# Newton's Laws and Momentum

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• Created by: lucysnell
• Created on: 29-01-16 20:44

## What is momentum?

• Momentum is a physical quantity that a moving object has
• The momentum of an object depends on its mass (m) and velocity (v)
• Momentum is a VECTOR quantity - therefore has magnitude and direction
• Momentum is always conserved
• Hence, total momentum before a collision = total momentum after a collision
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## Collisions - Elastic/inelastic?

Elastic Collision

• Momentum is always conserved
• Kinetic energy is conserved
• No energy is dissapated to the surroundings

Inelastic Collision

• Momentum is always conserved
• Kinetic energy is not conserved
• Energy is dissapated to the surroundings eg. heat, sound
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## Impulse = change in momentum

Newton's Second Law says - the force is equal to the rate of change of momentum

• F = (mv-mu)/t .... rearrange to get Ft = mv-mu
• Impulse = average force x time
• Therefore, the force of an impact can be reduced by increasing the time of the impact
• This is why cars have crumple zones
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## Newton's First Law

An object will continue in a state of rest of of uniform motion unless acted upon by a resultant force.

• If forces are not balanced eg. there is a resultant force, then the overall resultant force will cause the object to accelerate
• Acceleration of an object can mean a change in direction or speed, or both
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## Newton's Second Law

The rate of change of momentum of an object is directly proportional to the resultant force on the object and in the same direction.

• If mass is constant then F=ma applies
• The more force you have on a mass, the more the mass accelerates
• For a given force, the more mass you have, the smaller the acceleration
• F=ma cannot be used when the mass of an object is changing
• In this situation Newton's Second Law still applies but you must use Force=change in momentum/time
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## Newton's Third Law

If object A exerts a force on object B, then object B exerts an equal and opposite force of the same type of object A.

• The forces represent the same interaction but from different perspectives
• Newton's Third Law is a consequence of the conservation of momentum
• A resultant force acting upon an object means a change in mass or acceleration
• Hence this means a change in momentum
• Momentum is always conserved, so the forces must be equal so that the overall change in momentum is zero
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