Class 8 | Science | Ch 9 |Force and Pressure| NCERT Solutions
1. Give two examples each of situations in which you push or pull to change the state of motion of objects.
1) If we kick a stationary football kept on the ground, then the football starts moving. The force of our foot makes the stationary football to move.
2) Take a rubber ball and place it on table top. Now gently push the ball along the surface of table, the ball begins to move. Thus a ball at rest begins to move when a force is applied to it.
2. Give two examples of situations in which applied force causes a change in the shape of an object.
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.
(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
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.
(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.
In squeezing a piece of lemon, we make use of muscular force.As a result the shape of lemon changes and juice is extracted.
(b) Taking out paste from a toothpaste tube.
To take out toothpaste from tube we make use of muscular force.As a result the shape of tooth paste tube changes and toothpaste comes out.
(c) A load suspended from a spring while its other end is on a hook fixed
to a wall.
The suspended load exerts a force and pulls the spring downwards.As a result, spring gets stretched and its shape changes.
(d) An athlete making a high jump to clear the bar at a certain height.
The feet of an athlete exerts a muscular force on the ground which allows him to jump high and clear the bar.As a result, his state of motion gets changed.
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?
When a blacksmith hammers a piece of hot iron while making a tool, he uses his muscular force.The force changes the shape of iron so that it can be given a desired shape.
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?
If an inflated balloon is rubbed with a piece of synthetic cloth and then pressed against a wall ,the balloon sticks to the wall. It is the electrostatic force which is responsible for the attraction between balloon and the wall. When the balloon is rubbed with a piece of synthetic cloth the balloon gets electrostatic charges and hence gets electrically charged. The electrically charged balloon attracts the uncharge wall and sticks to it.
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.
We make use of muscular force to hold a bucket of water above the ground.
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.
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.
The two forces acting on the rocket are the force of gravity, which pulls the rocket towards the ground and ,the force of friction due to the earth’s atmosphere which opposes its motion.
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.