Chapter 7 . FORCE AND PRESSURE

Exercises :

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

Answer :  Push :

(i).  Pushing a shopping cart to move it forward in a supermarket.

(ii).  Pushing a bicycle pedal to generate forward motion.

Pull :

(i). Pulling a sled to make it move across the snow.

(ii).  Pulling a handle on a door to close it.

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

Answer : Two examples of situations where applied force causes a change in the shape of an object:

(i). When you push or compress a spring, it undergoes deformation and its shape changes. The spring gets shorter and becomes more compact as you apply force to compress it.

(ii). When you pull or stretch a rubber band, it elongates and changes shape. The rubber band becomes longer and thinner as you apply force to stretch 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) 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 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 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 a muscular force .

(c) The type of force responsible for a change in the state of motion of the arrow is an example of a non-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.
(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) Agent exerting the force: Fingers Object on which it acts: Piece of lemon Effect of the force: Extracting juice from the lemon

(b) Agent exerting the force: Hand Object on which it acts: Toothpaste tube Effect of the force: Squeezing out the paste from the tube

(c) Agent exerting the force: Spring Object on which it acts: Load Effect of the force: Supporting the load's weight and keeping it suspended   

(d) Agent exerting the force: Athlete Object on which it acts: Athlete's body Effect of the force: Propelling the athlete upwards to clear the bar

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 force due to hammering by the blacksmith causes deformation and reshaping of the hot piece of iron. It helps shape the iron into the desired form and also improves its strength and durability.

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 : The attraction between the balloon and the wall is likely due to static electricity. When the balloon is rubbed with the synthetic cloth, it acquires an electric charge, resulting in an electrostatic force that causes it to stick to the wall.

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 : The forces acting on a plastic bucket containing water held above ground level in your hand are:

Gravitational force: This force pulls the bucket and water downward toward the Earth.

Normal force: This force is exerted by your hand on the bucket to support its weight and prevent it from falling.

The bucket does not experience a change in its state of motion because the gravitational force pulling it downward is balanced by the normal force exerted by your hand in the opposite direction. These two forces are equal in magnitude and opposite in direction, resulting in a state of equilibrium. As a result, the bucket remains stationary or moves at a constant velocity without accelerating.

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:

Thrust force: This force is generated by the rocket engines, pushing the rocket upwards and propelling it forward.

Gravitational force: This force pulls the rocket and satellite downwards toward the Earth.

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.

The rise of water in the dropper after releasing the pressure on the bulb is due to atmospheric pressure. When we press the bulb, the air inside the dropper is compressed, and when released, the atmospheric pressure pushes the water into the dropper to balance the pressure inside and outside the dropper.