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science-physics

class VIII

#### Key Concepts

atmospheric pressure, contact force, electrostatic force, force, friction, gravitational force, gravity, magnetic force, muscular force, non contact force, pull, push

#### essential questions

1. How can we use forces and the laws of motion to understand the motion of objects?
2. What factors determine the friction force between two surfaces?

#### Remembering:

1. Define force. Give an example.
2. Define pressure.
3. Define atmospheric pressure.
4. Fill in the blanks:

a) An example of a non contact force is____

b) The pressure exerted by air around us is known as ____

c) SI unit of force is ____

d) Application of force can change the ___ or ___ of an object

e) Force exerted by our muscle is called ___ force

f) Direction of force of friction is always___ to the direction of motion.

g) Force per unit area is called____

h) The force exerted by a charged body on another charged or uncharged body is known as____

i) Force has____ as well as___

#### understanding:

1. What factors are involved for force to come into play?
2. Explain how force acts in the game of tug of war.
3. What do you understand by state of motion. Explain with an example
4. What is friction?
5. Moon revolves around Earth because of the pressure of: a) gravitational force  b) frictional force  c) magnetic force  d)  electric force
6. A ball rolling on the ground slows down and finally stops because: a) force   b) less force applied  c) friction   d) none of the above
7. The force involved in falling of an apple from a tree is known as: a) magnetic force  b) frictional force  c) gravitational force  d) electrostatic force
8. Which of the following is an example of non contact force: a) force exerted by us to lift a bucket  b) push a stationery car  c) force exerted by magnets  d) force due to friction
9. Write whether it is true or false:

a) Friction opposes the relative motion between two surfaces in contact

b) a force can act on an object with or without being in contact with it

c) The force exerted by a charged body on another charged body is known as gravitational force

d) Magnetic force is a contact force

e) force of gravity is an attractive force

f) atmospheric pressure at high altitudes is less than the pressure at sea level

g) Gases does not exert pressure on the walls of their container

#### Application:

1. It is said that application of force can increase the speed of the object. Is it true or false. Explain with an example.
2. Can you give reason why porter’s use a round piece of cloth when they carry heavy load on their heads?
3. Why don’t we feel the weight of the atmospheric pressure at sea level?
4. Even though there is no air in space to exert pressure, still astronauts wear special suits when in space. Give reasons for your explanation.
5. Why do deep sea divers have to wear special suits before going for diving at the depths?
6. Why tools meant for cutting and piercing always have sharp edges?
7. Why shoulder bags are provided with broad straps and not thin straps?
8. When magnet is brought near another magnet, the second magnet moves towards the first. The second magnet moves because of ___ force between them which acts____ respectively. a) gravitational, directly   b) gravitational, indirectly    c) magnetic, directly  d) magnetic, indirectly
9. During a dry day, when we comb our hair, we see it gets attracted towards the plastic comb during combing. the hairs are attracted towards the comb because of___ which is ____ force:  a) gravitational force, direct   b) electrostactic force, indirect   c) electrostatic force, indirect  d) muscular force, direct
10. When a stone is thrown up in the air and then it falls freely on the ground because of : a) magnetic pull of earth   b) gravity of earth  c) frictional force  d) air pressure
11. A force of 16N acts on an areas of 50cm². What is the pressure in pascal? a) 3200 Pa  b) 4200Pa  c) 5200Pa  d) 2200 Pa
12. What is the magnitude of force required in newton’s to produce a pressure of 27500Pa on an area of 200cm²  a) 650 N   b) 750N  c) 550N  d) 450N
13. What force will produce a pressure of 50,000 Pa on an area of 0.20m²?  a) 10,000N   b) 5,000N  c) 15,000N  d) 20,000N
14. The mass of a brick is 2.5kg. Its dimensions are 10cm ×5cm× 2cm. What will be its pressure exerted by it on the ground if it is resting in two different situation: ( take 1 kg wt= 10N) i)  10cm ×5cm base   ii) a 5cm × 2cm base  a) 5000Pa, 25000Pa   b) 6000Pa, 20,000Pa  c) 4000Pa, 15000 Pa  d) 10000Pa , 10000 Pa
15. A man weighing 50Kgf is standing on a wooden plank of 1m× 0.5m. What will be the pressure exerted by it on the ground: a) 300 Kgf/m²   b) 100kgf/m²  c) 200kgf/m²  d) 400 kgf/m²
16. A box of mass 10kg has a base area of 1m². What is the pressure exerted by it on the ground: ( take 1 kg wt = 10N)  a) 50Pa   b) 70Pa   c) 100Pa  d) 120Pa

#### Analysis:

1. Will the atmospheric pressure be higher at the poles than on the equator. Give reasons for your answer.
2. As we go higher on the mountains will the atmospheric pressure decrease than the atmospheric pressure at sea levels. Give reasons for your answer
3. There are two cylindrical tubes A and B height of 10m each with equal amount of water filled till the brim. There is a hole of 1 cm each on tube A at 3 m from top of the tube and in tube B at 4 m  from the bottom of the tube. Which of the holes will experience a higher atmospheric pressure. Give reasons for your answer.
4. Why does the inflated balloon deflate if we don’t close the hole of the balloon.
5. Do you think a woman weighing 60Kgs who is wearing 2 inch stilettoes will exert same pressure on the ground as when the same woman when she wears flat sandals? Give reasons for your answer
6. Two persons are applying forces on two opposite sides of a moving cart. The cart still moves with the same speed in the same direction. What do you infer about the magnitude and direction of the forces applied
7. The figure below shows a container filled with water. Which of the following is correct about pressure of water: a) pressure at A > pressure at B> pressure at C   b) pressure at A = pressure at B> pressure at C  c) pressure at A< pressure at B> pressure at C  d) pressure at A< pressure at B < pressure at C
8. Opening the door is an example of which type of force: a) contact force  b) spring force  c) non-contact force  d) magnetic force
9. A cement slab is kept in two different positions on a table as shown in the figure. The pressure exerted by the slab on the table will be: a) maximum in position A  b) maximum in position B  c) equal in both the situations
10. A water tank has four taps S,T, U and V as shown in the figure. The water will flow out at the same pressure from taps at:  a) U and V   b) U and T  c) S and V   d) S and T
11.  In the pulley system which indirect force is being applied when the load is being pulled up against it going up:  a) magnetic force   b) gravitational force   c) muscular force  d) electrostatic