Force & pressure class 8

This presentation will help the students of grade 8 to learn and understand the chapter Force & Press.ure

1. PHYSICS

2. CHAPTER -11 SESSION BY:MS . PREETI KHOSLA

3. CONTENTS

4. WHAT IS FORCE????

5. FORCE Force is the push or pull on an object. Eg:- If an object has to be moved, it has to be pushed or pulled. The SI unit of force is NEWTON (N).

6. CAUSE OF A FORCE Whenever a force is applied, there is interaction between two bodies. In other words we can say that an interaction between at least two bodies is necessary for application of a force.

7. FORCES ARE DUE TO INTERACTION :- An interaction of one object on another object results in a force between the two objects. Eg :- If a man stands behind a car, the car does not move. If he pushes the car, he applies force and the car begins to move in the direction of the applied force.

8. FORCE DEPENDS ON:- When two or more forces are involved, the strength of the force (net force) depends on the direction of each of the forces as well as the its magnitude. When forces are applied in same direction , they get add together whereas when the forces are in opposite direction , the net force is difference of the forces.

9. RESULTANT (OR NET) FORCE Case: 1

10. Case : 2

11. Case:3

12. EXPLORING FORCES :- a) If two forces act on a body in the same direction, the net force is the sum of the two forces. F 1 F 2 Net force = F 1 + F 2 b) If two forces act on a body in opposite directions, the net force is the difference between the two forces. F 1 F 2 Net force = F 2 – F 1 The effect on the object depends on the net force acting on it.

13. EFFECTS OF FORCE ON AN OBJECT :- i) Force can make an object to move from rest. ii) Force can stop a moving object. iii) Force can change the speed of an object if it is moving. iv) Force can change the direction of a motion of an object. v) Force can change the shape of an object. Force may cause one or more of these effects.

14. WHY?????

15. Let’s Test our self!!!!!! Q1. Which among the following can apply force? Lion, Human, Wall, Water A. Only lion and Human B. Only lion, Human and Wall C. Only lion, Human and Water D. All

16. Q2. The picture shows a goalkeeper trying to stop the goal. Which type of force is he applying on the ball? (Assume that he does not move his hand backwards). Push force Pull force None of the above

17. Q3. Hitting a ball with a bat is an example of push type of force. True or False ?

18. Q4. In a game of tug of war, the fat person in team B can apply 50 units of force. Three people in team A can apply 10, 20 , 15 units of force respectively. Which team is going to win? Team A Team B It’s a tie

19. Q5. Block A is pushed by two guys with 500 units force each and Block B is pushed by a single guy with 20 units force as shown in fig. Which block experiences greater net force ? Block A Block B Both experiences equal force

20. Contact forces :- are forces exerted by one object on another when they are in contact with each other. Contact forces are of two types. They are :- i) Muscular force : exerted by muscles of the body. ii) Force of friction : slows things down or prevents things from moving. It always opposes the motion. Non contact forces :- are forces exerted by one object on another without any contact with each other. Non contact forces are of three types. They are :- i) Magnetic force : exerted by magnet on certain metals like iron. ii) Gravitational force : the force with which the earth pulls everything towards itself. iii) Electrostatic force : the force exerted by electrostatic charge (positive and negative).

21. CONTACT FORCES :- i) Muscular force :- Muscular force is the force exerted by the muscles of our body. We use muscular force for our various activities. Animals like bullocks, horses, camels, donkeys, elephants also use muscular force to perform various tasks.

22. ii) Force of friction :- Force of friction is the force which opposes the motion of an object over a surface. Eg :- A ball rolling on ground gradually slows down and comes to rest due to force of friction. If we stop pedalling a bicycle, it gradually slows down and comes to a stop due to force of friction.

23. NON CONTACT FORCES :- i) Magnetic force :- Magnetic force is the force exerted by magnets. Eg :- Force of attraction and repulsion between two magnets. Force exerted by a magnet on a piece of iron or steel.

24. ii) Gravitational force :- Gravitational force is the force exerted by the earth on all objects. It pulls all objects towards the earth. Eg :- A coin or pen falls down when it slips off your hand. A ball thrown upward falls down . Leaves and fruits fall to the ground when they get detached from the plant. Water in rivers flow downwards.

25. iii) Electrostatic force :- Electrostatic force is the force exerted by a charged body on another charged or uncharged body. Eg :- A plastic straw charged by rubbing with paper attracts a suspended plastic straw. A plastic straw charged by rubbing with paper repels a suspended plastic charged by rubbing with paper.

26. Let’s Test our self!!!!!! Q1. A child slides down in a park as shown. Which force makes the child to slide? Muscular force Frictional force Gravitational force Magnetic force

27. Q2. An apple tree is shown on the right . In which of the following apples is gravitational force of earth acting? Apples on the ground Apple falling towards the ground A. Only I B. Only II C. Both I & II D. Neither I nor II

28. Q3. In the picture shown , a thief is carrying a metallic bag filled with metal coins and a policeman is trying to get back the bag with the help of a magnet. What all forces are acting on the bag at that moment? Only magnetic force Only magnetic force and gravitational force C. Magnetic force, muscular force of thief and gravitational force D. Magnetic force, muscular force of thief, muscular force of Policeman and gravitational force

29. WHAT IS PRESSURE???

30. Unit: Pascal (Pa) after the French scientist Blaise Pascal. - The greater the force ,the greater is the pressure. - The greater the area, the lesser is the pressure.

31. Numerical Q: Calculate the pressure exerted by a brick, which applies a force of 2.5 N, when It is placed on its widest base. It is placed upright on the soil . The length, breadth and height of the brick are 25cm, 10cm & 5 cm respectively. 25cm 10cm 5cm 10cm 5cm 25cm

32. When the brick is placed on its widest base: Area in contact with soil = (L X B) = 25cmX 10cm (1m = 100 cm so 1cm = 1/100m) = 25 m X 10 m = 0.025m 2 100 100 Force = 2.5 N Pressure exerted = F = 2.5 N A 0.025m 2 =100 N/m 2 or Pa Solution 25cm 10cm

33. B. When the brick is placed upright: Area in contact with soil = (L X B)= 5cmX 10cm = 5 m X 10 m = 0.005m 2 100 100 Force = 2.5 N Pressure exerted = F = 2.5 N A 0.005m 2 =500 N/m 2 or Pa 5cm 10cm

34. tyres

35. Let’s Test our self!!!!!! Q1. In which of the following case is the child exerting least pressure on the ground?

36. Q2. A cockroach and a spider are crawling on a desert. Both the insects have same weight and same size legs. Looking at the number of legs they have, which insect will leave deeper footprints after crawling on the desert sand? Cockroach Spider Both leave footprints of same depth.

37. DO LIQUIDS ALSO EXERT PRESSURE?????

38. PRESSURE EXERTED BY LIQUIDS :- Liquids exert pressure on the walls of a container. Liquids exert pressure on the bottom of a container. It depends upon the height of the liquid column. Liquids exert sideways pressure. Liquids exert equal pressure at the same height. Liquid exert pressure which increases with the depth. Rubber balloon

39. 1. A liquid exerts pressure on the walls of the container.

40. 2. Liquid exert pressure on the bottom of its container which depends on the height of its column

41. 3. Liquids exert equal pressure at the same height.

42. 4. Water pressure increases with depth.

43. MEASURING LIQUID PRESSURE An instrument used to measure liquid pressure is called pressure gauge. The simplest form of pressure gauge is a manometer , which measure pressure difference.

44. Q1. Assume that four containers with different base areas are filled with equal volumes of water . The pressure at the base of the containers is same for all the containers. greatest in case of C. greatest in case of A. Can’t be said. Let’s Test our self!!!!!!

45. ATMOSPHERIC PRESSURE Atmospheric pressure is the pressure of air around us. Atmospheric pressure is due to the weight of air. The weight of an air column of the height of the of the atmosphere and area 10 cm x 10 cm is as large as 1000 kg. WHY DO NOT WE GET CRUSHED UNDER THIS PRESSURE? REASON: The Pressure exerted by the fluids in our body is equal and opposite to the atmospheric pressure acting on our body and thus it cancels the effect of the atmospheric pressure. 10 cm 10 cm Atmospheric pressure is measured by barometer in mm of Hg. Air column

46. AIR EXERT PRESSURE IN ALL DIRECTIONS The steam inside the bottle has been condensed by cold water, the bottle now has very little air inside it. The atmospheric pressure acting on the bottle from outside is therefore much larger than the pressure exerted by the air inside it. This caused to bottle to get crushed.

47. FACTORS AFFECTING ATMOSPHERIC PRESSURE Altitude: As we go to higher altitudes, the air becomes thinner and the atmospheric pressure decreases. Temperature: Air expands on heating and becomes less dense. Therefore atmospheric pressure decreases with increase in temperature. Humidity: Water vapor is lighter than air. Therefore humid air is lighter than dry air. Hence air pressure decreases as humidity increases.

48. APPLICATIONS OF AIR PRESSURE

49. Sucker hook

50. Let’s Test our self!!!!!! Q1. An inflated balloon is hanging from ceiling of a room filled with air, as shown. What would happen to the balloon if the air inside the room is removed completely ? (Assume no leakage of air from balloon.) A. The balloon would remain as it is. B. The balloon would shrink. C. The balloon would burst.

51. Q2. Consider the following statements and identify the right ones. i )Low temperature at poles cause air to expand. ii ) High temperatures along equator cause air to contract. a. i only b. ii only c. Both d. None

52. Q3. Consider the following statements and identify the right ones. i ) Air pressure decreases when air is dry. ii) Air pressure at sea level is lower than at the mountain top. a. i only b. ii only c. Both d. None