Unit C - Forces and movement

1c (1.11-1.21)

Gravity

Gravity attracts all masses, but only noticeable when one of the masses is very big

This has three effects:

• On surface of planet, makes things accelerate towards ground

• Gives everything weight

• Keeps planets, moons, satellites in orbit

Weight vs Mass

• Mass is amount of ‘stuff’ in object - same value anywhere in universe

• Weight is caused by pull of gravity

• Object has same mass on Earth and Moon - but different weight

  1kg mass weighs less on Moon (1.6N) than Earth (10N) because force of gravity pulling on it is less

• Weight is force measured in newtons
  Mass is not a force

Equation: Weight, Mass, Gravity

W = mg

Weight = Mass x Gravitational field strength

[N] = [kg] / [N/kg]

Force

A push or pull

Vector quantity with size + direction

Gravity/Weight

When close to a planet this acts straight downwards

Reaction force

Acts perpendicular to surface and away from it (if surface is horizontal, reaction force acts straight upwards)

Electrostatic force

Between two charged objects

Direction depends on type of charge (like charges repel, opposites attract)

Thrust

e.g. push or pull due to engine/rocket speeding something up

Drag/air resistance/friction

Slows the object down

Lift

e.g. due to aeroplane wing

Tension

in a rope or cable

Drawing the forces acting on a body

• Many forces act on everything, but usually not noticed because they balance out

• Any object with weight feels reaction force back from the surface it’s on
  Otherwise it would just keep falling

• When an object moves in fluid (air, water etc.), it feels drag in opposite direction to motion

Friction

If an object has no force propelling it, it always slows down and stops due to friction (force that opposes motion)

To travel at steady speed, objects need driving force to counteract friction

Static friction

Friction between solid surfaces which are gripping

Can be reduced by putting lubricant (oil/grease) between surfaces

Sliding friction

Can be reduced by putting lubricant (oil/grease) between surfaces

Friction between solids often causes wear of two surfaces in contact

Drag

Keeping shape of object streamlined (sports car, boat hull) reduces drag in fluids
Lorries + caravans have ‘deflectors’ to make them more streamlines + reduce drag

Roof boxes on cars spoil their streamlined shape so slow them down
For a given thrust, higher drag = lower top speed of car

Opposite extreme is parachute (need as high drag as possible)

In fluid, friction always increases as speed increases

Newton’s First Law of Motion

As long as forces on object are balanced, it will stay still, or if already moving, it carries on at same velocity

Newton’s Second Law of Motion

If there is unbalanced force, object accelerates in that direction

Equation: Force, Mass, Acceleration

F = ma

Force = Mass x Acceleration

[N] = [kg] x [m/s²]

Newton’s Third Law of Motion

If object A exerts force on object B, then object B exerts an equal and opposite force on object A

e.g. swimming, push back against water with arms + legs, and water pushes you forwards with equal-sized force in opposite direction

Vector quantities

Have size and direction

e.g. force, velocity, acceleration, momentum

Scalar quantities

Only size, no direction

e.g. mass, temperature, time, length

Resultant force

When multiple forces act on object, you can find resultant force acting on object by adding/subtracting - need to know size of all different forces acting on object and their direction

Terminal velocity

Frictional forces increase with speed - but only up to a certain point

• When object first starts to fall, it has much more force accelerating it than resistance slowing it down

• As velocity increases, resistance builds up

• Resistance force gradually reduces acceleration until resistance force is equal to accelerating force
  At this point, object can’t accelerate any more, it has reached terminal velocity

Factors affect terminal velocity

• Accelerating force acting on all falling objects is gravity
  All objects would accelerate at the same rate without air resistance

• Air resistance causes things to fall at diff speeds, and terminal velocity of object is determined by its drag compared to its weight
  Drag depends on shape and area

Stopping distance

Distance covered in the time between driver first spotting a hazard and the car coming to complete stop

Stopping Distance = Thinking Distance + Braking Distance

Factors affecting thinking distance

• Reaction time - affected by tiredness, drugs, alcohol and old age

Factors affecting braking distance

• Speed - faster speed = further distance before stopping

• Mass of vehicle - larger mass = longer time to stop

• Quality of brakes - worn/faulty brakes increase braking distance

• Grip - depends on road surface, weather conditions (e.g. icy), tyres