Unit 3b: Dynamics and Space – Section 1: Newton’s law and Acceleration due to Gravity

1. Unit 3b: Dynamics and Space – Section 1: Newton’s law and Acceleration due to Gravity By Ms Hashemian

2. Learning intentions: • To investigation relationship between mass and weight. • To calculation using relationship between weight, mass and g in our solar system. • To learn how to calculate of weight, mass and gravitational field strength during interplanetary rocket flight. • Equivalence of acceleration due to gravity and gravitational field strength.

3. Investigate the relationship between mass and weight.

4. W m g Weight and Mass Earth’s Gravitational Field Strength is 10N/kg. In other words, a 1kg mass is pulled downwards by a force of 10N. Weight = Mass x Gravitational Field Strength (in N) (in kg) (in N/kg) • What is the weight on Earth of a book with mass 2 kg? • What is the weight on Earth of an apple with mass 100 g? • Dave weights 700 N. What is his mass?

6. Weight & Mass on other planets

7. 1) On the moon the gravitational field strength is 1.6 N/kg. What will Dave weigh if he stands on the moon? 2) A 6 kg astronomical instrument is taken to the moon. a) what is the mass and weight of this instrument on Earth? b) what is the mass and weight of the instrument on the Moon? c) what is the mass and weight in space where the gravitational strength is zero? 3) What is the weight a 50 Kg mass on Saturn if the gravitational field strength of Saturn is 6.9 N / kg ?

8. Success Criteria • I know how to carry out an investigation to find out the relationship between mass and weight. • I know how to use formula, W= mg in our solar system. • I know how to calculate of weight, mass and gravitational field strength during interplanetary rocket flight. • I know the equivalence of acceleration due to gravity and gravitational field strength.

9. Learning intentions: • To find out what is meant by Newton’s First and Second law of Motion. • Use of NII to explain the movement of objects in situations involving constant acceleration. • Carry out investigation to find out the relationship between unbalanced force, mass and acceleration. • To be able to carry out calculations involving the relationship between force, mass and acceleration in situations where only one force acting. • To carry out calculations using the relationship between unbalanced force, mass and acceleration where more than one co-linear force is acting.

10. Balanced and unbalanced forces Reaction Consider a camel standing on a road. What forces are acting on it? These two forces would be equal – we say that they are BALANCED. The camel doesn’t move anywhere. Weight

11. Balanced and unbalanced forces Reaction What would happen if we took the road away? Weight

12. Balanced and unbalanced forces What would happen if we took the road away? The camel’s weight is no longer balanced by anything, so the camel falls downwards… Weight

13. Balanced and unbalanced forces What would happen if we took the road away? The camel’s weight is no longer balanced by anything, so the camel falls downwards…

14. Balanced and unbalanced forces 1) This animal is either ________ or moving with _____ _____… 2) This animal is getting _________… 3) This animal is getting _______…. 4) This animal is…

15. Balanced and unbalanced forces

16. What is meant by Newton’s First Law of Motion? When equal forces act in opposite direction they are called balanced forces. Balanced force are equivalent to no force at all. When balanced forces (or no forces at all) act on an object it remains at rest or continue to move at a steady speed in a straight line. This is known as Newton’s First Law of Motion. The cyclist in the video is peddling flat out. He is going at a steady speed. The forces acting on the bike must be balanced. The force forwards of the cyclist equals the backwards force of friction. 20 N 20 N

17. Carry out an investigation to find out the relationship between unbalanced force, mass and acceleration. • Your mission is watch or participate in the demonstration. For each different force, measure the acceleration produced three times and find the average value. • Complete the table and graph as part of your report to find the relationship between Force and Acceleration. • Hint • Plot the force applied on the x axis. • Plot the acceleration produced on the y axis. • Remember to stick the graph in your jotter.

18. Your mission is watch or participate in the demonstration. For each different force, measure the acceleration produced three times and find the average value. • Complete the table and graph as part of your report to find the relationship between Mass and Acceleration. • Hint • Plot the Mass on the x axis. • Plot the acceleration produced on the y axis. • Remember to stick the graph in your jotter.

19. What is meant by Newton’s second law of motion? When unbalanced forces act on a object the object changes speed (or direction). The acceleration due to an unbalanced force depends on the mass of the object and can be calculated using Newton’s Second Law of Motion. When the force stays constant and the mass increases the acceleration decreases. When the mass stays the same and the force increases the acceleration increases. Unbalanced Force = mass x acceleration Fun = m x a

20. Fun m a Force and acceleration If the forces acting on an object are unbalanced then the object will accelerate, like these wrestlers: Force (in N) = mass (in kg) x acceleration (in m/s2)

21. Fun m a Force, mass and acceleration • A force of 1000 N is applied to push a mass of 500 kg. How quickly does it accelerate? • A force of 3000 N acts on a car to make it accelerate by 1.5 m/s2. What is the mass of the car? • A car accelerates at 5 m/s2. If it has a mass of 500 kg how much force is acting on it? • A force of 10 N is applied by a boy while pushing a 20 kg suitcase. How much does it accelerate by?

22. Mini Quiz: Newton’s Laws We have already looked at Newton’s laws. Now we are going to look at them in more depth. You may have to revise this work if you can not fully answer the following questions. 1. What apparatus do we use to measure Forces? Newton balance 2. What three things can a Force do to an object? Change an objects shape, speed or direction 3. How do you know the Forces acting on an object are balanced? Remains at rest or continues to move with a constant speed in a straight line. 4.Write down Newton's first Law A body will remain at rest or move with steady speed in a straight line unless acted upon by an unbalance force. 5. Write down Newton's second Law Fun = ma

23. More than one force In most situations there will be more than one force acting on an object. If we want to calculate the acceleration of the object we must first find the unbalanced force. The combination of forces is known as the resultant. Example: The engine of the car provides a driving force of 2500N. The frictional force acting on the car is 100N. The mass of the car is 800 kg. Calculate the acceleration of the car. Step 1: Find unbalanced force. Fun = 2 500 – 100 Fun = 2 400N Step 2: Calculate acceleration a = Fun/m a = 2 400/800 a = 3 m/s2. 2500N 100 N 800 kg

24. More than one force 1) A car of mass 1200 kg experiences friction equal to 500 N when travelling at a certain speed. If the engine force is 1400 N, what will be the car’s acceleration? 2) Two girls push a car of mass 1000 kg. Each pushes with a force of 100 N and the force of friction is 120 N. Calculate the acceleration of the car. 3) A careless driver tries to start his car with the hand brake still on. The engine exerts a force of 2500 N and the hand brake exerts a force of 1300 N. The car moves off with an acceleration of 1.2 m/s2. What is the mass of the car?

25. Learning intentions: • To be able to carry out calculations to find out the resultant of 2 vector quantities at right angles to each other. • To carry out calculations involving the relationship between resultant force, mass and acceleration where there are 2 forces acting at right angles.

26. 2N 2N 4N 4N Forces acting at right angles: In most of the situation there will be more than one force acting on an object. Sometimes these forces are no in straight line, they are acting at right angles. • Forces at right angle must be drawn head to tale. • Use Pythagoras for the size of the force resultant. • Use SOH CAH TOA for the angle of the force resultant. Θ= Tan̄ ¹(2/4) Θ=26.5°

27. 50 Kg 40Kg 3N 4N 4N 5N Calculate acceleration:

28. Success criteria: • I know how to carry out calculations to find out the resultant of 2 vector quantities at right angles to each other. • I can carry out calculations involving the relationship between resultant force, mass and acceleration where there are 2 forces acting at right angles.

29. Learning intentions: • To use of Newton’s laws to explain free- fall and terminal velocity. • Newton’s 2nd law and its application to space travel including rocket launch and landing. • Glycerine filled cylinder with ball bearings – terminal velocity. • Air resistance, dropping ball and sheet of paper parachutes investigation. • Identify forces acting on vehicles travelling with constant velocity-car, helicopter, plane, boat. • Frictionless motion – air hockey puck, linear air track, model hover craft.

30. Free fall & Terminal velocity Free fall is a downwards movement under the force of gravity only. The terminal velocity of a falling object is the velocity of the object when the sum of the drag force and buoyancy equals the downward force of gravity acting on the object. Since the net force on the object is zero, the object has zero acceleration. Therefore the velocity is constant.

31. Terminal Velocity Consider a skydiver: • At the start of his jump the air resistance is big/small so he _______ downwards. 2) As his speed increases his air resistance will _______ 3) Eventually the air resistance will be big enough to _______ the skydiver’s weight. At this point the forces are balanced so his speed becomes ________ - this is called TERMINAL VELOCITY

32. Terminal Velocity Consider a skydiver: • 4) When he opens his parachute the air resistance suddenly ________, causing him to _____ ____. 5) Because he is slowing down his air resistance will _______ again until it balances his _________. The skydiver has now reached a new, lower ________ _______.

33. Parachute opens – diver slows down Speed increases… Terminal velocity reached… On the Moon New, lower terminal velocity reached Diver hits the ground Velocity-time graph for terminalvelocity Velocity Time

34. Glycerine filled cylinder with ball bearings Falling through a high viscosity liquid Demonstration The higher the viscosity (density) of a liquid the more it resists motion of a body through it. The result can be very low terminal velocity. Apparatus and materials Measuring cylinder or tall and fairly wide glass tube, 1,000 ml, with firm stopper. Glycerine, heavy oil or liquid detergent Ball bearings (approximately 3 mm and 1.5 mm) Chinagraph pencil, water-based pen, or elastic bands Eye protection

35. Mini Quiz 2) A spacecraft of mass 1 500 kg "blasts off" from the surface of the moon where g = 1.6 N/kg. The forces acting on the spacecraft at the instant it "blasts off" are shown on the diagram. Calculate: (a) The weight of the spacecraft on the moon's surface. (2Marks) (b) The size and direction of the unbalanced force acting on the spacecraft as it "blasts off". (1 Mark) (c) The size and direction of the spacecraft's acceleration as it "blasts off”. (3Marks)

36. Mini Quiz • An aircraft on a test flight travels at a steady speed of mach 2 (twice the speed of sound). It travels a measured distance of 1.02 km in 1.5 s during its test flight. • Calculate the average speed of the aircraft in metres per second. (2 Marks) • b) What is the instantaneous speed of the aircraft at all times during the test flight? (1 Mark) • c) Calculate the speed of sound using the information given in the question. (2Marks)

37. Mini Quiz A car starts from rest and reaches a speed of 20 m/s in a time of 4 s. a) What is the initial speed of the car? b) What is the change in speed of the car during the 4s period? c) What is the acceleration of the car during this time?

38. Glycerine filled cylinder with ball bearings Falling through a high viscosity liquid Demonstration The higher the viscosity (density) of a liquid the more it resists motion of a body through it. The result can be very low terminal velocity. Apparatus and materials Measuring cylinder or tall and fairly wide glass tube, 1,000 ml, with firm stopper. Glycerine, heavy oil or liquid detergent Ball bearings (approximately 3 mm and 1.5 mm) Chinagraph pencil, water-based pen, or elastic bands Eye protection

39. Learning intentions: • To learn about Newton’s 3rd law and its application to explain motion resulting from a reaction force. • To understand how seat belts and airbags work.

40. Brain storm Explain by using Newton’s why do we always need to wear seatbelts?

41. Seatbelts A driver is driving with a constant speed of 60 mph. He suddenly saw a barrier in front of him on the road and had to break to avoid an accident. Explain in terms of the forces why seat belts are used in cars.

42. When a car brakes (or crashes) there is a force acting against the car slowing it down. If the passenger was not wearing a seatbelt they would (according to Newton’s First Law of Motion) continue to move forwards at constant speed (until they hit the window screen or dashboard). A seatbelt is therefore used to provide a backwards force to stop the passenger from continuing to move forward at constant speed.

43. What is meant by Newton’s 3rd Law of Motion? Newton's Third Law explains how things move. Below is an example of a typical situation that you could be expected to explain. Read the question and try to answer it. A balloon is taped to a straw. The straw is free to move along a length of tight thread. The balloon is held at the neck so that air does not escape from the balloon. The balloon is then released and it accelerates along the thread. What makes the balloon move?

44. You can easily do this at home:

45. The movement of the balloon along the thread is similar to the motion of a rocket at lift off. You should be able to explain rocket motion in terms of Newton's Third Law. At lift-off, hot burning gas is pushed downwards by the rocket motors. The hot gas pushes back on the rocket in an upwards direction. When this upwards push, or thrust, exceeds the weight of the rocket, the forces acting on the rocket are unbalanced and the rocket accelerates upwards. Launching a Rocket How does it work??

46. Example: The diagram shows the vertical forces acting on a rocket of mass 12 000 kg which is being launched from the earth's surface, where g = 10 N/kg (Air friction has been ignored). • Determine: • The weight of the rocket. • 120 000 N • (b) The size and direction of the unbalanced force acting on the rocket as it is launched. • 3000 N • (c) The size and direction of the rocket's acceleration as it is launched. • 0.025 m/s² • (d) What forces are acting on the rocket while waiting for lift-off? • The weight of the rocket acting downwards and the force of the launcher on the rocket keeping it upright and ready to go.

47. So, how can we describe Newton’s 3rd Law of Motion? For every action there is an equal and opposite re-action. This means that for every force there is a reaction force that is equal in size, but opposite in direction. That is to say that whenever an object pushes another object it gets pushed back in the opposite direction equally hard.

48. How to make a rocket balloon

49. Success Criteria: • I know what is meant by Newton’s 3rd law and its application to explain motion resulting from a reaction force. • I understand how seat belts and airbags work.

50. Friction • What is friction? • Give 3 examples where it is annoying: • Give 3 examples where it is useful: • What effect does friction have on surfaces?