CBSE Class 8 - Science - CH11 - Force and Pressure #class8Physics #eduvictors

Force and Pressure

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NCERT Chapter Solutions and Q & A


Q1: Give two examples of situations in which you push or pull to change the state of motion of objects.

Answer:

• A goalkeeper saving a goal.
• Hockey player flicking a ball.

Q2: What is a force?

Answer: Force is a push or a pull acting on an object which changes or tends to change the state of the object.

Actions like pulling, pushing, lifting, kicking, opening etc. are tasks we generally do in our daily life. These actions often result in a change in the position of the object. We say an effort is required either to push or pull an object, this effort is called force.

The SI (International System of Units) unit of force is Newton
 

Q3: Give two examples of situations in which applied force causes a change in the shape of an object.  

Answer:
      a. A lump of dough on a plate.
      b. A rubber band suspended from a hook.

Q4: Fill in the blanks in the following statements:

Answer:

(a) To draw water from a well we have to pull (force) 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.

Q5: 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

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.

Q5: What is the relation between Force and interaction?

Answer:  Whenever there is an interaction between two objects, there is a force acting upon each of the objects. When the interaction ceases, the two objects no longer experience the force. Forces only exist as a result of an interaction.

Q6: 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.

Answer:

S. No.	Situation	Agent	Object	Effect
1	Squeezing a piece of lemon between the fingers to extract its juice	Fingers	lemon	The effect of the force is that the juice is extracted from the lemon
2	Take out paste from a toothpaste tube.	fingers	toothpaste tube	The effect of the force is that the toothpaste is coming out of the tube
3	A load is suspended from a spring while its other end is on a hook fixed to a wall.	load	spring	The effect of the force is that the spring expands as the other end is on a hook fixed to a wall.
4	An athlete making a high jump to clear the bar at a certain height.	athlete	athlete’s body	The effect of the body is that the athlete will jump on the other side of the bar at a certain height.

Q6: 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 changes the shape of the piece of iron.

Q7: What force can do?

Answer: When a force is applied to an object, it:

• can move an object from rest.
• can change the speed of an object if it is moving.
• can change the direction of motion of an object.
• can bring about a change in the shape of an object.
• may or may not cause any of these or all of these effects

Q8: 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 force which is responsible for the attraction between the ball and the wall is electrostatic force. When we rub the balloon by a synthetic cloth, it gets charged. When it is taken near the wall, it will get attracted towards the uncharged wall because of the electrostatic force which is the force exerted by a charged body on another uncharged body.

Q9: 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 force acting on a plastic bucket containing water held above the ground level are :

1. Gravitational force: It is acting downwards.
2. Muscular force: It is applied by our hands to lift the bucket in the upward direction.

Although these forces are acting on the bucket no change is found in its state of motion because the forces are balancing each other and as a result net force becomes zero.

Q10: 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: Two forces acting on the rocket immediately after leaving the launching pad are:

1. Frictional force due to air,
2. Force of gravity pulling in the downward direction.

Q11: 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 a rubber bulb
(d) atmospheric pressure

Answer: Atmospheric pressure

Q12: What are balanced forces and unbalanced forces?

Answer: Balanced Forces: Two or more forces exerted on an object are balanced forces if their effects cancel each other and they do not cause a change in the object's motion. If the forces on an object are balanced, the net force is zero.

Unbalanced Forces: Two or more forces exerted on an object are unbalanced if their effects do not cancel each other. In this case, the net force is not zero which may result in a change in the state of motion of the body.

Q13: Why does the hair of the hand raise when the hand is brought near to television screen once the TV is switched off?
Answer: The television screen becomes electrically charged and it exerts an electrostatic force on the hair of your hand. This force is a non-contact force because there is no contact between the screen and the hair.

Q14: From the figure below, find out where the object will move in each case.

Answer: Case (i): The two forces acting are on the same direction. Thus the Net force is the sum of these two i.e. F = 5N + 7N = 12N. The net force is non-zero, therefore the object will experience motion in the right direction.

Case (ii): In this case, the two forces acting are in opposite directions. Therefore the net force is the difference between the two
i.e. 5N - 5N = 0N. Since the net force is ZERO, the forces are balanced and its state of motion will not change. Initially, the object is at rest, and it will remain at rest.

Case (iii): Two forces acting in opposite directions. Therefore, net force is 15 - 5N = 10N. Since the net force is non-zero, the object will experience a state of motion in the direction of a larger force i.e. a Net force of 10N will move the object upwards.

Q15: Define Pressure.

Answer: The force acting per unit surface area is called pressure. The SI unit of pressure is measured in newton/metre², which is equal to 1 pascal (Pa).

                                      Pressure = Force ÷ Area

Q16: How do fluids exert pressure?

Answer: Fluids exert pressure in all directions. Fluids exert pressure on all bodies immersed in them and also on the walls of the container that holds them.


Q17: A force of 50N is applied to an area of 2m². Compute the pressure being applied to the area.

See NCERT Page 145 (extended learning)

Answer:  Given, Force = 50N and Area = 2m².
Pressure = Force/Area = 50/2 = 25 Pa.


Q18: What is atmospheric pressure?

Answer: Air exerts pressure on all objects. The pressure exerted by air is called atmospheric pressure.






Q19: What are the factors pressure exerted by liquids depend on?

Answer: Pressure exerted by liquids depends upon:

• The pressure exerted by liquids at the bottom depends on the height (h) of the liquid column.
• Liquid exerts pressure on the walls of the container.
• Liquid exerts equal pressure at the same depth.
• The pressure of the liquid increases with the increase in depth(h).
• The pressure depends on the density(d) of a liquid.
• The pressure of a liquid depends on gravitational force (g).

Mathematically, the pressure exerted by liquid can be expressed as:
                            p = h × d × g

where,
   p = pressure of a liquid,
   h = height of the liquid column.
   d = density of liquid and
   g = gravitational force.



Q20: Name the instrument used to measure atmospheric pressure.


Answer: Barometer.


Q21: Why do sea divers wear specially designed suits?

Answer: Since the pressure of liquid increases with an increase in depth. Specially designed suits protect scuba divers from the huge pressure of the water underneath.


Q22: Who invented the first barometer?

Answer: Torricelli, an Italian Scientist (1643)


Q23: There is no atmosphere around many planets, then why there is an atmosphere around the earth?

Answer: Earth and some inner planets have atmospheres due to the force of gravity on the planet. Due to the force of gravity, gases in the atmosphere do not escape. In the case of the moon and some other planets, the force due to gravity is comparatively too weak to hold the gases.


Q24 Have you been to a place where you feel atmospheric pressure?

Answer: When we go higher in the mountains or in an aeroplane during flight,  our ears pop. As we go higher, the atmospheric pressure decreases, and to balance internal body pressure our ears pop. Stronger winds are also due to the low-pressure region created.