P4 - Forces

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  • Created by: 11mbrady
  • Created on: 21-02-16 12:42



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Forces and interaction

-When an object exerts a force on another object, it always experiences a force in return. These two are called the, 'partner forces' or 'interaction pair' 

-If you push against a wall, the wall will push back in the opposite direction with exactly the same force otherwise you and the wall would fall over

-Although the forces are equal, they can still cause things to move because they act upon different objects

-For example, a jet engine exerts a force on the exhaust gases making them accelearte backwards. The exhaust gases exert an equal but OPPOSITE force on the jet, giving it a forward momentum

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Reaction Force

An object resting on a surface experiences a reaction force

If you put a book on a table, the book pushes down on a table with a force equal to its weight - an the table exerts an equal and opposite force upwards on the book

This upward force is called the reaction force - it is the table's 'reation' to the force exerted by the book


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Friction = Both objects expereincing a force that opposes the movement (REACTION FORCE)

The frictional force will match the size of the force trying to move an object, up to a maximum point - after this point friction will be less than the other force and the object will move


  • The friction between your shoes and the ground allows you to push against it and move forwards


  • Eg. The moving bits and pieces in a car engine
  • You can reduce sliding friction and gripping friction by adding a lubricant like oil/grease


  • An object moving through a fluid has to force its way past alll the molecules in the fluid and that causes friction . Drag only happens if the object is moving through a fluid.
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Size and Direction Of a Force

Drawing a diagram of an object to show the forces acting on it:

  • LENGTH of the arrow shows the SIZE of the force
  • DIRECTION of the arrow shows the DIRECTION OF FORCE
  • If the arrows come in opposite pairs and they're all the same size, then the forces are balanced


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Steady Speed

If an object is moving at a steady speed the forces must be balanced


A jet aircraft is moving at a steady speed and at a constant altitude (Height)

The forces acting on it therefore must be balanced

The drag caused by air resistance balances by the thrust produced by engines. The lift prouced by the air moving over the plane's wings balanced the weight

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Resultant Force

RESULTANT FORCE = The overall force acting on an object (when you add up all the individual forces and their directions)

The resultant force decides the motion of an object - whether it accelerates, deccelerates or stays at a speady speed

If there's a resultant force acting on an object, its speed OR direction (or both) changes

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Acceleration- Unbalanced Forces

The bigger the resultant force = the greater the acceleration

Eg. If a car's engine exerts a bigger driving (forward ) force than the drag counter force (backwards) the car will accelerate . If the driving force was less than the drag, the car would slow down. But the forces in the up and down directon are still balanced

A rocket

  • A rocket taking off accelerates away from the ground , so the upward force (thrust) must be greater than the downward force slowing it down (gravity and drag)
  • If the thrust stopped (rocket runs out of fuel) then the downward force would be greater than the upward force and the rocket would slow down until it stopped and then would accelerate downwards
  • Motion is all about the relative sizes of the up and down forces
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Momentum= how hard it is for something to stop moving (kg m/s)

Mass = kg      Velocity = m/s

"The greater the mass of an object , or the greater the velocity it has, the more momentum it has"

Momentum is a vector quantity - has size and direction but not speed

A resultant force of zero = object stays still   

If the resultant force isn't zero, the momentum changes in the direction of force

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Change in Momentum and Force


"A rock with a mass 1kg is travelling through space at 15m/s. A comet hits the rock, giving it a resultant force of 2500N for 0.7 seconds. Calculate the rocks initial momentum and the change in momentum"

Initial momentum = mass x velocity = 1 x 15 = 15 kg m/s

Change of momentum = force x time = 2500 x 0.7 = 1750 kg m/s

"The bigger the force and the longer it acts for, the bigger the change in momentum"

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Cars and Forces

"The greater the time for a change in momentum, the smaller the force"

If your momentum changes slowly, the forces acting on your body are small and you are less likely to be hurt

In a collision, you can't really affect the change in momentum . The cars mass and its change in velocity stays the same.

However the average force of an object can be lowered by slowing the object down over a longer time.

Safety features in cars INCREASE the collision TIME to reduce the forces on the passengers

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Safety Features in Cars


  • Crumple on impact
  • Increases the time taken for the car to stop


  • Slow you down more gradually


  • They stretch slightly (increasing the time taken for the passenger to stop)
  • Reduces the force acting on the chest


  • Provide padding that increases the time taken for your head to come to a stop if it hits something hard
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