Motion And Force - Change In State Of Motion

What is Force ?

Before we dwell into how force can affect the state of motion of a body, let’s take a glance at force. Force is an interaction between two or more bodies that when unopposed causes a change in the state of motion of the objects. So in the definition of force itself, it explicitly states that force is something that causes a change in the state of motion. Force can also be thought of as a push or a pull. So let’s see how force affects the state of motion of a body. But what exactly is a change in the state of motion? A change in the state of motion corresponds to either a change in the speed of the body and/ or a change in the direction of motion of the body. The change in speed could mean a speeding up (acceleration) or speeding down (deceleration).

A ball cannot move on its own, neither can a chair or a table. All non-living objects are stationary unless a force is acting on them. You need to have an engine to generate force in order to make it move, you need to kick the ball which is a form of applied force in order for the ball to fly into the goal. The chair will not move unless you move it. All these are examples of how force plays a huge role in our daily life.

A force can result in the following changes in the state of motion of a body;

  • Force can cause Motion and Acceleration:

    There are myriad of examples in real life of force resulting in motion. When you kick a stationary ball. You are applying force to it through your legs which causes the ball to start moving. This is not the only effect though, force can also cause an increase in motion if it is applied in the same direction in which the body is moving. For example, if you cycling and you’re already in motion, by pedalling some more you are effectively using force to increase the speed in which you are travelling. This is an example of force being used to speed up or cause acceleration. We should remember that the acceleration happens only as long as force is being applied. If the force is removed, the acceleration stops.

State Of Motion

The force applied by the snow bike through string causes the man on skis to move.

  • Force can cause Deceleration or stop Motion:

    Brakes are an extremely important part of a vehicle. Can you imagine how dangerous your cycle would be if you could not slow down in the face of an accident. Luckily force comes into the rescue once again. Force can sometimes also cause motion to stop or in other words cause deceleration. Brakes use the force you applied to slow your vehicle down or to stop it. If you throw a ball up at a given velocity, it will slow down as it travels upward due to the force of gravity.

State Of Motion

A goalkeeper applies force to stop an incoming ball.

  • Force can cause a Change in the Direction:

    Do you play cricket? Cricket and almost every other sport relies on force to change the direction of  the ball. The bowler ball the ball towards the batsman but on coming in contact with the bat the ball travels in a completely different path. This is because the batsman applies force on the ball in order to change the direction of the ball. The steering wheel of your car works similarly. By applying force to turn the wheel, you are changing the direction in which your car is travelling.

State Of Motion

A cricketer uses force to change the direction of an incoming ball.

To summarize, a force is a push, pull, or dragging on an object that affects the object’s state of motion. The force may also cause a combination of the above effects, it may both accelerate/ decelerate and change the direction of the object. Force in real life most often affects both speed and direction of the motion. The action from a force can cause an object to accelerate, to decelerate, to stop or to change direction.

Watch this video to get a clear picture of motion:

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Practise This Question

Which of the following statements is/are true?

1. Forces in the same direction add up.

2. Two forces in opposite directions result in a net force, equal to their difference in the direction of the larger force.

3. Change in either direction or magnitude of force changes its effect.

4. When more than one force is acting on an object, the effect on the object is due to a net force acting on it.