FORCE AND LAWS OF MOTION, Class IX
FORCE
When we want to open a door, we have to push the door handle and when we want to close the door, we have to pull the door handle with our hand. This means that to move a body or an object it has either to be pushed or pulled. A push or pull on a body is called force. The direction of force is the direction of pulled or pushed object. Forces are used in our everyday actions like pushing, pulling, lifting, stretching, twisting and pressing. For example,
1. A force is used when we push or kick a football.
2. A force is used when we pull a table.
3. A force is used when we lift a bag from the floor.
4. A force is used when we stretch a rubber band.
5. A force is used when we twist a wet cloth to squeeze the water out.
6. A force is used when we press the brake pedal of a bus.
Effects of force
which it can produce in various bodies or objects and us.
A force can move a stationary body
If we kick a football kept on the ground with
our foot, then the football starts moving, the force of our foot moves a
stationary football. The force of engine can move a stationary car, from these
examples we conclude that a force can make a stationary body move.
A force can stop a moving body
Let a football moving on the ground stops after some time. In this case, the force of friction of ground stops the moving football from examples we conclude that a force can make a moving body stop.
A force can change the speed of a moving body
A
bicycle is moving with a certain speed. Now if someone pushes the moving
bicycle from behind, then the speed of bicycle increases and it will move faster.
We can say that a force can change the speed of a moving bicycle or moving body.
If someone pulls the moving bicycle from
behind, the speed of moving bicycle decreases and it will move slower. This
pull or push is known as force, so we can say that the force can change the
speed of moving body or object.
If
the force is applied in the direction of motion of a body, its speed increases.
On the other hand, if the force is applied in the opposite direction. The speed
decreases.
The
speed of a falling ball or any other falling body increases because the earth
applies a pulling force on it which is called the force of gravity.
A force can the change the direction of a moving body
When a moving tennis ball is hit by a racket, then the direction of tennis ball changes and it goes in a different direction. If we blow air form our mouth on the smoke rising up from a burning incense stick then the direction of motion of smoke changes. In the case, the force exerted by the blowing air change the direction of moving smoke. From these examples we conclude that a force can change the direction of motion of a moving body.
A force can change the shape and size of a body
If we take a light spring and pull it at both the ends with our hands, then shape and size of the spring change. The turns of the spring become farther apart and its length increases. In this case, the force of our hands changes the shape and size of the spring. For this example, a force can change the shape and size of a body or object.
BALANCED AND UNBLANCED FORCES
Forces are of two types: Balanced force and unbalanced force.
Balanced forces
If the resultant of all the forces acting on a body is zero, the forces are called balanced forces. A body under the action of balanced forces doesn’t change its position of rest or of uniform motion and it appears as if no force is acting on it. This will become clearer from the following example. Suppose a heavy box is lying on the ground let us push this box with our hands. We find that the box doesn’t move and remains in its state of rest though as many as four forces are acting on it. The four forces acting on the box are.
1. Force of our push.
2. Force of friction (which opposes the push and doesn’t allow the box to move).
3. Force of gravity (which pulls the box downwards).
4. Force of reaction (exerted by the ground in the box upwards which balanced the force of gravity).
Now through the box four forces are acting on it. Since the box doesn’t move at all, we conclude that the resultant of all the forces acting it is zero. The box, therefore, behaves as if no force is acting on it. The force acting on this stationary box are an example of balanced forces that the force of our push on the box is balanced by the force of friction and the force of gravity is balanced by the force of reaction of the ground. If a number of balanced forces act on a stationary body, the body continues to remain in its stationary position.
Unbalanced
forces
If the resultant of all the forces acting on a body is not zero, the forces are called unbalanced forces. When unbalanced forces act on a body, they produced change in its state of rest or of uniform motion. That is unbalanced forces can move a stationary body or they can stop a moving body. In other words, unbalanced forces acting on a body can change its speed or direction of motion. This will become clearer from the following example. Suppose a toy car is lying on the ground. Let us push this car with our hand. We find that the toy car starts moving.
(i) Force of our push.
In this case also, the force of gravity on the car acting downwards and the force of reaction of ground acting upwards are equal and opposite, so they balance each other. Now due to wheels of the toy car, the opposing force of friction is much less here. The force of our push is, therefore greater than the force of friction in this case, so they can’t balance each other. The resultant of all the forces acting on the toy car is not zero. There is a net unbalanced force acting on the toy car which makes the car move from its position of rest.
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