Forces in motion

Air resistance definition

A drag force that opposes the motion of objects moving through air

Factors affecting drag force on an object

The shape of the object (area)

The speed

The viscosity of the fluid (how easily fluid flows past a surface)

Drag definition

A force that acts on objects moving through a fluid so as to oppose the motion

Define lift

Force that acts upwards on surfaces, such as an aircraft wing, that are moving through a fluid

Friction acts to…

Prevent solid surfaces sliding across each other

Define terminal speed

Top speed of a falling object, reached when the opposing forces of weight and drag are equal

Conditions for an object falling at terminal velocity

Resultant force on object is zero

Acceleration is 0

Object travels at constant velocity

Why object reaches terminal velocity falling through air

Initially only force acting is weight so object accelerates at g

Drag force increases with increasing speed

Therefore resultant force decreases, magnitude of acceleration decreases

Eventually forces balance

So resultant force is 0

As F=ma, acceleration is 0 so object falls at a constant speed

Use of parachute

Parachute opens, increases surface area and immediately increases air resistance so it is greater than weight

Resultant force acts in opposite direction to motion causing person to decelerate

Drag decreases with decreasing speed

Resultant force decreases, magnitude of deceleration decreases

Eventually forces balance

Resultant force = 0 by F=ma. Acceleration is 0 so object falls at constant speed but smaller than previous terminal velocity

Newtons first law

A body will remain at rest or constant velocity unless acted on by an external force

Newtons second law

The acceleration of a body is proportional to the resultant force acting on it and inversely proportional to the mass of the body in the direction of the resultant force. F=ma

Newtons third law

If body A exerts a force on body B, then body B will exert an equal and opposite force on body A

Newtons 2nd law but cooler

Rate of change of momentum of an object is directly proportional to the resultant force which acts on an object

2nd law formula

F = ∆mv / ∆t

Momentum description formula and units

Vector quantity, the more momentum an object has, the harder it is to stop

Mass X velocity

Kgms^-1

The principle of conservation of linear momentum

In a collision or explosion, total momentum before equals total momentum after, provided that no external forces are acting

Elastic Vs inelastic collisions

Kinetic energy is conserved in elastic collisions. It is transferred to other forms in inelastic

Impulse equation and units

Force X time force acts for F∆t which is also ∆mv

Ns or Kgms^-1

How to find impulse graphically

Area under graph on force time graph

Rate of change of momentum formula and units

F = ∆mv / ∆t

Kgms^-2

Ethical transport design

Features protecting people in a crash

Crumple zones crumple on impact. Longer stop/impact time and decreased force on passengers

Seat belts stretch slightly, increasing time taken for wearer to stop. Reduces force on chest

Air bags. Slows passengers gradually and prevent from hitting hard surfaces inside car

Work done equation and units

Force X distance moved in direction of force. J

When is work done relevant

When energy is transferred

Energy transferred (work done) forrumla

W = Force X displacement X cosx (angle between direction of force and displacement)

Power definition formula and units

Work done per second, rate at which energy is transferred. P = Fv or ∆W/∆T. Js^-1 or W

Work done on force displacement graph

Area under graph

Efficiency equation

Useful output power / total input power X 100

Principle of conservation of energy

Energy is neither created or destroyed, only converted from one form to another

Gravitational potential energy definition and formula

Energy stored by a mass due to its position in a gravitational field. mg∆h

Kinetic energy definition and formula

Energy of a mass moving with velocity. ½mv²

Energy conservation of an object falling in presence of resistive forces

Loss in gpe = gain in ke + work done against resistance (typically appears as heat)

Contact force (reaction F)

Force exerted between two objects when they are in contact with each other

Tension force

Force applied to an object that acts to stretch it

Define a newton

the force required to accelerate a mass of 1kg by 1ms^-2