KS3 Forces and Motion

1. KS3 Forces and Motion

2. Contents Motion Weight, mass and gravity Balanced and unbalanced forces Friction Moments Pressure and Hydraulics

3. Distance, Time and Speed • To work out the speed of an object you need to know: • the distance traveled • how long it took to travel that distance distance traveled time taken

4. Formula triangle Use this equation: SPEED = d distance time s x t Speed is measured in many different units, e.g. m/s, km/h, km/s, miles per hour. The units of distance and time used will give the units to be used for speed.

5. Examples A boy travels from his home to the cinema, a distance of 10 km in 1 hour. Calculate his speed in km/h. d (distance in km) Speed in km/h = t (time in h) d 10 km = 1 h s x t = 10 km/h Cover the quantity you want to calculate - s (speed)

6. 1x60x60 You sometimes have to change the units in this type of problem - here is the same problem again A boy travels from his home to the cinema, a distance of 10 km in 1 hour. Calculate his speed in m/s. (1km = 1000m) d (distance in m) Speed in m/s = t (time in s) d 10,000 m = 3600 s s x t = 2.8 m/s Cover the quantity you want to calculate - s (speed) Click for solution

7. d s x t Questions 1. A group set off from home and walk at an average speed of 3.6 km/h. How far would they travel in 2 hours? Give your answer in km. Click for solution Distance (km) = Speed (km/h) x time (h) = 3.6 km/h x 2 h = 7.2 km

8. distance Time = Time = Time = 1.85 hours speed d 10 km 5.4 km/h s x t 2. How long would it take a woman to walk 10 km if her average speed is 5.4 km/h ? Click for solution

9. Experiments - Speed 1. Time how long it takes you to run 100m. 2. Then calculate your speed for the run. SPEED (m/s) = 3. Repeat the experiment for each member of your group. What was the fastest speed for your group ? Distance (m) Time (s)

10. Results Name distance (m) time (s) speed (m/s) 100 100 100 100 100 Conclusion The fastest member of the group with a speed of ______ was _______.

11. Weight, mass and gravity

12. Weight and Mass Weight and mass are not the same. Mass is the amount of matter in an object. This will have the same value anywhere in the Universe including space. Weight is a force and it is caused by the pull of gravity. In fact, weight is the pull of gravity acting on a mass.

13. Weight and Mass Weight is a force so is measured in Newtons. Like other forces it has both magnitude and direction. Mass is not a force, it is measured in kilograms. A 1 kg mass will weigh less on the moon than it does on Earth. This is because the force of gravity is less on the moon because the moon is smaller than the Earth. An astronaut could jump 20 feet into the air on the moon because gravity is less. However, he still has the same body, and the same mass, it just weighs less, because he is on the moon and gravity is weaker.

14. Weight and Mass So, a scientist should never say “He weighs 50kgs” but, should say “He has a mass of 50kgs”, or the scientist could say: “the gravitational force acting on his mass is about 500 Newtons”. This is the same as saying: “his weight is about 500 Newtons”.

15. Gravity Gravity is an attractive force that acts between all masses. The force depends on the mass of the object. All objects produce a gravitational force but it is only significant when the mass is about the size of a moon or planet. Think about it: When you jump the gravitation force of the Earth pulls you down. Your gravitation force pulls the Earth up!

16. Space shuttle - smaller mass Force of gravity The force of gravity depends on the mass of the planet and how close you are. Objects will have higher weight on Jupiter because it has a larger mass than Earth. Earth - large mass

17. Balanced and unbalanced forces

18. Forces Balanced Forces 10 N 10 N If you link two newton meters and pull equally hard from both ends, the forces recorded on both will be the same. We say that the forces acting on the central hooks cancel each other out - they are equal in magnitude and opposite in direction. Because the forces are balanced the hooks do not move.

19. Forces unbalanced Forces 11 N 10 N movement What happens if the pull on one end is harder than on the other? The forces acting on the hooks are no longer balanced. Both hooks will start to move to the left, that is, their speed will change. This is called acceleration. Unbalanced forces lead to a change in speed or direction.

20. More Balanced and Unbalanced Forces Think of a car traveling at a constant 50 mph. The engine provides sufficient force to just overcome all the frictional forces that are acting to decrease the speed. 50 mph 500 N 500 N

21. Now a cross-wind acting on the car produces a sideways force. 50 mph Cross wind This causes the direction of the car to change. This happens because the sideways forces on the car are not balanced. If the car turns left so that the wind is now BEHIND the car, what will happen to the speed?

22. The air resistance will decrease because the car has a “tail wind” (it is being blown from behind). This means the forces acting on the car are no longer balanced. The car will increase in speed (accelerate) until the forces are balanced again. > 50 mph 400 N 500 N 60 mph 500 N 500 N

23. Summary • If the forces on an object are balanced : • If it is stopped it will remain stopped. • If it is moving then it will continue to move at the same speed. • In other words, it will continue to do what it is already doing without any change. • If the forces are unbalanced two things can happen • The speed will change. • The direction of motion will change. • This is called acceleration.

24. A resultant force of 100 N is accelerating the car. Resultant Forces The sum effect of more than one force is called the resultantforce. You can find out the resultant force by calculating the difference between opposing forces. 100 N 400 N 500 N

25. Find the resultant force: 1. 5N 5N Click for solution 10N 20N Resultant force = 20N -10N = 10N down The block will accelerate down.

26. 5N 2. 5N Click for solution 5N 5N Resultant force = 5N - 0N = 5N right. The vertical forces are equal in size and opposite in direction so there is no resultant force in the vertical direction. The block will accelerate to the right.

27. 17N 7N 3. 3N 10N 20N 13N 10N Click for solution Resultant force = 30 - 13 = 17N right. The vertical forces are equal in size and opposite in direction so there is no resultant force in the vertical direction. The block will accelerate to the right.

28. Friction Friction always tries to slow moving object down. We say it opposes motion. Friction is created whenever two touching objects or surfaces move past each other. Friction also occurs when things move through air. This is called air resistance or drag.

29. Brake pad and rim Pedal bearing Wheel bearing Wheel bearing Links in chain tyre and road Friction On the diagram label all sources of friction. Click for answers Air resistance, or “Drag” One more? Probably the most important…

30. 400 N 300 N Air Resistance or drag • Air resistance is a type of friction caused when objects move through the air. • Cars are designed so that they are streamlined. The flow of air around the body is made as smooth as possible so that air resistance is minimized. • Air resistance depends on: • the size of the car; • the shape of the car; • the speed of the car.

31. Other Sources of Friction in Cars One of the most important sources of friction in cars in that between the tyre and the road. When the car brakes, the maximum possible amount of friction is desirable so that the car does not skid. • The friction between the tyre and the road is affected by: • inflation pressure of the tyre • the road surface • whether the surface is wet.

32. Force and Rotation

33. Force and Rotation pivot 5N A force acting on an object can cause it to turn about a pivot. What would happen to the see-saw above ? Would it turn? If so, clockwise or anti-clockwise?

34. Force and Rotation pivot The left goes down - an anticlockwise turn. A turning force is called a moment.

35. Moments Pivot Distance from force to pivot Force Suppose you were trying to unscrew a nut using a spanner. The spanner exerts a moment or turning force on the nut. If the moment is big enough it will unscrew the nut. If not there are 2 ways of increasing the moment.

36. Increasing the moment 1. Increase the distance from the force to the pivot - apply the force at the end or use a longer spanner. Pivot Distance from force to pivot Force

37. 2. Increase the force applied - push/pull harder or get someone stronger to do it! pivot Distance from force to pivot Force

38. Moment The moment of a force is given by the relationship: Moment = Force (N) x Distance (cm or m). Moments are measured in Newton centimeter (Ncm) or Newton metre (Nm). moment F x d

39. A 500 N woman stands on one end of a see-saw. She is 0.5m from the pivot. What moment does she exert? Click for solution 0.5m pivot 500N Moment = 500 x 0.5 = 250 Nm.

40. Principle of Moments pivot The green girl exerts an anti-clockwise moment equal to ... her weight x distance from pivot. The yellow girl exerts a clockwise moment equal to... her weight x distance from pivot.

41. Principle of Moments pivot If the two moments are equal then the seesaw is balanced. This is known as the principle of moments. When balanced Total clockwise moment = total anti-clockwise moment “c.m.” = “a-c.m.”

42. The principle of moments can be investigated using the balance shown below with 10g masses: Moment (right) = (3 x 10) + (4x10) = 70gcm Moment (left) = 7 x 10 = 70gcm Both moments are equal therefore the seesaw is balanced.

43. At balance total “c.m.” = total “a-c.m.” 200 x 1.5 = 150 x distance 200 x 1.5 distance = 2 m = distance 150 Using Moments in Calculations 1. Two girls are on a seesaw. One weighs 200N and is 1.5m from the pivot. Where must her 150N friend sit if the seesaw is to balance ?. Click for solution “c.m.” = clockwise moment “a-c.m.” = anti-clockwise moment

44. Pressure and Hydraulics

45. Pressure Pressure is exerted whenever a force is applied over an area. 2. 1. Which one exerts the biggest pressure, 1 or 2?

46. 1. Case 1. The arm applies a force onto a board via a finger tip. The force applied produces a high pressure because the force acts over a small area.

47. 2. Case 2. The arm applies the same force onto the board. The force is now acting over a larger area - the area of the palm is greater than the finger tip. Thus, a lower pressure is produced.

48. F Force Pressure = Area P x A Pressure Pressure is the force per unit area so is calculated using the expression shown below: Pressure is measured in: Newtons per metre squared (N/m2) which is called a PASCAL (Pa) Pressure can also be measured in: Newtons per millimetre squared (N/mm2); Newtons per centimeter squared (N/cm2).

49. The same force spread over a big area means low pressure. Which shoes would you choose for walking over a muddy field?

50. The boots on the right spread the weight over a larger area. Therefore, the pressure exerted on the ground is low. In contrast, fashion shoes have a smaller area and exert a higher pressure. These shoes are likely to sink into soft ground.