# Physics P3.

OCR GATEWAY.

HideShow resource information

## Mass, weight and gravity.

• Gravity is the force of attraction between all masses.
• Gravitaional field strength is acceleration due to gravity or g.
• The value of g is about 10m/s2 on Earth.
• Mass is the amount of matter in an object and is the same wherever you go.
• Weight is caused by the pull of gravity.
• Weight is a force measured in newtons.
• Mass is not a force it is measured in kilograms.
• Weight = mass x gravitational field strength.
1 of 13

## Forces.

• Gravity or weight act straight downwards.
• Reaction force from a surface, usually acts straight upwards.
• Thrust or push or pull due to an engine or rocket speeding something up.
• Drag or air resistance or friction is slowing the thing down.
• Lift is a force acting upwards that keeps something up.
• Tension how tight something is.
• Stationary object = all forces in balance...
• The force of gravity is acting downwards this causes a reaction force pushing up on the object. This means it's in balance as these forces are equal.
• Steady horizontal speed = all forces in balance.
• Steady vertical speed = all forces balanced.
• Horizontal acceleration = unbalanced forces and vertical acceleration = unbalanced forces...
• You only get acceleration with an overall resultant (unbalanced) force.
• The bigger the unbalanced force the greater the acceleration.
• On a force diagram the arrows will be unequal.
2 of 13

## Friction forces and terminal speed.

• When an object is moving friction acts in the direction that opposes movement.
• The frictional force will match the size of the force trying to move.
• To travel at a steady speed you need a driving force to overcome the friction.
• Friction occurs in three main ways...
• Friction between solid surfaces which are gripping.
• Friction between soild surfaces which are sliding past each other.
• Resistance or drag from fluids - the larger the area of the object the larger the drag.
• FRICTION ALWAYS INCREASES AS THE SPEED INCREASES.
• When objects first set off they have more froce accelerating them than resistance slowing them down.
• As the speed increases so does the resistance which gradually reduces the acceleration untill the forces are equal.
• It will have reached its maximum or terminal speed.
• The terminal speed of any moving object depends on its drag compared to its driving force.
• The greater the drag, the lower the terminal speed.
3 of 13

## Forces and acceleration.

• If the forces on an object are all balanced, then it'll keep moving at the same speed in the same direction.
• If there is an unbalanced force, the object will acceleratein the direction of the force. The size of the acceleration is decided by F = ma which is newtons second law of physics.
• The overall unbalanced force is often called the resultant force.
• Any resultant force will produce acceleration and the formula is...
• Force = mass x acceleration.
• If the forces are parallel, the resultant force is found by just adding or subtracting them.
4 of 13

## Stopping distance.

• The stopping distance of a car is the distance covered between the driver first spotting the hazard and the car coming to a complete stop.
• Stopping distance = thinking distance = braking distance.
• Thinking distance - the distance the car travels in the time between the driver noticing the hazard and applying the brakes. It's affected by two factors...
• How fast your going - the faster you're going the further you'll go.
• Your reaction time - this is affected by the tiredness, drugs, alcohol, distractions and a lack of concentration.
• Braking distance - the distance taken to stop once the brakes have been applied. It's affected by four main factors...
• How fast your going - the faster your going the further it takes to stop.
• How heavily loaded the car is - with the same brakes, the heavier the vehicle the longer it takes to stop.
• How good the brakes are - braking depends on how much force you apply. Also if brakes are worn or faulty you won't be able to brake as quickly.
• How good the grip is which depends on three things - road surface, weather conditions and tyres.
5 of 13

## More on stopping distances.

• As a car speeds up the thinking distance increases at the same rate as speed .
• The graph is linear - a straight line.
• This is because the thinking time stays constant - but the higher the speed, the more distance you cover in the same time.
• Braking distance, however increases faster the more you speed up.
• The relationship between the speed and braking distance is a squared relationship.
• This means as speed doubles, braking distance increases 4-fold.
• If the speed trebles, braking distance is 9-fold - see card 10 for explanation.
6 of 13

## Momentum

• The greater the mass of an object the greater its veleocity, the more momentum the object has.
• Momentum (kg m/s) = mass (kg) x velocity (m/s).
• Momentum has size and direction.
• When a force acts on an object, it causes a change in momentum.
• Force acting (N) = change in momentum (kg m/s) / time taken for change to happen (s)
• Its the amount of time taken  for a change in momentum that determines how big or small an force is.
• If the change in momentum stays the same and t is small f will be big ans vice versa.
7 of 13

## Car safety

• Car safety feactures reduce the forces acting in accidents.
• In a collision, the force on an object can be lowered by slowing down the object over a long period of time.
• This is because the longer it takes for a change in momentum, the smaller the forces acting.
• These features increase the collision time to reduce the forces of deceleration to try and reduce injury...
• Crumple zones crumple and change shape on impact, (absorb kinetic energy).
• Seat belts stretch slightly.
• Air bags also slow you down more gradually.
• Roads can be made safer by having crash barriers and escape lanes.
• ABS brakes help drivers keep control of a cars steering when braking. They automatically pump on and off to prevent skidding.
• Safety features have to be tested using dummies and crash test cars.
8 of 13

## Work done and gravitational potential energy.

• When a force makes an object move, energy is transferred and work is done.
• When energy is supplied work is done and energy is transferred.
• Work done = force x distance.
• Gravitational potential energy (G.P.E.) is the energy that something has because of its height above the ground.
• The energy used to raise it is stored and can be changed into kinetic energy if it falls.
• G.P.E. = mass x g x height
• g is gravitational field strength.
9 of 13

## Kinetic energy.

• The kinetic energy (K.E.) of something is the energy it haswhen moving.
• It depends on both its mass and speed.
• Kinetic energy = 1/2 x mass x (speed x speed).
• To sptop a car the kinetic energy has to be converted to heat energy at the brakes and tyres.
• Kinetic energy transferred = work done by brakes.
• 1/2 x m x (v x v) = F x d
• v = speed, F = maximum braking force, d = braking distance.
• The braking distance increases as speed squared increases.
• This means if you double the speed you double the value of v but the v x v means the K.E. is then increased by a factor of four.
• F is the maximum possible braking force d must increase by a factor of four to make the equation balance.
• In other words if you go twice as fast the braking distance must increase by a factor of four.
• Doubling the mass of an object doubles the K.E. it has which will double the braking distance.
10 of 13

## Falling objects and roller costers.

• When something falls its G.P.E is converted into K.E.
• Kinetic energy gained = gravitational potential energy lost.
• 1/2m(v x v) = mgh.
• h = height, g = 10
• When a falling object reaches terminal speed its speed can't increase anymore so neither can its K.E. Instead the G.P.E. is transferred to internal energy of the object.
• The formula can be rearanged to give h = (v x v) / 2g.
• Roller coasters transfer energy...
• At the top of a roller coaster the carriage has lots of G.P.E.
• As the carriage descends G.P.E. is transferred to K.E. and the carriage speeds up.
• As the carriage continues to descend it keeps accelerating.
11 of 13

## Power.

• Power is the rate of doing work.
• A powerful machine is one which transfers a lot of energy in a short space of time.
• Power = work done / time taken.
• Power is measured in watts.
• Power means how much energy per second so watts are the same as joules per second.
• Sometimes you want to find the power of something based on force and speed.
• Power = force x speed.
12 of 13

## Fuel consumption and emissions.

• The size and design of car engines determine how powerful they are and the more powerful and engine the more energy it transfers.
• The fuel consumption of a car is usually stated as the distance travelled using a certain amount of fuel.
• A car with a low value for l/100km has a low fuel consumption but a car with a low value for mpg has a high fuel consumption.
• Heavy cars have a high fuel consumption.
• Driving style can affect fuel consumption for example frequently braking and accelerating.
• The higher the fuel consumption the greater the emissions.
13 of 13