# Forces and Motions

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## Velocity and Distance-time graphs

• speed and velocity are both how fast you're going.
• speed is just how fast you're going (e.g. 30mph or 20 m/s) with no regard to the direction.
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## distance graphs

• flat sections are where it's stationary- it's stopped
• straight up hill or downhill sections mean it is travelling at a steady speed.
• the steeper the graph, the faster it's going
• downhill sections mean it's going back towards its starting point
• curves represent acceleration or deceleration
• a steeping curve means it's speeding up (increasing gradient)
• a levelling off curve means it's slowing down (decreasing gradient)
• speed = gradient = vertical / horizontal
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## Velocity-time Graph

• flat sections represent steady speed
• the steeper the graph, the greater the acceleration or deceleration
• uphill sections (/) are acceleration
• downhill sections (\) deceleration
• the area under any section of the graph (or all of it) is equal to the distance travelled in that time interval.
• a curve means changing acceleration.
• acceleration = gradient = vertical change / horizontal change
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## Acceleration

• acceleration is how quickly velocirty is changing
• acceleeration is not the same as veloctiy or speed.
• this shange in veloctiy can be a change in speed or a change in direction or both
• acceleration= change in velocity / time taken.
• in a triangle: (v-u) / a x t. v= final velocity. u= initial velocity. a= acceleration. t= time.
• unit of acceleration is m/s squared
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## Resultant force

• resultant force is the overall force on a point or object
• there are at least two forces acting on a object along any direction.
• the overall effect of these forces will decide the motion of the object- weather it will accelerate, decelerate or stay at a steady speed.
• if the forces all act along the same line (they are parallel and act in the same or the opposite direction), the overall effect is found by just adding or subtracting them.
• a resultant force means a change in velocity
• if there is a resultant force acting on an object, then the object will change its state of rest or motion.
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## Forces and Acceleration Part 1

• an object needs a force to start moving
• if the resultant force on a stationary object is zero, the object will remain stationary.
• no resultant force means no change in velocity.
• if there is no resulant force on a moving object it'll just carry on moving at the same velocity
• a resultant force means acceleration
• if there is a non-zero resultant force, then the object will accelerate in the direction of the force.
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## Forces and Acceleration Part 2

• f=ma
• f= resultant force (N)
• m= mass (kg)
• a= acceleration (m/s squared)
• reaction forces are equal and opposite. when two objects interact, the forces they exert on each other are equal and opposite.
• when skater A puches on skater B (the 'action' force), she feels an equal and opposite force from skater B's hand (the 'reaction' force). both skaters feel the same sized force, in the opposite directions, and so accelerate away from each other.
• sxater A will be accelerated more than skater B, through, because she has a samller mass - remember a=f/m
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## Stopping Distances

• if you need to stop in a given distance, then the faster a vehichle's going, the bigger braking force it'll need.
• for any givien braking force, the faster you're going, the greater your stopping distance
• the total stopping distance of a vehicle is the distance covered in the time between the driver first stopping a hazard and the vehicle coming to a complete stop.
• stopping distance = thinking distance + braking distance.
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## Thinking Distance and Braking Distance

• thinking distance is affected by two factors:
• how fast you're going - whatever your reaction time, the faster you're going the futher you'll go.
• how doepy you are - this affected by tiredness, drugs, alcohol and a careless attitude.
• braking distance is affected by four main factors:
• how fast you're going- the faster you're going, the futher it takes to stop
• how good you're brakes are- all brakes must be checked and maintained regularly. if not it can lead to a bad acciedent.
• how good the types are- tyres should have a minimum tread depth of 1.6mm in order to be get rid of water in wet conditions.
• how good the grip is- this depends on three things: road surface, weather conditions, tyres
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## Forces and Elasticity

• when you apply force to an object, it made cause it to strech and change shape.
• any object that can go back into it's original shape after a force has been removed is an elastic object.
• this energy is not lost but stored by the object as elastic potential energy. can be converted into kinetic energy.
• extension of an elastic object is directly proportional to force
• f = k x e
• f= force (N). k= spring constant (N/m). its value depends on the material that you are streching. e= extension (m)
• extension of an elastic object is directly proportional to force but stops working when the force is great enough.
• there is a maximum force that the elastic object can take and still extend proportionally. this is known as the limit of proportionally.
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