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Forces

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Distance, Speed and Time Speed = distance (in metres) time (in seconds) D TS 1)Dave walks 200 metres in 40 seconds. What is his speed? 2)Laura covers 2km in 1,000 seconds. What is her speed? 3)How long would it take to run 100 metres if you run at 10m/s? 4)Steve travels at 50m/s for 20s. How far does he go? 5)Susan drives her car at 85mph (about 40m/s). How long does it take her to drive 20km?

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Distance-time graphs 40 30 20 10 0 20 40 60 80100 4) Diagonal line downwards = 3) Steeper diagonal line = 1)Diagonal line = 2) Horizontal line = Distance (metres) Time/s

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40 30 20 10 0 20 40 60 80100 1)What is the velocity during the first 20 seconds? 2)How far is the object from the start after 60 seconds? 3)What is the velocity during the last 40 seconds? 4)When was the object travelling the fastest? Distance (metres) Time/s

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Acceleration S TA Acceleration = change in speed (in m/s) (in m/s 2 ) time taken (in s) 1)A cyclist accelerates from 0 to 10m/s in 5 seconds. What is her acceleration? 2)A ball is dropped and accelerates downwards at a rate of 10m/s 2 for 12 seconds. How much will the ball’s velocity increase by? 3)A car accelerates from 10 to 20m/s with an acceleration of 2m/s 2. How long did this take? 4)A rocket accelerates from 1,000m/s to 5,000m/s in 2 seconds. What is its acceleration?

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Velocity-time graphs 80 60 40 20 0 10 20 30 4050 Velocity m/s T/s 1) Upwards line = 2) Horizontal line = 3) Steeper line = 4) Downward line =

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80 60 40 20 0 1)How fast was the object going after 10 seconds? 2)What is the acceleration from 20 to 30 seconds? 3)What was the deceleration from 30 to 50s? 4)How far did the object travel altogether? 10 20 30 4050 Velocity m/s T/s

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

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

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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

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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

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1) This animal is either stationary or moving with constant speed… 4) This animal is also either stationary or moving with constant speed.. 2) This animal is getting faster… 3) This animal is getting slower….

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Force, mass and acceleration 1)A force of 1000N is applied to push a mass of 500kg. How quickly does it accelerate? 2)A force of 3000N acts on a car to make it accelerate by 1.5m/s 2. How heavy is the car? 3)A car accelerates at a rate of 5m/s 2. If it weighs 500kg how much driving force is the engine applying? 4)A force of 10N is applied by a boy while lifting a 20kg mass. How much does it accelerate by? F am

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Terminal Velocity Consider a skydiver: 1)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

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

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Work done When any object is moved around work will need to be done on it to get it to move (obviously). We can work out the amount of work done in moving an object using the formula: Work done = Force x distance moved in J in N in m W DF Work done against frictional forces is transferred mainly as heat.

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Force and acceleration If the forces acting on an object are unbalanced then the object will accelerate, like this bear: Force (in N) = Mass (in kg) x Acceleration (in m/s 2 ) F AM

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Kinetic energy Any object that moves will have kinetic energy. The amount of kinetic energy an object has can be found using the formula: Kinetic energy = ½ x mass x velocity squared in J in kg in m/s KE = ½ mv 2

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Some example questions… 1)A 70kg boy is running at about 10m/s. What is his kinetic energy? 2)A braking force of 1000N is applied by a driver to stop his car. The car covered 50m before it stopped. How much work did the brakes do? 3)What is the kinetic energy of a 100g tennis ball being thrown at a speed of 5m/s? 4)A crane is lifting a 50kg load up into the air with a constant speed. If the load is raised by 200m how much work has the crane done? (The answer isn’t 10,000J)

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Momentum Depends on mass And velocity Momentum = mass x velocity Is a measure of how difficult it is to change the movement of an object. mom = mv

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Momentum Is conserved (kept the same) in all collisions.

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Forces in all directions A force is needed to start anything moving. An interaction force arises between two objects. The green arrow shows the force.

Forces in all directions A force is needed to start anything moving. An interaction force arises between two objects. The green arrow shows the force.

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