CIE IGCSE PHYSICS Forces

1. CIE IGCSE PHYSICSForces July 13th 2011 Physics for IGCSE Physics pages 36-41

2. Section 1: Effects of Force State that a force may produce a change in size and shape of a body. Describe the ways in which a force may change the motion of a body. Find the resultant of two or more forces acting along the same line. Demonstrate an understanding that mass is a property which ‘resists’ change in motion. Inertia Recall and use the relation between force, mass and acceleration (including the direction). Describe, and use the concept of, weight as the effect of a gravitational field on a mass. Vectors and Scalers Demonstrate an understanding of the difference between scalars and vectors and give common examples Add vectors by graphical representation to determine a resultant Determine graphically the resultant of two vectors CIE IGCSE Specification

3. To write in your books 100 words each on the lives of Galileo and Newton 200 word total Include the following When he was born When he died Where he lived What three things was he known for One interesting fact Homework due Wednesday 19th October

4. To write in your books 100 words each on the lives of Galileo and Newton 200 word total When he was born – 1 mark B When he died – 1 mark D Where he lived - 1 mark L What three things was he known for – I mark for each mentioned K1, K2, K3 One interesting fact - 1 mark I 100 handwritten words – 1 mark 100 Quality 1 or 2 marks ( Plagiarism 1 mark max) Homework Markscheme

5. Force A force is a push or a pull, it is a vector it has magnitude and direction. A force can cause an object to: speed up slow down change direction change shape Force is measured in newtons (N). Force is measured with a newtonmeter. newtonmeters

6. What is a force? A force is a “push” or a “pull” or a “twist”. Q. How many different forces do you know?

7. Learning today Newton 1 Balanced Forces What inertia is Newton 2 Unbalanced Forces cause a body to speed up slow down or change direction. Also given as F=ma Unit N Gravity Mass and Weight W=mg Unit Newton – N g= 10 m/s2 Newton 3 For every action there is an opposite and equal reaction

8. Some types of force 1. Gravitational This is the attractive force exerted between bodies because of their masses. This force increases if either or both of the masses is increased and decreases if they are moved further apart. Weight is the gravitational force of the Earth on an object. Bathroom scales measure weight. A mass of 1kg weighs about 10N 1 stone is about 63N.

9. 2. Normal reaction or contact This is the repulsive force that stops two touching bodies moving into each other. The word ’normal’ means that this force acts at 90° to the surfaces of the bodies. It is caused by repulsive molecular forces. normal reaction forces weight The two upward reaction forces on the tyres balance the downward weight of the car

10. 3. Friction This is the force that opposes motion. The kinetic energy of the moving object is converted to heat energy by the force of friction. Friction is needed for racing cars to grip the road Friction is needed for walking!

11. 4. Air resistance or drag This is the force that opposes the movement of objects through air. Drag is a more general term used for the opposition force in any gas or liquid. Objects are often streamlined to reduce this force. streamlined car a parachute maximises drag force

12. 5. Upthrust This is the force experienced by objects when they are placed into a fluid (liquid or gas). An object will float on a liquid if the upthrust force equals its weight. A hot air balloon rises when the upthrust from the surrounding air is greater than the balloon’s weight.

13. 6. Magnetic Between magnets but also the force that allows electric motors to work. 7. Electrostatic Attractive and repulsive forces due to bodies being charged. Electrostatic force causes the girls’ hair to rise when they touch the Van der Graaff generator.

14. What is a force? A force is a “push” or a “pull”. Some common examples: ___ ________ (drag) – acts against anything moving through air ________ – pulls things downwards _____ – acts against anything moving ______ – keeps things afloat Words – upthrust, air resistance, friction, weight

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

16. 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

17. 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…

18. Balanced forces- Newton 1st When forces are balanced, it means that: If an object is stationary, it will continue to be stationary. If you are sitting on a chair, your weight is balanced by an upwards force from the chair. If an object is moving, it will move at constant velocity.  If you are pedalling a bicycle at a constant speed, the force you put into the pedals is balanced by the air resistance and friction.

19. Inertia When is at rest it takes a force to make it move. All objects resist a change in velocity, this is called inertia. Mass and paper demonstration

20. Learning today Newton 2 Unbalanced Forces cause a body to speed up slow down or change direction. Also given as F=ma What a resultant force is Gravity Mass and Weight W=mg Unit Newton – N g= 10 m/s2 Newton 3 For every action there is an opposite and equal reaction

21. 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…

22. Balanced and unbalanced forces

23. Newton 2 Unbalanced Forces cause a body to speed up slow down in the direction of the force or they change direction. Also given as F=ma Resultant force is the overall sum of many different forces

24. Force and acceleration F M A If the forces acting on an object are unbalanced then the object will accelerate or change direction, like these wrestlers: Force (in N) = Mass (in kg) x Acceleration (in m/s2)

25. Force, mass and acceleration F M A • A force of 1000N is applied to push a mass of 500kg. How quickly does it accelerate? • A force of 3000N acts on a car to make it accelerate by 1.5m/s2. How heavy is the car? • A car accelerates at a rate of 5m/s2. If it weighs 500kg how much driving force is the engine applying? • A force of 10N is applied by a boy while lifting a 20kg mass. How much does it accelerate by?

26. Resultant force A number of forces acting on a body may be replaced by a single force which has the same effect on the body as the original forces all acting together. This overall force is called resultant force. In the example opposite, 5N is the resultant force of the 3N and 2N forces. 3N 2N 5N

27. Determine the resultant force in the cases below: TRIPLE ONLY 4N 1. 2. 3N 4N 6N 1N 10N 6N 4N 4N 4N There is no resultant force in this case 2N 4N 3. 4. 5. 3N 7N

28. Mass and weight Mass is the amount of matter in an object. Mass is measured in kilograms. The mass of an object is the same on the Moon as on the Earth. Weight is the force of gravity on an object. Weight is measured in newtons. The weight of an object on the Moon is about one sixth that on the Earth. A newtonmeter is used to determine the weight of the parcel

29. The acceleration due to gravity (g) The acceleration due to gravity (g) varies with planet, moon and star and depends on the height of an object. Some examples of the acceleration due to gravity:

30. Gravitational field strength (g) This is an alternative way of measuring the strength of gravity. The gravitational field strength is equal to the gravitational force exerted per kilogram. Near the Earth’s surface, g = 10 N/kg In most cases gravitational field strength in N/kg is numerically equal to the acceleration due to gravity in m/s2, hence they both use the same symbol ‘g’.

31. Calculating weight weight = mass × gravitational acceleration W= m xg weight is measured in N mass is measured in kg gravitational acceleration is measured inm/s2 On the Earth’s surface a mass of 1kg has a weight of 10N.

32. Falling objects When an object falls through air or some other fluid initially the only significant force acting on it is the downward pull of gravity. On Earth, it will initially accelerate downwards at 10 m/s2.

33. Learning today Terminal Velocity in a fluid Forces acting – Weight and Drag Newton 3 For every action there is an opposite and equal reaction Explain using Newton 3 how a rockets produces a force The difference between vectors and scalars Yacapaca results

34. As the object speeds up frictional forces such as air resistance become greater the faster the object moves. Eventually the resultant force on the object will be zero when the frictional forces equal the weight of the object. The object then moves at a constant speed called terminal velocity.

35. Terminal VelocityVelocity-time graphs for a falling object in fluids The forces are Weight and Drag

36. Newton 3 • For every action there is an equal and opposite reaction. • Forces act in pairs Two bodies are involved so forces do not cancel out A person in a lift Earth and the moon Task 1 Explain how a rocket produces a force. Task 2 Question 4

37. Vectors and Scalars All physical quantities (e.g. speed and force) are described by a magnitude and a unit. VECTORS – have magnitude, direction and a unit. examples: displacement, velocity, acceleration, force. SCALARS – have magnitude only examples: distance, speed, mass, work, energy.

38. Scalar and Vectors Draw a T-table at the back of your books Scalar Vectors

39. Scalar and Vectors Now draw a T-table in the front of your books. Scalar Vectors

40. Parachuting A parachutist will have two different terminal velocities. Before opening the parachute it is about 60 m/s (140 m.p.h..). Afterwards, due the much greater drag force, the terminal velocity is about 5 m/s (12 m.p.h.)

41. Velocity-time graph of a parachutist velocity first terminal velocity parachute opened ground reached initial acceleration = 10 m/s2 second terminal velocity time

42. Choose appropriate words to fill in the gaps below: A _____ is a push or a pull. A force can cause an object to ___________ or change shape. Force is measured in _______ (N) with a newtonmeter. There are many types of force. ________ force occurs when two bodies touch each other. Friction is a force that _______ the _______ of one body relative to another. It is caused by the _________ forces between ___________. force accelerate newtons contact motion opposes attractive molecules WORD SELECTION: newtons accelerate force attractive opposes molecules contact motion

43. Vectors and Scalars All physical quantities (e.g. speed and force) are described by a magnitude and a unit. VECTORS – have magnitude and direction examples: displacement, velocity, acceleration, force. SCALARS – have magnitude only examples: distance, speed, mass, work, energy.

44. Representing Vectors An arrowed straight line is used. The arrow indicates the direction and the length of the line is proportional to the magnitude. Displacement 25m at 45o North of East Displacement 50m EAST

45. Addition of vectors 1 4N 4N 6N 6N object object resultant = 10N object 6N 4N 4N 6N object object resultant = 2N object The original vectors are called COMPONENT vectors. The final overall vector is called the RESULTANT vector.

46. Resultant force and motion

47. Examples 1 & 2 The box will move when the man’s push force is greater than the friction force. The plane will accelerate provided that the engine force is greater than the drag force.

48. Examples 3 & 4 The brakes exert a resultant force in the opposite direction to the car’s motion causing the car to decelerate. Once released, the glider moves at a near constant velocity as it experiences a nearly zero horizontal resultant force.

49. Choose appropriate words to fill in the gaps below: A single force, called _________ force, can be used to replace a _______ of forces that act on a body. If the resultant force is _____ then the body will either remain at _____ or continue to move at a constant ________. If the resultant force is in the same _________ as an object’s motion, the object will __________. A car is decelerated when the braking force acts in the _________ direction to the car’s motion. resultant number zero rest velocity direction accelerate opposite WORD SELECTION: number rest direction zero opposite velocity accelerate resultant

50. Changing shape Force can also change the shape of an object. A stretching force puts an object such as a wire or spring under tension. A squashing force puts an object under compression.