Chapter 11 Force and Pressure

Force is simple called as push or pull.

Examples of push:

• Pushing a car.
• Inserting a plus in socket.
• Close the door
• Pushing a trolley.
• Kicking a ball.

Examples of pull force are as follows:

• Opening of door.
• Horse pulling the cart.
• Drawing a bucket of water from the well.
• Plucking the string of the guitar.
• Dog pulls its piece of the meat towards itself.

By applying force, shape of the object got changed. Some examples are as follows:

1. Change in the shape of dough occurs by applying force.
2. By using muscular force, we can change the shape of a plastic ball by squeezing it.
3. Deformation of clay by pressing it in between the hands.
4. Elongation and contraction of a rubber band when force is applied on it.

(a) Pull

Pull or push is simply called as force. To draw water from a well, we have to pull whereas we have to apply push force on bucket to throw it in the well.

(b) Attracts

When a charged body attracts another charged or uncharged body towards it, is called as the electrostatic force of attraction.

(c) Push

If we want to change in the direction of trolley, we have to apply either push force or pull force on it. In the pull force, the direction changes in object is same to the applying force.

(d) Repels

Magnet have two poles: North and South. Same pole repels each other whereas opposite poles of magnet attracts each other.

(a) Shape

Applied force can change the shape and the direction of the object. So, when we stretch a bow, pull force is applied on it, so its shape is changed.

(b) Muscular

Stretching of bow is the example of a contact force. In this, muscular force is applied on it. With the help of muscular force, we stretch the bow.

(c) Contact

Contact force are the forces that act between two objects that are physically touching each other. Here, we touch bow physically, this is an example of contact forces.

(d) Gravity, friction

The friction force is applied by the air is known as the drag force. When arrow moves towards the target, earth attracts the arrow towards itself due to gravity and the frictional force of air also come into play.

1. In squeezing a piece of lemon, the muscular force is used to extract juice from it. It is an example of contact force. Our muscles apply force on lemon, then due to squeezing its shape got changed.
2. In taking out paste from the toothpaste tube, we use muscular force here. Hence its shape got changed due to applied muscular force.
3. Here, push force is applied by the load on the spring. Due to it, spring goes downward. Hence, the spring is stretched and the shape of the spring is change due to the applied push force.
4. Here, athlete applies push force on the ground with his feet. By doing this, his muscular feet feels some muscular force on the ground. Due to this, he jumps over the bar. Change in the motion of the athlete occurs by applying push and muscular force.

When a blacksmith hammers a hot piece of iron, he uses its muscular force on the hot piece of iron. This is the example of contact force also. By applying muscular force, blacksmith is able to change in the shape of the hot iron piece.

When an inflated balloon is rubbed with a piece of synthetic cloth, then it becomes charged. The balloon sticks to the wall because of the attractional force present between them. When a charged body attracts an charged or uncharged body towards itself then it is known as electrostatic force of attraction. Hence, the balloon sticks on the wall due to the presence of electrostatic force of attraction between the balloon and the wall.

Here, muscular force is acting on the plastic bucket containing water held above the ground level. Muscular force is applied in the opposite direction of gravity; here which pulls the bucket towards the ground. Here both muscular and gravity are equal in magnitude. So, both the forces cancel out each other. Here the net force on the bucket is zero. Hence there is no change in the state of the motion of the bucket.

When a rocket is fired upwards to launch a satellite in its orbit, then the force of gravity is acting on it. The force of gravity pulls the rocket towards the earth but the force of friction which is due to the earth’s atmosphere, opposes its motion.

(d) An atmospheric pressure

It is because, when we press the bulb of a dropper with its nozzle kept in water, air in the dropper is seen to escape in the form of bubbles. So, rise of the water in the dropper is due to the atmospheric pressure. When all the air escapes from the nozzle, then the atmospheric pressure, act on water, forces to fill the dropper.