Forces and Motion

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A force is a push or a pull on an object and affect the object in different ways:

  • A force can make an object change shape
  • A force can make an object change direction
  • A force can make an object change speed

There are four main types of forces used in physics:

  • Weight
  • Reaction
  • Drag 
  • Thrust

Weight is a force and is NOT the same as mass

Weight is measured in newtons and mass is measured in kg

Your mass is always the same but your weight changes depending on the gravitational field strength of the planet.  weight = mass x gravitational field strength (on earth this is 10 N/kg)

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Motion depends on interacting forces which will be equal in size and opposite in direction. For example you are on ice skates and push your friend who is also on ice skates you will both move with the same force but in opposite directions.


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When an object is stationary the weight and reaction forces are equal

When an object is at a constant speed horizontally the thrust, weight reaction and drag are all equal.

When an object is accelerating horizonatally the thrust is larger than the drag but the reaction and weight forces are equal

When an object is at a constant speed vertically the drag and weight are equal

But when the object is accelerating vertically the weight is larger than the drag

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Jumping out of a plane

1) As soon as you jump the weight will be greater than the drag which causes you to gain speed as you fall

2) After falling for a while you reach terminal velocity (travelling at a constant seed, the fastest you will go) because the drag is equal to the weight

3) Once you open the parrachute the drag is larger than the weight because there is more air resistance so you begin to slow down

4) Eventually the larger drag will equal the weight and you will gain a new terminal velocity that is much slower and safer

Always remember that when travelling at a constant speed the forces are balanced

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Resultant force

A resultant force is the overall force acting on a body.

If there is no resultant force the object is either stationary or moving at a constant velocity. This is because the forces are balanced so when you take one from its oppostie force you get a total of 0N.

If there is a resultant force the object is accelerating or deccelerating (for decceleration when you take one away you will get a negative velocity). 

To work out resultant force you simply take the two opposing forces i.e thrust and drag or weight and reaction and take them away from eachother

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