# Science : Force (Part-2)

## Key learning

• Force is a push or pull acting upon an object.
• Balanced forces: – The resultant of all the forces acting on a body is zero.
• Unbalanced forces:- The resultant of all the forces acting on a body is greater than zero.
• Force is also defined as “The product of mass and acceleration.

Friction  – Fraction is the force that resists the motion of one surface relative to another with which it is in contact

• It is parallel to the contact surface and opposite to the direction of motion.
• I. unit is Newton (N)
• Vector Quantity

Methods to reduce friction:-

• Lubricate the surfaces in contact
• Smoothen the surface in contact
• Place ball bearings, rollers between surface

Moment of a force or Torque

• The Turning effect of a force is known as the moment
• It is the product of the force (F) and the perpendicular distance (d) from the line of action of the force and pivot.
• I. unit is Newton metre (N-m)
• It is vector quantity

### Newton’s laws of motion

First law of motion – “A body will continue in its state of rest or uniform motion in a straight line unless an external resultant force acts on it.

Newton’s First Law of Motion implies that –

1. The state of rest requires no resultant force to maintain.
2. The state of uniform velocity also requires no resultant force to maintain
• Inertia is the tendency of an object to continue in its original of motion.
• Newton’s second law:- “The rate of change of momentum of a body is proportional to the resultant force acting on it and the change takes place in the direction of the forces”

Momentum of a body is proportional to the resultant force acting on it and the change takes place in the direction of the force”

• Momentum (P):- Momentum of a moving body is the product of its mass (m), and its velocity (v).
• I. unit is Newton –Metre/Second (nm/s)
• Vector quantity
• Newton’s third law:- “If body A exerts a force on body B, then body B exerts a force of equal magnitude but in the opposite direction on body A.”

## Previous years Questions

Q. 1 The sliding friction is ———— than the static friction.

(a) Double            (b) Same             (c) Greater                  (d) Smaller

Ans. (d) Sliding Friction:- It is that kind of friction that happens when a body slides over another body.

Static friction – It is the force of friction between two surfaces so long as there is no relative motion between ­­ them.

Q.2       For an object, the state of rest is considered to be the state of ———– speed.

(a) Decreasing              (b) Increasing                (b) zero             (d) Negative

Ans.:- The state of rest is considered to be the state of zero speed.

Q.3       Action and Reaction ———-.

(a) Always act on same body

(b) Are equal in magnitude

(c) Are in same direction

(d) Always act independently

Ans. (b)  According to the third law of motion “for every action there is an equal and opposite reaction:

Q.4      The S.I. unit of acceleration is ———

(a) Meters/sec. sq.

(b) Meters/sec.

(c) Seconds/meter

(d) Seconds/meter2

Ans. (a)  Acceleration is the change in velocity per unit time. Its S.I. unit is m/sec. sq.

Q.5      The mass of an object ———-.

(a) Changes from place to place

(b) Remains same every where

(c) Is equal to its weight

(d) Is greater at mountains.

Ans. (b) The mass of an object is constant, it does not change unless it gains or losses matter.