Friday, September 14, 2012

CBSE Class 8 - Science - CH11 - Force and Pressure

Force and Pressure
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NCERT Chapter Solutions and Q & A


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

Answer:
  • A goal keeper 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 change in 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 Taking out paste from a toothpaste tube. fingers toothpaste tube The effect of the force is that the toothpaste is coming out from the tube
3 A load 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 on 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 upward direction.
Although these forces are acting on the bucket but 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 downward direction.
Q11: 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: 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 cance each other. In this case, the net force is not zero which may result in change in state of motion of the body.

Q13: Why does 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 right direction.

Case (ii): In this case, the two forces acting are in opposite direction. Therefore the net force is the difference of 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, it will remain in rest.

Case (iii): Two forces acting are in opposite direction. 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 larger force i.e. 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 on an area of 2m2. Compute the pressure being applied on 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 depend 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 density(d) of a liquid.
  • The pressure of a liquid depends on gravitational force (g).
Mathematically, pressure exerted by liquid can be expressed as:
                            p = h × d × gwhere,
   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 atmosphere around earth?

Answer: Earth and some inner planets have atmosphere due to force of gravity on the planet. Due to force of gravity, gases in atmosphere do not escape. In 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 at 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 also due to low pressure region created.


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