# 9th Class Science Force and laws of motion Force

## Force

Category : 9th Class

### Introduction

The Greek thinker, Aristotle held the view that if a body is moving, something external -is required to keep it moving. The Aristotelian law of motion may be phrased for our purpose here, : An external force is required to keep a body in motion.

Force

Force is that agency which can change the state of rest or of uniform motion or shape or direction of motion of any object. The unit of force in M.K.S. system is newton. It is a vector quantity.

Effects of force

• Force is required to change the shape and size of a body.
• Force is required to move or stop a body.
• Force is required to change the speed or direction of a moving body.

Balanced and Unbalanced Forces

Balanced forces cannot change the speed of a body. They can change the shape of the object. If the resultant of the forces acting on a body is zero, then the forces are said to be balanced.

In tug of war you must have observed balanced forces in action, if the two teams have equal strength, or force, the rope will stay pretty much in the same place.

Unbalanced forces can cause the change in speed or direction of motion of the body. If the resultant of the forces acting on the body is not zero, the forces are said to be unbalanced.

The trolley will accelerate if $T>F$

When two unbalanced forces are exerted in opposite directions, their combined effect is equal to the difference between the two forces and is exerted in the direction of the larger force.

Resultant Force

When two or more forces act on a body simultaneously, then the single force which produces the same effect as produced by all the forces acting together is known as the resultant force.

 Description of motion Net Force : Yes or No ? Yes No Yes No Yes Yes

A 2-kg object is moving horizontally with a speed of 4 m/s. How much net force is required to keep the object moving at this speed and in this direction?

(a) 8N

(b) ON

(c) 20 N

(d) 7N

(e) None of these

Suppose you were in space in a weightless environment, would it require a force to set an object in motion?

(a) Yes

(b) No

(c) Both A and Bare partly correct

(d) All of these

(e) None of these