# CLASS 8 SCIENCE CHAPTER-11 FORCE AND PRESSURE

Question 1. In which situations, do you need to pull or push to change the state of motion of an object? Give two examples.
Solution :
Following are the examples of pull force:
(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.

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

Question 3. Fill in the blanks with the appropriate words :
a) We need to ________the loaded trolley in order to move it.
b) We need to ________the rope to draw water from the well.
c) The north pole of the magnet________the north pole of another magnet.
d) A charged body________an uncharged body.
Solution :
a) We have to either push or pull the loaded trolley in order to move it.
b) We have to pull the rope to draw water from the well.
c) The north pole of the magnet repels the north pole of another magnet.
d) A charged body attracts an uncharged body.

Question 4. While taking the aim at the target, an archer stretches her bow. She then releases the arrow which begins to move towards the target. In the given information, fill in the blanks using the words given below.
Non-contact, muscular, attraction, contact, friction, gravity, shape
a) _______ force is the force which the archer applies to stretch the bow.
b) To stretch the bow, the archer applies a force that causes a change in its_________.
c) While the arrow moves towards its target, the force acting on it is because of_______and which is due to _______of air.
d) _________ force is an example of changing the state of motion of the arrow.
Solution :
a) Muscular force is the force which the archer applies to stretch the bow.
b) To stretch the bow the archer applies a force that causes a change in its Shape.
c) While arrow moves towards its target, the force acting on it because of gravity and which is due to the friction of air.
d) Contact force is an example of changing the state of motion of the arrow.

Question5. In the following situation, identify the object on which the force is acting and the object exerting the force. State the effects of the force in it.
a) Tooth paste is taken out from the tube.
b) To clear the bar at a certain height, an athlete making a high jump.
c) To extract the juice of lemon, it is to be squeezed between the finge
Solution :
(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.
(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 changed.
(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.

Question-6. While making a tool, a blacksmith hammers a hot piece of iron. How does the force due to hammering affects the piece of iron?
Solution : While hammering a hot piece of iron blacksmith changes the shape of iron using his muscular force.

Question 7. When an inflated balloon is rubbed with a piece of synthetic cloth and pressed against a wall, then it sticks to the wall. Find the force responsible for the attraction between the wall and balloon.
Solution : This is an electrostatic force.

Question 8. Which force is acting on a plastic bucket containing water held above ground level in your hand? Why does the force acting on the bucket not bring a change in its state of motion? Discuss.
Solution : For holding the bucket of water above the ground, we use muscular force. This muscular force acts against the force of gravity that pulls the bucket towards the ground. The two forces are equal in magnitude but opposite in direction. Therefore, the net force on the bucket is zero. Hence, there is no change in the state of motion..

Question 9. Name the forces acting on the rocket if the rocket is launched upward to launch a satellite into its orbit.
Solution :
The two forces on the rocket immediately after leaving the launching pad are:
(i) Gravitational Force: It is force acting on rocket pulling it in the downward direction.
(ii) Frictional force: It is the force acting due to presence of earth’s atmosphere, which opposes its motion in upward direction.

Question 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
Solution : (d) atmospheric pressure.

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