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Chapter Review:

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• A push or a pull on an object is called a force.

• A force arises due to the interaction between two objects.

• Force has magnitude as well as direction.

• The force resulting due to the action of muscles is known as the muscular force.

• Muscular force can be applied only when it is in contact with an object, it is also called a contact force.
• The force acts opposite to the direction of motion and this is responsible for changing the state of motion of objects. It arises due to contact between the surfaces is called frictional force.
• The force which is felt by the objects without any contact is known as non-contact force. 
• The force exerted by a charged body on another charged or uncharged body is known as electrostatic force.
• Every object attracts each other and pulls toward itself. Such force which applied on objects is known as Gravitational force and gravitation.
• The force acting on a unit area of a surface is called pressure.                                Pressure = force / area on which it acts 

• The pressure exerted by this air is known as atmospheric pressure.

• Liquids and gases exert pressure on the walls of their containers.

Exercise - Question:

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

Answer:

Following are the pull force examples:

a) In order to open a drawer, we have to pull it. This action changes the state of motion of the drawer.

b) To draw water from the well, the rope is pulled. This action changes the state of motion of the bucket.

Following are the examples of push force:

a) A football is pushed by the foot of a player. This action changes the state of motion of the ball.

b) In order to change the place of the heavy box from one room to another, we have to push it. This action changes the motion of the box.

2. Give two examples of situations in which applied force causes a change in the shape of an object.

Answer:

The forces which change the shape of an object are as follows:

i) By pressing the clay between the hands, it deforms.

ii) The shape of the plastic bottle changes by squeezing 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) Pull 

(b) Attracts (Pull) 

(c) Pull 

(d) Repules (Pushes)

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) Shapes

(b) Muscular

(c) Contact

(d) Gravity,friction

 

Text Book Exercise:

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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) In squeezing a piece of lemon, we make use of muscular force to extract its juice. This muscular force is exerted on the lemon by our fingers. As a result, the shape of the lemon gets changed

Concept insight: Muscular force is a type of contact force

(b) We use our muscular force to take out paste from a toothpaste tube. The muscular force is exerted on the toothpaste tube by our fingers. As a result, the shape of the tube gets change

(c) Here, the suspended load exerts a force on the spring and pushes the spring downwards. As a result, the spring gets stretched. Hence, its shape gets changed.

(d) An athlete pushes the ground with his feet. His feet exert a muscular force on the ground. This allows him to jump over the bar. As a result, his state of motion gets changed.

Concept insight: Every action has an equal and opposite reaction

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:

While hammering a hot piece of iron blacksmith changes the shape of iron using his muscular force.

Concept insight: Force brings change in the shape of an object.

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:

Electrostatic force is responsible for the attraction between the balloon and the wall. The balloon rubbed with a synthetic cloth gets electrically charged and sticks to the wall. This is because a charged object attracts uncharged object.

Concept insight: Electrical force is a type of non contact force. A charged object attracts an uncharged object

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:

A bucket filled with water held above the ground experiences two types of forces:

1. Muscular force (acting upwards)

2. Gravity (acting downwards)

These two forces are balanced because they are equal in magnitude but act in opposite directions. So, the net force on the bucket is zero and hence the state of motion of the bucket remains unchanged.

Concept insight: Non-zero net force is required to change the state of motion of a body.

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:

The two forces acting on the rocket immediately after leaving the launching pad are:

 (i) Upward force applied by the rocket engine.

(ii) Downward gravitational force applied by the Earth

 (Ignoring frictional force due to air resistance)

 The upward force due to rocket engine exceeds the downward gravitational force which makes the rocket move upward.

Concept insight: Unbalanced forces can change the state of motion of a body.

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

Additional Questions With Solutions:

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Ques: Define force.

Ans: A push or a pull on an object is called a force.

Ques: Define muscular force.

Ans: The force resulting due to the action of muscles is known as the muscular force.

Ques: Write the kinds of force.

Ans: There are two kinds of force.

(i) Contact force

(ii) Non-contact force

Ques: What is contact force? Write its types.

Ans: Muscular force can be applied only when it is in contact with an object, it is also called a contact force.

This is two types:

(i) Muscular force – The force resulting due to the action of muscles is known as the muscular force.

(ii) Frictional force  -  The force acts opposite to the direction of motion and this is responsible for changing the state of motion of objects. It arises due to contact between the surfaces.

Ques: Why is frictional force known as contact force?

Ans: The frictional force known as contact force as it arises due to contact between the surfaces.

Ques: What is non-contact force? Write its types.

Ans: The force which is felt by the objects without any contact is known as non-contact force.

This is two types:

(i) Electrostatic force: The force exerted by a charged body on another charged or uncharged body is known as electrostatic force.

(ii) Gravitational force: Every object attracts each other and pulls toward itself. Such force which applied on objects is known as Gravitational force and gravitation

Ques: Define Pressure.

Ans: The force acting on a unit area of a surface is called pressure. 

Ques: Why shoulder bags are provided with broad straps and not thin strap?

Ans: Broad straps reduce pressure on back. If the strap will be thin the value of pressure will be raised as per unit area of surface gets reduced.

Ques: Why have nails sharp edges?

Ans: Nails have sharp edge because if per unit area of surface is reduced the pressure on it is increased. Hence nails push easily.

Ques: What is atmosphere ?

Ans: Envelop of air is known as the atmosphere.

Ques: What is atmospheric pressure?

Ans: The pressure exerted by this air is known as atmospheric pressure.

Additional Questions With Solutions:

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Ques: Define force.

Ans: A push or a pull on an object is called a force.

Ques: Define muscular force.

Ans: The force resulting due to the action of muscles is known as the muscular force.

Ques: Write the kinds of force.

Ans: There are two kinds of force.

(i) Contact force

(ii) Non-contact force

Ques: What is contact force? Write its types.

Ans: Muscular force can be applied only when it is in contact with an object, it is also called a contact force.

This is two types:

(i) Muscular force – The force resulting due to the action of muscles is known as the muscular force.

(ii) Frictional force  -  The force acts opposite to the direction of motion and this is responsible for changing the state of motion of objects. It arises due to contact between the surfaces.

Ques: Why is frictional force known as contact force?

Ans: The frictional force known as contact force as it arises due to contact between the surfaces.

Ques: What is non-contact force? Write its types.

Ans: The force which is felt by the objects without any contact is known as non-contact force.

This is two types:

(i) Electrostatic force: The force exerted by a charged body on another charged or uncharged body is known as electrostatic force.

(ii) Gravitational force: Every object attracts each other and pulls toward itself. Such force which applied on objects is known as Gravitational force and gravitation

Ques: Define Pressure.

Ans: The force acting on a unit area of a surface is called pressure. 

Ques: Why shoulder bags are provided with broad straps and not thin strap?

Ans: Broad straps reduce pressure on back. If the strap will be thin the value of pressure will be raised as per unit area of surface gets reduced.

Ques: Why have nails sharp edges?

Ans: Nails have sharp edge because if per unit area of surface is reduced the pressure on it is increased. Hence nails push easily.

Ques: What is atmosphere ?

Ans: Envelop of air is known as the atmosphere.

Ques: What is atmospheric pressure?

Ans: The pressure exerted by this air is known as atmospheric pressure.