Exercises
Question 1
Give two examples each of situations in which you push or pull to change the state of motion of objects.
Answer
(a) Pull situations (in which state of motion of objects is changed) :
- Moving a book placed on a table towards ourself to read.
- We pull the clothes out of the washing machine after they are clean.
(b) Push situations (in which state of motion of objects is changed) :
- A football player taking a penalty kick, hits (or pushes) the ball towards the goal.
- A cricket ball hit by a batsman.
Question 2
Give two examples of situations in which applied force causes a change in the shape of an object.
Answer
- When we press a lump of dough kept on a plate with our hands, its shape changes.
- When we press a smiley ball, its shape changes.
Question 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 pull or push it.
(d) The north pole of a magnet repels the north pole of another magnet.
Question 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.
Question 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
| S. No. | Agent | Object | Effect | Reason |
|---|---|---|---|---|
| a. | Fingers | Lemon | Shape of lemon changes | Muscular force is exerted on the lemon in order to extract its juice. As a result, the shape of the lemon changes. |
| b. | Fingers | Tooth paste tube | Shape of tooth paste tube changes | Muscular force is exerted on the tube in order to take out paste. As a result, the shape of the tube changes. |
| c. | Suspended load | Spring | The spring gets stretched. | The suspended load pushes the spring downwards. Hence, its shape changes. |
| d. | Athlete | Ground | Pushes the ground backwards and as a result takes a high jump to clear the bar at a certain height. | When the athlete pushes the ground with his feet, his feet exert a muscular force on the ground. Same magnitude of force is applied back by the ground on the athlete. This force allows them to jump over the bar. As a result, their state of motion gets changed. |
Question 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
The muscular force applied by the blacksmith on the hot piece of iron changes the shape of the iron and hence, it can be moulded in any desired shape.
Question 7
An inflated balloon was pressed against a wall after it had been rubbed with a piece of synthetic cloth. It was found that the balloon stuck to the wall. What force might be responsible for the attraction between the balloon and the wall?
Answer
An electrostatic charge develops on the balloon after it has been rubbed with a synthetic cloth. Due to this charge the balloon sticks to the wall. The electrostatic force is responsible for the attraction between the balloon and the wall.
Question 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
We use muscular force in order to hold the 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 and opposite in direction. As a result, the net force on the bucket is zero. Hence, there is no change in the state of motion.
Question 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
When a rocket has been fired upwards to launch a satellite in its orbit, two forces acting on it are:
- Force of gravity acting downwards
- Frictional force due to the earth's atmosphere, which opposes its motion.
Question 10
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
- pressure of water
- gravity of the earth
- shape of rubber bulb
- atmospheric pressure
Answer
atmospheric pressure
Reason — When all the air escapes from the nozzle, a low pressure develops inside the tube. Due to this the atmospheric pressure, acting on water becomes more than the pressure inside the tube. As water moves from a higher pressure area to a lower pressure area, hence on releasing the bulb, water gets filled in the dropper.