Topic 2 + 6 - Mechanics

Newton's First Law

Newton's First Law

"Every body perseveres in its state of rest, or of uniform motion in a straight line unless it is compelled to change that state by forces impressed upon it." This was how Sir Isaac Newton originally stated his first law of motion (Galileo Galilei was also onto this).

Newton's Second Law

The net force exerted on an object equals the rate of change of momentum. Fnet = dp/dt = Δp/Δt When the mass is constant: F = ma

Newton's Third Law

If object A exerts a force on object B then object B exerts an equal but opposite force on object A.

Newton's Universal Law of Gravity (unit: N)

Every mass exerts an attractive force on every other mass (along a line joining their centers) and this force is proportional to the product of the masses and inversely proportional to the square of the distance between their centers. F = GmM/r^2

Gravitational Field Strength (unit: N kg

The force per unit mass on a small mass.

Work (unit: J = N m)

The component of force in the direction of motion times the displacement W = FΔxcosθ

Conservation of Energy

In an isolated system, the total energy is conserved.

Momentum (unit: kg m s

Mass times velocity.

Conservation of Momentum

In an isolated system, the total momentum is conserved.

Impulse

Force multiplied by time. Also equal to change in momentum. FΔt = Δp

Explain, with reference to the change in momentum, why a flexible safety net is less likely to harm the skier than a rigid barrier

• Same change in momentum/impulse

• A flexible safety net extends stopping time/time taken to stop would be greater with net

• force is proportional to rate of change of momentum therefore F is smaller «with safety net»