proportional to its mass.

**Equation:** F = m a

**Ans.** Centripetal force is a force that keeps a body to move in a circle.

**Mathematical form:** F* _{c = }*mv²/r

**Ans.
Action:** Action is always accompanied by a reaction force and the two forces must always be equal and opposite.

**Reaction:** To every action, there is always an equal but opposite reaction.

**Ans.**

- Friction causes wear and tear of the moving parts of machines.
- Most of the useful energy is lost due to friction

**Ans.** A force moves or tends to move stops or tends to stop the motion of a body. The force can also chane the direction of motion of a body.

**Unit:** Its SI unit Newton (N)

Ans.

F = 100 N, m=50Kg

Formula: F=ma

a=F/m = 100/50 = 2mS-²

Ans. To every action, there is always an equal but opposite reaction.

**Example:** Interaction Forces Pairs.

Ans. The ratio between the force of limiting friction F* _{s}* and the normal reaction R is constant. This constant is called the coefficient of friction.

**Symbol:** µ

**Ans.** The branch of mechanics that deals with the study of the motion of an object and the causes of its motion is called dynamics.

**Ans.** Rolling friction is the force of friction between a rolling body and a surface over which its rolls.

Solved MCQs of Physics Chapter 3 for Class 9 are also available.

Above available Short Questions are taken from Al-Hassan Super Notes Physics (English Medium) – IX

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