Force and Motion

What Is Force?

You push a cart or squeeze a piece of clay. You pedal a bicycle or kick a football. In all these actions you are applying force. Force is something that changes the motion or shape of an object.The examples mentioned are all Mechanical Forces, where one object touches another. There are many other types of forces – electricity, gravitation, magnetism, which act without contact among objects. For now, we will focus only on mechanical forces.

Force and Motion are related

An object at rest can only be put into motion by a cause. When you push a stationary cart, it starts to move. This cause is the force you’ve applied. Force can cause an object to move, it can cause an object to change direction, and it can cause an object to slow down and stop.

When you hit a ball coming towards you, you are applying force to change the direction of that ball. When you roll a ball on a rough carpet, it eventually slows down because Friction (a kind of force) between the carpet and the ball slows the ball down.

There cannot be moved without a force. And, a force always causes motion or a change in motion.

What is Speed?

Speed is how we measure motion. It is a function of time and distance – how far an object has moved in a period of time. It is the ratio of distance over time. A car that has traveled 60 kilometers in one hour implies that its speed is 60 kilometers per hour.

In reality, the car would have moved at different speeds over the hour. It would have started from rest (zero speed) and then gradually increased its speed over the hour so as to cover 60 kilometers. Hence, speed is often referred to as Average Speed.

Speed (or Average Speed) = Distance/Time

What is Uniform and Non-Uniform Motion?

When both the speed and direction of a moving object are constant, the motion of the object is said to be Uniform – it covers equal distance in equal intervals of time.

A car that covers 30 meters in every second is said to have a uniform speed of 30 meters per second.

An object that covers unequal distances in equal intervals of time is said to have Non-uniform motion. A horse that runs a race at different speeds at different points of time would be a good example.

What are Acceleration and Deceleration?

If the velocity of an object changes (increases) from one time period to another, the object is undergoing an Acceleration. Or, if the velocity decreases from one time period to another, the object is undergoing a Deceleration (also called Negative Acceleration).

A car that moves at 10 miles per hour for the first minute, then at 20 miles per hour for the second minute, and finally at 30 miles per hour for the third minute, would be said to have a Uniform Acceleration of “10 miles per hour per minute”. If the car slowed down at the same rate it would be a Uniform Deceleration. If the rates of acceleration or deceleration vary from one time period to another, then it is said to have Variable Acceleration or VariableDeceleration.

Acceleration = Final velocity – Starting velocity

Elapsed Time

For example, if a car accelerates from 0 miles per hour to 60 miles per hour in 30 seconds then the acceleration of the car would be –

Check Point

1. ______ can change the motion or shape of an object.
2. When an object changes its location in space, it is referred to as its ______.
3. Motion is always in relation to some other place or object in the universe – it is with respect to a ________.
4. ______ is the ratio of distance over time.
5. When the velocity of an object increases from one time period to another, the object is said to undergo an _______.

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