Forces Revision

1. Forces Revision Syllabus Motion Forces

2. Syllabus: Movement and position use the following units: kilogram (kg), metre (m), metre/second (m/s), metre/second2 (m/s2), newton (N), second (s) (P1.1). understand and use distance–time graphs (P1.2) recall and use the relationship between average speed, distance moved and time average speed = distance moved / time taken (P1.3) recall and use the relationship between acceleration, velocity and time acceleration = change in velocity / time taken a = (v-u)/t (P1.4) interpret velocity–time graphs (P1.5) determine acceleration from the gradient of a velocity–time graph and the distance travelled from the area between the graph and the time axis (P1.6).

3. Syllabus Double Award • express a force as a push or pull of one body on another • identify various types of force (e.g. gravitational, electrostatic, etc.) • understand that friction is a force that opposes motion • recall and use the relationship between unbalanced force, mass and acceleration: • force = mass × acceleration • F = m × a • recall and use the relationship between weight, mass and g : • weight = mass × g • W = m × g • describe the forces acting on falling objects and explain why falling objects reach a terminal velocity • describe the factors affecting vehicle stopping distance including speed, mass, road condition and reaction time • recall and use the relationship between the moment of a force and its distance from the pivot: • moment = force × perpendicular distance from pivot • recall that the weight of a body acts through its centre of gravity • describe how extension varies with applied force for helical springs, metal wires and rubber bands • recall that the initial linear region of a force – extension graph is associated with Hooke’s law

4. Syllabus Separate Science • express a force as a push or pull of one body on another identify various types of force (e.g. gravitational, electrostatic etc) • distinguish between vector and scalar quantities • appreciate the vector nature of a force • add forces that act along a line • understand that friction is a force that opposes motion • recall and use the relationship between unbalanced force, mass and acceleration • force = mass × acceleration • F = m × a • recall and use the relationship between weight, mass and g : • weight = mass × g • W = m × g • describe the forces acting on falling objects and explain why falling objects reach a terminal velocity • describe the factors affecting vehicle stopping distance including speed, mass, road condition and reaction time • recall and use the relationship between the moment of a force and its distance from the pivot: • moment = force × perpendicular distance from pivot • recall that the weight of a body acts through its centre of gravity • recall and use the principle of moments for a simple system of parallel forces acting in one plane • understand that the upward forces on a light beam supported at its ends vary with the position of a heavy object placed on the beam • describe how extension varies with applied force for helical springs, metal wires and rubber bands • recall that the initial linear region of a force - extension graph is associated with Hooke’s law

5. Motion Motion

6. Speed vs. Velocity This car is travelling at a speed of 20m/s This car is travelling at a velocity of 20m/s east Speed is simply how fast you are travelling… Velocity is “speed in a given direction”…

7. Speed Definition: The rate of change of distance Equation: speed = distance time Units: m/s Symbol: v (final) or u (initial) Mnemonic: Xylophones are Very Terrible x v t

8. Acceleration16/11/2014 Definition: How quickly velocity changes Equation: acceleration = (final velocity – initial velocity) ÷ time a = (v-u) t Symbol: a Units: m/s2 Mnemonic: A Changing Vulture undergoes Therapy v-u a t

9. Distance Time Graphs x x x x t t t t • a stationary object • An object with a fast constant velocity • An object with a slow constant velocity • an object which is accelerating • The gradient of a distance time graph is its velocity • ie distance divided by time

10. What does the graph show?

11. Velocity Time Graphs v v v v t t t t • An object with a constant velocity • An object with a fast constant acceleration • An object with a slow constant acceleration • an object which has a changing acceleration • The gradient of a velocity time graph is its acceleration • ie velocity divided by time

12. What does the graph show?

13. Calculating acceleration from graphs

14. Calculating Distance Travelled Sketch the graph in your books and write the answers

15. v (m/s) B C A D E F t (s) The graph is of a sky diver, describe what is happening: ________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________ Distance Time Graphs Gradient = _____________ Velocity Time Graphs Gradient = ___________, Area under = ____________ x (m) v (m/s) B F 10 10 C G 5 A 5 0 0 t (s) t (s) Distance Travelled = 0 5 10 15 0 5 10 15 x(m) v (m/s) 10 10 E I K 5 5 H D 0 0 t (s) t (s) Distance Travelled = 0 5 10 15 0 5 10 15 Worksheet

16. v (m/s) B C A D E F t (s) The graph is of a sky diver, describe what is happening: At A he is accelerating due to his weight, as his speed increases the air resistance increases until at B they balance and he reaches a constant velocity called the terminal velocity. At C he opens his parachute and the air resistance increases causing him to decelerate at D. At E the forces are balanced again and he falls at a new lower terminal velocity until at F he lands Distance Time Graphs Gradient = Velocity Velocity Time Graphs Gradient = acceleration, Area under = distance x (m) v (m/s) B F 10 10 C G 5 A 5 0 0 t (s) t (s) Distance Travelled = 125 m 0 5 10 15 0 5 10 15 x(m) v (m/s) 10 10 E I K 5 5 H D 0 0 t (s) t (s) Distance Travelled = 75 m 0 5 10 15 0 5 10 15

17. Exam Question

18. Forces Forces

19. What is a Force?

20. Drawing Vectors Small Force Big Force Going Right Going Right Tiny Force Huge Force Going Left Going Down

21. Cardsort: Types of Forces • Upthrust: • upwards force on a body caused by the water being displaced around it. This is sometimes called buoyancy. • Lift: • upwards force on an aeroplane. • Thrust: • forwards force created by a body. • Weight: • Force due to gravitational attraction. • Friction: • Force caused by rubbing of surfaces that opposes motion. • Air/Water Resistance: • Frictional Force caused by particles colliding with the body that opposes motion. • Magnetic: • Force caused by magnetic attraction. • Electrostatic: • Force caused by attraction between charges. • Reaction: • Force created by a body as a reaction to a force being applied.

22. What are the Balanced Forces?

23. Newton’s First Law Definition: An object will remain at rest or at a constant velocity in a straight line unless there is a resultant force acting on it. Isaac Newton 1643-1727

24. Calculating resultant forces

27. Newton’s Second Law F m a If the forces acting on an object are unbalanced then the object will accelerate, like these wrestlers: • Definition: • The net force acting on a body is directly proportional to the mass x the acceleration of the body and acts in the same direction. • Equation: • F = m x a • N kg m/s2 • Mnemonic: • Fear my anger Isaac Newton 1643-1727

28. Gravitational Field Strength W m g • Definition • Acceleration due to a gravitational field. Can be measured in N/kg or m/s2. • Is 10 N/kg on Earth. • Symbol: • g • Equation: • W = m x g • N kg m/s2 • Mnemonic: • Wow my goodness • Wazzup my G Worksheet

29. Direction of Friction • Friction always acts in the opposite direction to the way an object is moving. • What is the direction of friction for the following moving objects?

30. Terminal velocity of a skydiver

31. Velocity–time graph of skydiver

32. Stopping Distance Thinking Distance Braking Distance Braking Distance Stopping Distance = = Thinking Distance + +

33. Factors affecting stopping distances Copy into your book and add: mass of the car

34. Stopping distances

35. Moments • a moment is a turning force about a point. • moment about a point = force x distance • If an object is balanced the sum of the moments on it must add up to zero. • Moments clockwise = moments anticlockwise “Give me a place to stand and I will move the Earth” Archimedes (287 – 212 BC)

36. Principle of Moments Separate Science Drag and drop any of the masses onto the “see – saw” and try to get it to balance. The masses are in kilograms and the distance in metres.

37. Centre of Gravity • The weight of an object acts through the centre of gravity. • If you balance an object the moments on it are zero. • Therefore the centre of gravity must be at the pivot point when balanced

38. Extension of a metal Load (N) Plastic region Elastic limit Load  extension Extension (cm)

39. Definition: • A force is a _____, _____ or _____ that can cause a change in motion, shape, or direction. • Symbol: • F (_______) • W (_______) • Units: • N (_______) Load (N) ______region ______ limit ________________________________ Extension (cm) Worksheet

40. Definition: • A force is a push, pull or twist that can cause a change in motion, shape, or direction. • Symbol: • F (force) • W (weight) • Units: • N (newton) Find the Resultant Force: Load (N) Plastic region Elastic limit Load  extension Hooke’s Law is Obeyed Extension (cm)

41. Crossword