GCSE Forces and Motion W Richards Worthing High School Edited by J. Catchpole, Hockerill AEC

Distance, Speed and Time Speed = distance (in metres) time (in seconds) Dave walks 200 metres in 40 seconds. What is his speed? Laura covers 2km in 1,000 seconds. What is her speed? How long would it take to run 100 metres if you run at 10m/s? Steve travels at 50m/s for 20s. How far does he go? Susan drives her car at 85mph (about 40m/s). How long does it take her to drive 20km?

Distance-time graphs 2) Horizontal line = 40 30 20 10 4) Diagonal line downwards = Distance (metres) 3) Steeper diagonal line = Time/s Diagonal line = 20 40 60 80 100

40 30 20 10 20 40 60 80 100 Distance (metres) Time/s Distance (metres) Time/s 20 40 60 80 100 What is the speed during the first 20 seconds? How far is the object from the start after 60 seconds? What is the speed during the last 40 seconds? When was the object travelling the fastest?

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

Acceleration V-U T A Acceleration = change in velocity (in m/s) (in m/s2) time taken (in s) A cyclist accelerates from 0 to 10m/s in 5 seconds. What is her acceleration? A ball is dropped and accelerates downwards at a rate of 10m/s2 for 12 seconds. How much will the ball’s velocity increase by? A car accelerates from 10 to 20m/s with an acceleration of 2m/s2. How long did this take? A rocket accelerates from 1,000m/s to 5,000m/s in 2 seconds. What is its acceleration?

Velocity-time graphs 1) Upwards line = 80 60 40 20 4) Downward line = 4) Downward line = Velocity m/s 3) Steeper line = 2) Horizontal line = T/s 10 20 30 40 50

80 60 40 20 Velocity m/s T/s 10 20 30 40 50 How fast was the object going after 10 seconds? What is the acceleration from 20 to 30 seconds? What was the deceleration from 30 to 50s? How far did the object travel altogether?

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

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

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

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…

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…

Balanced and unbalanced forces What is happening to these cars?

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) F A M

Force, mass and acceleration 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? F A M

Terminal Velocity Consider a skydiver: At the start of his jump the air resistance is _______ 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

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

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

Newton’s Laws of Motion These are my three laws of motion (summarised): 1) If an unbalanced force acts on an object that object will either accelerate or change direction: F A M 2) That force is given by F=ma 3) When a force acts on an object there is an equal force acting in the opposite direction (“Action and reaction are equal and opposite”)

Newton’s 2nd Law Instead of F=ma Newton actually said that the force acting on an object is that object’s rate of change of momentum. In other words… mv T F Force = Change in momentum Time (in N) (in kgm/s) (in s) For example, David Beckham takes a free kick by kicking a stationary football with a force of 40N. If the ball has a mass of 0.5kg and his foot is in contact with the ball for 0.1s calculate: The change in momentum of the ball, The speed the ball moves away with

Example questions A golfer strikes a golf ball with a force of 80N. If the ball has a mass of 200g and the club is in contact with it for 0.2s calculate a) the change in momentum of the golf ball, b) its speed. A tennis player strikes a serve with a force of 30N. If the ball has a mass of 250g and the racket is in contact with it for 0.15s calculate the ball’s change in momentum and its speed. A footballer takes a goal kick by kicking a 0.4kg football away at 10m/s. If his foot was in contact with the ball for 0.1 seconds calculate the force he applied to the ball. A golfer strikes a 200g golf ball away at 50m/s. If he applied a force of 50N calculate how long his club was in contact with the ball for.

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

Stopping a car… Tiredness Too much alcohol Thinking distance Too many drugs Poor visibility Wet roads Icy roads Stopping distance Tyres/brakes worn out Driving too fast