Forces and Motion

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  • Created by: Polly
  • Created on: 16-11-13 11:28

Resultant Force

Resultant Force is the force left over after all of the forces have cancelled each other out. 


Resultant Force=2N^

If the Resultant Force on an object is 0 and the object is not moving...

The object stays still

If the Resultant Force on an object is 0 but its moving already...

The object continues moving at a constant velocity

If the Resultant Force on an object is not 0... 

It accelerates or decelerates

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Weight, Mass, and Gravitational Field Strength

Mass is a measure of the amount of matter (or stuff) in an object. Its measured in kg, and it does not change when gravitational field strength does.

Weight is a force caused by gravity. Its measured in N, and it changes as gravitational field strength does.  

Gravitational Field Strength (g) is the weight (N) of a 1kg mass. On earth g=10

Weight = Mass x Gravitational Feild Strength

W        = M     x  G

(N)        (kg)       (N/kg)

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Terminal Velocity

Falling objects accelerate if their weight is bigger than their drag because there is a resultant force acting on them.

As the falling object accelerates its drag increases until the forces are balanced. At this point there is no resultant force, so it falls at a constant velocity.

This is called its terminal velocity

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Momentum = Mass x Velocity

p    = m       x v

(kgm/s)        (kg)       (m/s)

Momentum is a vector. This means it has a direction. Negative momentum is going backwards. 

Momentum is a measurement of how difficult it is to stop a moving object

POCOM means the Principle of Conservation of Momentum. This means that if two objects collide in a closed system, the total momentum after the collision is the same as the total momentum before the collision. 

Closed System= No external forces 

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Momentum 2


p=massA+velocity A p=mass(A+B)+velocity(A+B)


u=?m/s u=0m/s  v=3m/s 

m=10,000       m=2500                                                 m=12,500 

37500=ux10,000  =37500/10,000 =3.75m/s       p=mxv =37500

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Momentum 3

If two objects collide or interact, the forces acting on each one will be the same size but in opposite directions. This means that momentum lost by one of the objects will be gained by the other object. Therefor, whenever two objects collide or interact, momentum is conserved. 

EG: Firing a gun

0m/s   ?   105.2m/s

0.75kg   0.75kg   0.00054kg

Bullet:p=mxv =0.0568kgm/s 

Gun:0.0568kgm/s= ?x0.75 =0.0568/0.75 =0.0757m/s

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