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## Forces – A result of interaction

### Key Concepts

1. Force – A result of interaction

### Introduction

We do a lot of activities in our daily lives which involve application of force in one way or the other such as moving from place to place, playing outdoors, school activities etc. An object applies force on another object by certain actions such as pushing, pulling, lifting, kicking, flicking, throwing etc. All these actions involve two objects, one which applies force and the other on which the force is applied. In this section we will be learning about the way force is applied by a body on another.

### Force:

The pictures below show some of the activities that one performs on a day to day basis. All these activities involve the application of a force on another object. For example, in the first picture, the man is applying a force on the trolley in the forward direction by pushing it, in the third picture the body is applying a force on a football and in the fifth picture the lady is applying a force on the bag. In other words, the application of force involves more than one object interacting with each other.

Force has a particular direction of application, which is in the direction of push or pull. The force applied on to an object can be measured in the SI unit of Newton (N)

## Force – A result of interaction:

Suppose a car does not start by following the usual method. Will the car start if a person stands near its back as shown in the picture below? No, it would not start this way. The person needs to push the car hard in order to make it start moving.

It can be concluded that,

When the objects are not interacting with each other, the force is not applied

When the objects are interacting with each other, the force is applied

(3)                                                              (4)

In picture (1), both the girls are pushing each other. The forces applied by them are in opposite direction to and towards each other.

In picture (2), both the boys are pulling each other. The forces applied by them are in opposite direction to and away from each other.

In picture (3), the boy and the dog are pulling each other. The forces applied by them are in opposite direction to and away from each other.

[Fig 2.4: Direction of forces]

Here the boy and the girl are pulling a toy towards each other. There are three objects involved in this act. The forces involved are opposite in direction and are directed away from each other. The forces come to play only when the girl, the toy and the boy interact with each other as shown.

Identify the agent that applies the force and the object on which it acts in the following cases. Also identify the directions of all the forces applied.

In picture (1), the hand squeezes the lemon from both sides. Therefore, the agent that applies force is the hand and the force acts on the lemon, squeezing the juice out of it. The forces are applied from opposite directions as shown by the red arrows.

In picture (2), the girl lifts the weight by pushing it upwards using her hands. Therefore, the agent that applies force is the girl and the force acts on the weight. The forces are applied from the bottom in the upward direction as shown by the red arrows.

In picture (3), the man applies a force on the slingshot by pulling it towards himself. Therefore, the agent that applies force is the man and the force acts on the slingshot. The force is applied in the backward direction as shown by the red arrow.

## Summary

1. Force is a result of interaction between objects.
2. The force applied on to an object can be measured in the SI unit of Newton (N).
3. Force has a particular direction of application, which is the direction of push or pull.
4. No force is applied when the objects are not interacting with each other.
5. For a force to come to play at least two objects should interact with each other.

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