Force, Mass and Momentum

A Force is anything which can cause an object to accelerateAccelerationdirectionexternalforcegravityKilogrammassmomentumNewtonrelativeSecondvelocity.

The unit of force is the accelerateAccelerationdirectionexternalforcegravityKilogrammassmomentumNewtonrelativeSecondvelocity (N)*.
The Newton is the unit of force which, when applied to a accelerateAccelerationdirectionexternalforcegravityKilogrammassmomentumNewtonrelativeSecondvelocity of 1 kg, gives it an acceleration of 1m/s/s.

The Mass of an object is a measure of how difficult it is to accelerateAccelerationdirectionexternalforcegravityKilogrammassmomentumNewtonrelativeSecondvelocity that object.

The unit of mass is the accelerateAccelerationdirectionexternalforcegravityKilogrammassmomentumNewtonrelativeSecondvelocity .

Relationship between Force, Mass and Acceleration

Force = Mass × accelerateAccelerationdirectionexternalforcegravityKilogrammassmomentumNewtonrelativeSecondvelocity

F = ma




Weight
The weight of an object is a measure of the accelerateAccelerationdirectionexternalforcegravityKilogrammassmomentumNewtonrelativeSecondvelocity of the Earth’s gravity acting on it.

Weight = Mass × Acceleration due to accelerateAccelerationdirectionexternalforcegravityKilogrammassmomentumNewtonrelativeSecondvelocity

W = mg

Because weight is a force, it follows that the unit of weight is also the Newton.



Momentum


Momentum = Mass x Velocity



The Unit of Momentum is the accelerateAccelerationdirectionexternalforcegravityKilogrammassmomentumNewtonrelativeSecondvelocity metre per second


Newton’s Laws of Motion

Newton’s First Law of Motion
States that every object will remain in a state of rest or travelling with a constant accelerateAccelerationdirectionexternalforcegravityKilogrammassmomentumNewtonrelativeSecondvelocity unless an external accelerateAccelerationdirectionexternalforcegravityKilogrammassmomentumNewtonrelativeSecondvelocity acts on it.

Newton’s Second Law of Motion
States that the rate of change of an object’s accelerateAccelerationdirectionexternalforcegravityKilogrammassmomentumNewtonrelativeSecondvelocity is directly proportional to the force which caused it, and takes place in the direction of the force.

Newton’s Third Law of Motion*
When body A exerts a force on body B, B exerts a force equal in magnitude (but) opposite in accelerateAccelerationdirectionexternalforcegravityKilogrammassmomentumNewtonrelativeSecondvelocity (on A).

Applications:
• Seat belts
• Rocket travel
• Ball games



F = ma is a special case of Newton’s accelerateAccelerationdirectionexternalforcegravityKilogrammassmomentumNewtonrelativeSecondvelocity Law



Friction

Friction is a accelerateAccelerationdirectionexternalforcegravityKilogrammassmomentumNewtonrelativeSecondvelocity which opposes the accelerateAccelerationdirectionexternalforcegravityKilogrammassmomentumNewtonrelativeSecondvelocity motion between two objects.

Examples of Friction:
Brakes
Walking
Air Resistance


The Principle of Conservation of Momentum

States that in any collision between two objects, the total accelerateAccelerationdirectionexternalforcegravityKilogrammassmomentumNewtonrelativeSecondvelocity before impact equals total momentum after impact, provided no accelerateAccelerationdirectionexternalforcegravityKilogrammassmomentumNewtonrelativeSecondvelocity forces act on the system.

In symbols m1u1 + m2u2 = m1v1 + m2v2

u1 is initial accelerateAccelerationdirectionexternalforcegravityKilogrammassmomentumNewtonrelativeSecondvelocity of m1
v2 is final accelerateAccelerationdirectionexternalforcegravityKilogrammassmomentumNewtonrelativeSecondvelocity of m2

Note that if the two objects coalesce (stick together) after collision, then there is only one final velocity, and the above equation becomes

m1 u1 + m2 u2 = (m1 + m2)v3

Areas where the principle of conservation of momentum applies:
• Collisions (ball games)