# P4- Explaining Motion

A summary of the Physics module P4- from the OCR 21st Century Science textbook

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• Created by: R_Hall
• Created on: 09-05-11 17:28

## Forces in all Directions

• To start anything moving requires a force
• Forces ALWAYS arrive from an interaction between two objects, they always come in a pair called an interaction pair
• The forces in an interaction pair are equal in size but opposite in direction
• The two forces act on different objects
• Objects can push and pull- anything can exert a force
• Where interaction object touch each other the forces are known as contact forces
• Gravity and magnetism act at a distance.
• An apple falling to the Earth exerts an opposite but equal force on the Earth as the gravity does upon it
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## How things start moving

• The interaction pair used to make rockets take off; gases are pushed down and the rocket is propelled up
• Jet engines use the same idea- air is drawn in and pushed out, the jet moves forwards
• An engine moves the wheels to make the car move forwards, that is why it is hard to start a car in mud
• The wheel causes a backwards force on the ground surface and the tyre on the road makes the car move forwards
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## Friction- A responsive force

• Friction is the force exerted on an object due to interaction between two sliding objects. The roughness of a surface decides how much friction is present.
• Friction adjusts its size in response to a situation- to a limit
• All surfaces have lumps and hollows
• When two surfaces are put together, the lumps fit into the hollows. when one object slides over another, it has to ride up over these lumps- this requires a force
• As all surfaces are rough, they only touch at a few points (where one lump meets another)- only a few real points of contact.
• The pressure at these points is large, "cold welding" them together. Force is needed when sliding to break these welds.
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## Reaction of surfaces

• A ball on a table in stationary (unlike letting it go from arms length), but gravity has not been stopped, another force cancels it out
• The table exerts an upward force on the ball that balances the downward force of gravity
• A cushion exerts force on a bag,the springy foam pushes upwards on the bag. The bag sinks into the cushion until it reaches the point where the push of the foam= the pull of gravity
• The upward force that hard surfaces exert is called the reaction of the surface
• The material of the surface limits its upward push. If the force exerted on the surface is too big, the surface become distorted and cannot spring back
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• If there is a force acting on an object, but it is not moving, then there must be another force balancing (or cancelling out) the first one
• If the forces acting on an object balance each other, they add to 0
• The sum of all the forces acting on an object is called the resultant force (you must take into account the direction.
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## How fast are you going?

• Average speed = distance travelled/ time taken
• For most journeys, the speed isn't constant, so the average speed is not very useful
• The speed at a particluar instant is called the instantaneous speed
• If you were able to calculate average speeds over a short interval, it would be close to the instantaneous speed
• The speedometer in a car shows the instantaneous speed
• The police use several methods to find the speed of a vehicle- Gatso Speed Cameras (cameras use radar to take photos at intervals and compare to distance scale on road), Truvelo Speed Cameras (cameras are triggered by detector cables and pressure sensors) and Police Radar Guns (bounce microwaves off cars and teh change in frequency determines speed)
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## Picturing motion

• Distance- time graphs show how far an object is from its starting point at every instant  during its journey
• Speed- time graphs show the speed of a moving object at every instant during its journey
• Distance-time graphs- The slope of the line shows the speed, steeper= faster, horizontal= stationary
• Speed-time graphs- These graphs have smooth gradual changes from one speed to another
• Tachographs monitor the speed and distance travelled by a vehicle
• When you want to talk about speed in a particular direction, you use the term velocity
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## Force, Interaction and Momentum

• If there is not interaction between two objects, there is no movement
• If two objects interact, the heavy one moves slower than the light one
• Momentum is this property of any moving object. Momentum (kgm/s)= Mass (kg) x velocity (m/s)
• If an object is moving in one direction it is positive, negative if in the other
• A change in momentum depends on the size of the force you push with and how long the force lasts
• Change of momentum (kgm/s)= Force (N) x time for which it acts (s)
• When two objects interact, the total change in momentum of the two objects (taking into account direction) is 0
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## Car safety

• In a collision, if the car comes to a sudden stop, there will be a sudden change of momentum, causing injury
• The momentum of a moving car depends on its mass and its velocity
• The bigger the time of collision, the smaller the force and the change of momentum
• Crumple zones are used in cars as they crumple gradually in a collision, to make the force less
• Seatbelts work the same way, they make the change of momentum take longer
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## Laws of motion

• If the resultant force acting on an object is 0, the momentum of the object does not change
• If there is a resultant force acting on an object, the momentum will changes. Change of momentum is given by and in the same direction as the resultant force
• A driving force is needed to keep an object moving.
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## Forces acting on a cyclist

• The driving force is you pedalling to push the wheels forward. The counter force is air-resistance and friction and it acts in the opposite direction.
• When you start cycling, the counter force is small and your driving force is bigger. You move forward and your speed increases
• As you go faster, the air resistance force on you gets bigger- the counter force increases. You are still getting faster, but not as quickly
• Eventually, you reach a speed where your counter force equals you driving force, you have a steady speed
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## Work and energy

• Work is done whenever a force makes something move. The amount of work (J)= force (N)x distance moved in direction of the force (m)
• The amount of work depends on the force you have to exert and the distance moved in the direction of the force
• Amount of work done= energy transferred
• Gravitational Potential Energy is increased when you do work by lifting. GPE (J)= Weight (kg) x vertical height difference (m)
• Some energy is wasted through friction and heating
• Kinetic energy is the energy something has owing to motion.Kinetic energy (J)= 1/2 x mass (kg) x velocity x velocity
• When a roller coaster runs down a slope it loses GPE but gains kinetic
• Conservation of energy is the principle that the total amount of energy at the end of a process is equal to the energy at the start
• Amount of GPE lost= Amount of kinetic gained
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