“Force and Pressure” is Chapter 9 taken from the JKBOSE textbook of Science for students of Class 8th. In the previous post, I provided you with Questions and Answers for Chapter 8 Chemical Effects of Electric Current. This post is about the Class 8 Question Answers Force and Pressure. Before we move on to solutions you must have your basics clear. So, let me begin by giving you a brief overview of what you have studied in this chapter.
Force and Pressure Class 8 Question Answers
Overview of Chapter
- FORCE-A PUSH OR A PULL.
- FORCES ARE DUE TO AN INTERACTION.
- EXPLORING FORCES.
- A FORCE CAN CHANGE THE STATE OF MOTION.
- FORCE CAN CHANGE THE SHAPE OF AN OBJECT.
- CONTACT FORCES.
- Muscular Force.
- Frictional Force.
- NON-CONTACT FORCES.
- Magnetic Force.
- Electrostatic Force.
- Gravitational Force.
- PRESSURE EXERTED BY LIQUIDS AND GASES.
- ATMOSPHERIC PRESSURE.
FORCE-A PUSH OR A PULL: A push or pull acting on a body or object is called force and the direction in which an object or body is pulled or pushed is called the direction of a force.
FORCES ARE DUE TO AN INTERACTION: Force between two objects is caused by an interaction of one object with another object. So, in case there is no interaction between two objects, then no force can show its effect.
EXPLORING FORCES: After exploring forces we can conclude that:
- The net force is the sum of two applied forces if both forces are applied in the same direction.
- The net force is the difference between two applied forces if two applied forces act in opposite directions.
- The net force is zero if two forces applied on an object are equal in magnitude and opposite directions.
A FORCE CAN CHANGE THE STATE OF MOTION: If force is applied to an object then it may change its speed. If force is applied in the direction of motion of a moving object, its speed increases. If force is applied against the direction of motion of a moving object, its speed decreases. Force can move a stationary object and stop a moving object.
FORCE CAN CHANGE THE SHAPE OF AN OBJECT: Force can change the shape and size of an object. For Example, pulling and releasing a spring, squeezing toothpaste, and hammering a hot block of iron to form agricultural implements.
CONTACT FORCES: Contact forces are those forces which come into action only when two objects are in actual contact with each other.
- Muscular Force:The force exerted by the muscles of the body of a human or animal is called muscular force.
- Frictional Force: The force which always opposes the direction of motion of one body over another body is called friction frictional force.
NON-CONTACT FORCES: Non-Contact forces are those forces which come into action only when two objects are not in actual contact with each other.
- Magnetic Force: The force exerted by a magnet is called magnetic force.
- Electrostatic Force: The force exerted by an electrically charged object is called electrostatic force.
- Gravitational Force: The force with which every object that has mass attracts it is called gravitational force. The force with which the earth pulls objects towards itself is called the force of gravity or simply gravity.
PRESSURE: Force acting per unit area of an object is called pressure. It depends on two factors. The magnitude of force applied and the area over which force is applied.
PRESSURE EXERTED BY LIQUIDS AND GASES: All liquids exert pressure on the base and walls of the container in which they are kept. The pressure exerted by a liquid increase with increasing depth inside the liquid. All liquids exert equal pressure at the same depth.
ATMOSPHERIC PRESSURE: The tremendous weight of air in the atmosphere exerts pressure on the surface of the earth and this pressure is known as atmospheric pressure.
- Give two examples of each of the situations in which you push or pull to change the state of motion of objects.
Ans. Examples of Push
a. The door is pushed to open it.
b. In a football game a player pushes the ball to score a goal.
Examples of Pull
a. The bucket full of water is pulled by pulling the rope tied to it from the well.
b. The drawer is pulled to open it.
- Give two examples of situations in which applied force causes a change in the shape of an object.
Ans. The two examples are as under:
- The size of the rubber band changes when it is pulled with the hands.
- The size of the dough changes when it is pressed with a hand.
- Fill in the blanks in the following statements:
(a) To draw water from a well we have to apply pull at the rope.
(b) A charged body attracts an uncharged body towards it.
(c) To move a loaded trolley, we have to push it.
(d) The north pole of a magnet repels the north pole of another magnet.
- An archer stretches her bow while aiming 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, and attraction.
(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 contact force.
(c) The type of force responsible for a change in the state of motion of the arrow is an example of a muscular 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.
- 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.
|Situation||Agent exerting a force||The object on which force acts||Result of applied force|
|a||Fingers of hand||Piece of lemon||Lemon juice squeezes out|
|b||Fingers of hand||Tube of toothpaste||The paste comes out of the tube|
|c||Suspended load||Spring||The size of the spring elongates|
|d||Muscles of legs||Body of athlete||The high jump is made to clear the bar|
- A blacksmith hammers a hot piece of iron while making a tool. How does the force due to hammering affect the piece of iron?
Ans. The force of hammering flattens the hot piece of iron and moulds it into a useful tool by changing its shape and size.
- 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?
Ans. When an inflated balloon is rubbed with a piece of synthetic cloth, it gets charged. When the same charged balloon is pressed against a wall it gets attracted by the wall because the wall is uncharged. The force which acts between the charged balloon and an uncharged wall is the electrostatic force of attraction.
- 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.
Ans. Two forces are acting on a plastic bucket containing water held above the ground level in our hand. These two forces are the gravitational force of the earth which acts in a downward direction towards the earth and the muscular force of our arm which acts in an upward direction and keeps the bucket held above the ground level. These forces do not bring any change in the state of motion of the bucket because they are equal in magnitude and are in opposite directions so cancel the effect of each other.
- 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.
Ans. When a rocket is fired in an upward direction to launch a satellite in its orbit two forces act on it. These are:
- The gravitational force of the earth.
- The frictional force of the air.
- When we press the bulb of a dropper with its nozzle kept in water, the 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) the gravity of the earth
(c) the shape of the rubber bulb
(d) atmospheric pressure
Ans d) Atmospheric pressure
That’s all about Force and Pressure Class 8 Question Answers. Hope you found it useful. Do share your views about this post in the comment section below.