Chapter 11: Force and Pressure

Extra Internal Question and Answer:

Question: What is force? Give its definition.

Answer: Force is a push or a pull acting on an object. It arises due to the interaction between two objects. A force can change the state of motion of an object, its direction, or even its shape.

Question: What is meant by contact force? Give two examples.

Answer: A contact force is a type of force that acts only when two objects are in physical contact with each other.

Two examples of contact force are:

(i) When we push or lift an object using our muscles.

(ii) The force that opposes the motion of an object when it moves over a surface.

Question: Can force change the direction of motion of an object? Give an example.
Answer: Yes, force can change the direction of motion. For example, when a batsman hits a cricket ball, its direction changes.
Question: What is a non-contact force? Name its types.

Answer: A non-contact force is a force that acts on an object without any physical contact between the objects. The main types of non-contact forces are gravitational force, magnetic force, and electrostatic force.

Question: What is electrostatic force? Give one example from daily life.
Electrostatic force is the force of attraction or repulsion between charged objects. For example, a rubbed comb can attract small pieces of paper.

Answer: What is the difference between a push and a pull?
Question: The difference between a push and a pull is :

Question: What is muscular force? Give a example.

Answer:  Muscular force is the force exerted by the muscles of our body. It is a contact force because it acts only when there is physical contact. For example, lifting a school bag or pushing a table involves muscular force.

Question: Why is muscular force called a contact force?

Answer: Muscular force is called a contact force because it can act only when there is direct physical contact between the object and the person or animal applying the force. Without touching the object, muscular force cannot be applied.
Question: What is friction? What are the factors affecting friction?

Answer: Friction is the force that opposes the motion of one surface over another when they are in contact. It acts in the opposite direction of motion. Friction depends on the nature of surfaces in contact and how hard the surfaces are pressed against each other.

Question: What is the main difference between contact and non-contact forces? 

Answer: The main difference is that contact forces act only when objects are in physical contact, whereas non-contact forces act without any physical contact between objects. For example, muscular force is a contact force, while gravitational force is a non-contact force.
Question: What is electrostatic force? Give a example .

Answer: Electrostatic force is the force of attraction or repulsion between charged objects. It acts without direct contact. For example, a rubbed balloon attracts small pieces of paper, and a charged balloon sticks to a wall due to electrostatic force.

Question: What is gravitational force? Give two examples of gravitational force from daily life.

Answer: Gravitational force is the force of attraction between any two objects in the universe. Examples: An apple falling from a tree to the ground, and a ball thrown upwards coming back to the Earth.

Question: What is pressure? Given a example

Answer: The force acting on a unit area of a surface is called pressure. For example, when pushing a nail into a wooden plank, the sharp end exerts more pressure, so it easily goes into the wood.

Question: What is atmospheric pressure? What will happen if there were no atmospheric pressure on the Earth?

Answer: Atmospheric pressure is the force exerted by the air in the atmosphere on the surface of the Earth. If there were no atmospheric pressure, liquids would not rise in straws or droppers and living organisms would not survive as normal life processes depend on it.

Exercises

1. Give two examples each of situations in which you push or pull to change the state of motion of objects.

Answer: (i) Examples of pushing objects:

(a) Pushing a swing to make it move.

(b) Pushing a door to open it.

(ii) Examples of pulling objects:

(a) Pulling a trolley to move it forward.

(b) Pulling a rope in a tug-of-war game.
2. Give two examples of situations in which applied force causes a change in the shape of an object.

Answer: Applied force can change the shape of an object in many situations. Two examples are:

(i) When we press a rubber ball with our hands, its shape gets compressed or flattened due to the applied force.

(ii) When we pull a rubber band, it becomes longer and its shape changes because of the force applied on it.
3. Fill in the blanks in the following statements.
(a) To draw water from a well we have to __________ at the rope.
(b) A charged body __________ an uncharged body towards it.
(c) To move a loaded trolley we have to __________ it.
(d) The north pole of a magnet __________the north pole of another magnet.

Answer: (a) To draw water from a well we have to pull at the rope.

(b) A charged body attracts an uncharged body towards it.

(c) To move a loaded trolley we have to push or pull it.

(d) The north pole of a magnet repels the north pole of another magnet.

4. An archer stretches her bow while taking aim at the target. She then releases the arrow, which begins to move towards the target. Based on this information fill up the gaps in the following statements using the following terms.
muscular, contact, non-contact, gravity, friction, shape, attraction
(a) To stretch the bow, the archer applies a force that causes a change in its __________.
(b) The force applied by the archer to stretch the bow is an example of __________ force.
(c) The type of force responsible for a change in the state of motion of the arrow is an example of a __________ force.
(d) While the arrow moves towards its target, the forces acting on it are due to __________ and that due to __________ of air.

Answer: (a) To stretch the bow, the archer applies a force that causes a change in its shape.

(b) The force applied by the archer to stretch the bow is an example of muscular force.

(c) The type of force responsible for a change in the state of motion of the arrow is an example of a contact force.

(d) While the arrow moves towards its target, the forces acting on it are due to gravity and that due to friction of air.
5. In the following situations identify the agent exerting the force and the object on which it acts. State the effect of the force in each case.
(a) Squeezing a piece of lemon between the fingers to extract its juice.
(b) Taking out paste from a toothpaste tube.
(c) A load suspended from a spring while its other end is on a hook fixed to a wall.
(d) An athlete making a high jump to clear the bar at a certain height.

Answer: (a) When a piece of lemon is squeezed between the fingers, the fingers act as the agent applying force on the lemon. The lemon is the object. Due to this force, the shape of the lemon changes and juice is extracted from it.

(b) While taking out paste from a toothpaste tube, our fingers act as the agent and the tube is the object. The applied force changes the shape of the tube and pushes the paste out.

(c) In the case of a load suspended from a spring, the load acts as the agent applying force on the spring. The spring is the object. Because of this force, the spring gets stretched, showing a change in its shape.

(d) When an athlete makes a high jump, the athlete’s legs act as the agent applying force on the ground. The ground is the object. As a result of this force, the athlete is pushed upwards and is able to jump over the bar.
6. A blacksmith hammers a hot piece of iron while making a tool. How does the force due to hammering affect the piece of iron?

Answer: When a blacksmith hammers a hot piece of iron, the hammer applies force on the iron. Due to this force, the shape of the iron changes. The iron gets flattened or moulded into the desired shape of the tool. Thus, the force of hammering helps in changing the shape and size of the iron piece.
7. An inflated balloon was pressed against a wall after it has been rubbed with a piece of synthetic cloth. It was found that the balloon sticks to the wall. What force might be responsible for the attraction between the balloon and the wall?

Answer: When the inflated balloon is rubbed with a piece of synthetic cloth, it becomes electrically charged. When this charged balloon is pressed against the wall, it attracts the wall due to an electrostatic force.This force of attraction between the charged balloon and the wall makes the balloon stick to the wall.
8. Name the forces acting on a plastic bucket containing water held above ground level in your hand. Discuss why the forces acting on the bucket do not bring a change in its state of motion.

Answer: The forces acting on the bucket are the gravitational force (pulling it downward) and the force applied by our hand (holding it upward). These two forces are equal and opposite, so they balance each other. Hence, there is no change in the state of motion of the bucket.
9. A rocket has been fired upwards to launch a satellite in its orbit. Name the two forces acting on the rocket immediately after leaving the launching pad.

Answer: Immediately after the rocket leaves the launching pad, two main forces act on it. One is the gravitational force of the Earth, which pulls the rocket downward. The other is the thrust force produced by the burning fuel, which pushes the rocket upward.

10. 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. Once we release the pressure on the bulb, water gets filled in the dropper. The rise of water in the dropper is due to
(a) pressure of water.
(b) gravity of the earth.
(c) shape of rubber bulb.
(d) atmospheric pressure.

Answer: (d) atmospheric pressure.

[ When we press the bulb of the dropper, the air inside it is forced out in the form of bubbles. When we release the pressure, the space inside the dropper becomes empty, and the pressure inside decreases. Due to this, the atmospheric pressure acting on the water outside pushes the water into the dropper. ]