1. Forces and Newton’s Laws of Motion

2. 4N Sketch a labelled vector diagram to show the net force being applied to the box. 10N 2N 3N Box Starter

3. 10N 3N Box Starter Sketch a labelled vector diagram to show the net force being applied to the box.

4. What is a “force”? Force is “push or pull” Force is a vector quantity Measured in N (Newtons)

5. Examples of Forces Thrust Force (an accelerating car) Buoyancy Force (a floating ship) Lift Force (a flying plane) Tension Force (springs, ropes, BOTH ENDS) Friction Force (against movement) Weight Force (always downwards) Support Force (usually upwards)

6. Q: What is your weight? Q: What is your mass? Weight = Mass Weight is a type of force, ( measured in Newtons) F w = mg g is the acceleration due to gravity g = 9.8 ms -2 (on Earth)

7. Newton’s Laws of Motion

8. It’s all about the “Net Force”, the sum of all forces. Net force (F net ) is also called the Resultant force or the Total force.

9. Adam Austin

10. 40 N 20 N Forces are balanced Net Force = 0 Support Force Weight Force Austin’s push force Adam’s push force

11. Newton’s 1 st Law: When the net force on an object is zero (forces are balanced), the acceleration is zero. It means the object is either stationary or moving at a constant velocity.

12. Question: “If the Net force is zero, it must mean the object is not moving (the velocity = 0)” Would you agree with the statement? F net is zero = acceleration is zero (the velocity is constant) Does NOT necessarily mean that the velocity is zero

13. Support Force Weight Force Forces are balanced Stationary Velocity is constant (at zero) Acceleration is zero

14. Support Force Weight Force Friction ForceThrust Force Forces are balanced Velocity is constant (moving at a constant velocity) Acceleration is zero

16. Newton’s 1 st Law Net Force = 0 means that: Velocity is constant. There is no acceleration. Question: What about when the Net Force is NOT zero?

17. Adam Austin

18. 30 N 70 N 20 N Support Force Weight Force Adam’s push force Austin’s push force Forces are “unbalanced” Net Force = 40 N towards left It accelerates towards Austin

19. Newton’s 2 nd Law: If the net force on an object is NOT zero, the acceleration will NOT be zero. The relationship between F net and the acceleration is: F net = ma (m = mass) F net and a will always be in the same direction.

20. Friction ForceThrust Force Support Force Weight Force Forces are unbalanced The car accelerates (speeds up)

21. Friction ForceThrust Force Support Force Weight Force Forces are unbalanced The car decelerates (slows down)

22. 1.What net force is needed to keep a 5 kg mass moving at a constant velocity of 10 ms -1 ? 2.Calculate the net force needed to accelerate a 8 kg mass at 2 ms -2 3.Calculate the acceleration of the mass shown below.

23. A tow truck pulls a car of mass 1250 kg with a force of 1500 N. The friction between the truck and the road is 600N. a)Sketch a force diagram b)Sketch a vector diagram to find the net force. c)Calculate the car’s acceleration, using Newton’s 2 nd Law.

24. Mr Chu’s “coin drop” story

26. Weight Force Evan has just jumped off the plane E There is very little air friction (force) at first The only major force present is the weight force The forces are UNBALANCED Evan accelerates

27. Weight Force A few seconds later… E Friction Force (drag) The air friction force has increased a bit (because the air friction increases proportionally to Evan’s speed) The forces are still UNBALANCED Evan continues to accelerate

28. Weight Force After a while E Friction Force (drag) The friction force has grown as large as the weight force The forces are BALANCED Evan does not accelerate Evan falls at the maximum, constant velocity (Terminal Velocity)

29. Terminal Velocity The terminal velocity is the maximum velocity of a falling object when the drag (air friction) force equals the weight force in magnitude. Since the net force on the object is zero, the object has zero acceleration and falls at a constant velocity.

30. He opens his parachute! Friction Force (drag) The friction force suddenly becomes much larger than the weight force The forces are UNBALANCED Evan decelerates (slows down) Weight Force

31. What if there was no air? Free fall = unlimited speed?

32. A parachutist of mass 100 kg (including parachute) jumps from a great height. After a few seconds she reaches a terminal velocity of 50 ms -1. a)What is the total weight (of the parachutist and her parachute)? b)What is the force of air resistance on her when she is falling at the terminal velocity?

33. The parachutist now pulls her ripcord and as her parachute unfurls she slows to a steady speed of 5.0 ms -1 in 2.0 seconds. a)Calculate the acceleration over the 2 seconds. b)Calculate the net force acting on her over the 2 seconds. c)What is the force of air resistance now?

34. A car is accelerating at 2 ms -2. The mass of the car and its passengers is 1500 kg. 1.Find the weight of the car 2.Find the support force acting on the car 3.Draw a free body diagram showing these four forces: weight, support, thrust, friction 4.If the thrust force is 4000 N, find the friction force by using Newton’s 2 nd Law.

35. The three best mates - Matt, Liam and Chris are all looking at an airplane flying across the clear blue sky. Matt says, “There are three forces acting upon that plane, thrust force, lift force and friction force” Liam replies, “I agree, but the thrust force must be greater than the friction force, because the plane is moving forward” Chris cries, “Dudes! You are both wrong!!” Who do you think is (are) correct?

36. Newton’s 3 rd Law: For every action force there is an equal and opposite reaction force. (e.g. a recoil of a gun)

39. A suitcase is pulled across the floor by a strap at an angle of 40 o as shown. The tension force on the strap is 50 N and the mass of the suitcase is 7 kg. If the suitcase is moving at a constant velocity, calculate the weight force, the support force and the friction force (between the wheels of the suitcase and the floor). 7 kg 50 N

40.  Activity 9A Q1, 2, 5  HW Booklet – “Forces” Due: Monday next week MARK YOUR HOMEWORK