Newton's laws of motion

Newton's First Law

• "An object at rest stays at rest and an object in motion stays in motion with the same speed and in the same direction unless acted upon by an ubalanced force"

Newton's Second Law

• F = MA
• F = Force
• M = Mass
• A = Acceleration

• If there are two forces in opoosition then they cancel out: F1-F2 = MA
• Exam questions usually have two components e.g. weight and thrust
• T - MG = MA => T = + MG + MA
• M1 > M2
• Resultant Force on M1: M1A = M1G - T
• Resultant Force on M2: M2A = M2G - T
• M1G - M2G = (M1 + M2)A

Calculations on Slopes

• Recalling the forces acting on a block at rest on a slope you can now write out a formula for the acceleration as the forces are unbalanced: MGSin(θ) - F0 = MA
• F0 = Friction

• As an object falls through a fluid it feels a force acting against it called drag

The magnitude of the drag force depends upon:

• The shape of the object
• The speed of the object
• The viscosity of the fluid

• As long as the forces of weight and drag are uneven the object will accelerate. When they are equal and opposite then terminal speed is reached, the resultant force is zero. F = MG - D (drag)
• Drag also exists when a vehicle is moving and this combines with friction. The total resistive forces will eventually equal the engine force. Fr-Fe = MA, F = P/V

Thinking Distance

• The distance it takes a car to travel whilst the driver is "thinking" or reacting to a situation is:    S1 = VT0
• Concentration, tiredness, alcohol and drag can all affect reaction times

Braking Distance

• This is the distance it takes the car to stop under constant deceleration (A) S2 = (v^2)/2A

Stopping Distances

• Stopping Distance - Thinking Distance + Braking Distance

Skidding

• If the force applied by the brakes is greater then the maximum possible friction between the tyres and the road surface then a car will shed
• Different road surfaces and different tyres will change the maximum friction between the two and hence change the maximum possible braking force

• The Impact Time T = (2S)/(U+V)
• The Acceleration A = (V-U)/T
• The Impact Force F= MA