# Physics P3.

OCR GATEWAY.

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- Created by: Robyn Smith
- Created on: 29-05-12 16:54

## 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.

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## 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.

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## 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.

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## 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.

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## 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.

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## 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.

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## 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.

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## 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.

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## 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.

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## 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.

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## 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.

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## 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.

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## 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.

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