# GCSE P2 Chapter 2

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• Created by: emma998
• Created on: 26-02-14 17:46

## Forces between objects

• Forces are measured in Newtons (N)
• Objects always exert equal and opposite forces on each other
• These are sometimes called action and reaction forces
• If a car hits a barrier, the barrier will then exert an equal and opposite force on the car
• The weight of a book on a table causes a downward force of gravity from the book and there is an equal and opposite force from the table pushing the book upwards.
• The wheels of a forward moving car push backwards against the road and the road creates an equal and opposite force on the tyre pushing the car forwards.
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## Resultant force

• Most objects have more than one force acting on them
• The forces can be balanced or unbalanced
• If the forces are unbalanced there is a resultant force acting on the object
• A resultant force is the single force which has the same effect on an object as all the forces acting on the object together.

If the resultant force on an object that is able to move is zero then forces are balanced and:

• if the object is stationary, it will remain stationary
• if the object is moving, it will continue to move at a constant speed

If resultant force acting on object that can move is not zero then forces are unbalanced and:

• if the object is stationary, it will begin to move in the direction of the resultant force
• if the object is moving in direction of the force, the object will accelerate in that direction
• if the object is moving in the opposite direction of the resultant force, the obect will deccelerate
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## Forces and acceleration

• Resultant forces always cause an acceleration (positive or negative)
• Resultant forces are needed to accelerate and to change direction (acceleration is caused by a change in velocity which can be caused by a change in direction)
• Equation linking force and acceleration:
• Force(N) = Mass(Kg) X Acceleration(m/s)squared
• For a given acceleration the larger the mass the greater the force generated/needed
• For a given force the larger the mass the smaller the acceleration
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## Stopping distance

• The greater the speed of a vehicle the greater the decceleration needed to stop the vehicle in a given distance. So the bigger the braking force needed
• The stopping distance is the distance the vehicle travels during the driver's thinking distance and breaking distance
• Thinking distance is the distance the vehicle travels during the time it takes for the driver to react and is increased by tiredness, distractions and the influence of alcohol or drugs
• Braking distance is the distance the vehicle travels under the braking force and is increased by poorly maintained roads and bad weather and is also affected by the condition of the car
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## Falling objects

• If an object is falling freely through a fluid the resultant force acting on it is the force of gravity
• It will make an object close to the Earth's surface accelerate at about 10m/s/s
• As the object continues to accelerate, the drag force acting on the object will increase until the forces opposing the motion of the object are equal to the weight of the object. The resultant force is then zero so the object no longer accelerates and we say that is has reached its terminal velocity.
• The equation: Force = mass X acceleration becomes:
• Weight = mass X acceleration due to gravity
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## Stretching and squashing

• An elastic object is an object which returns to its original shape once the force deforming it is removed
• The spring constant of a spring is a measure of how stiff the spring is. The greater the spring constant the greater the force which must be applied to produce a specific extension
• Hooke's law states that extension of an elastic object which is being stretched or squashed is directly proportional to the force applied providing the limit of proportionality has not been exceeded. Elastic objects that follow this rule as said to be obeyers of Hooke's law
• Force(N) = Spring constant(NM) X extension(M)
• When an elastic object is stretched or squashed work is done and energy is transfered to the object as elastic potential energy which is then released when the force deforming it is removed.
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## Force and speed issues

• Fuel economy is reducing the amount of fuel a vehicle uses when travelling a paticular distance. Fuel economy involves:
• Reducing the speed of a vehicle
• Fitting a wind deflecter to make the vehicle more streamlined
• Speed cameras are used to discourage motorists from speeding. Two cameras can be placed next to a road one after the other to calculate the average speed of a vehicle.
• Skidding occurs when brakes are applied to harshly and the wheels lock together increasing the stopping distances. Anti-skid surfaces are rougher than normal roads and therefore increase the friction between the road and the wheels and they are put in place to reduce skidding at places where drivers have to stop quickly such as traffic light junctions.
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