Physics describe questions

These revision cards should help you answer describe questions. Just remember the bullet points!!!

  • Created by: luke
  • Created on: 03-12-11 20:02

An experiment to determine the acceleration of fre

Describe an experiment to determine the acceleration of free fall g of a falling body


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  • Method 1 - Electronic timer
  • Steel ball held by an electromagnet
  • Current to magnet is switched off, ball begins to fall and electric timer starts
  • Ball falls through a trapdoor and a circuit to stop the timer
  • Now we know time taken for the ball to fall from rest through the distance h between the bottom of the ball and the trapdoor


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  • Pivot object near one edge
  • draw vertical line on object through pivot
  • Pivot object from second pivot point
  • Draw verical line from pivot
  • Where two lines cross is the centre of gravity
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Describe the factors that affect thinking distance and breaking distances

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  • Thinking distance depends on speed and thinking time
  • The greater the vehicles speed, the greater the thinking distance
  • Thinking time increases with age (reactions are slower), and also if the driver is under the influence of drugs (alcohol, recreational, medical)
  • Breaking distance depends on the speed at which the vehicle is travelling, and the acceleration with which it slows down
  • Because the distance travelled while braking depends on the square of the speed (u^2), double the speed gives four times the braking distance.
  • If the cars tyres are bald, and if the road surface is smooth or wet, friction (and hence deacceleration) will be reduced and the braking distance will be greater.
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Describe and explain how air bags, seat belts and crumple zones in cars reduce impact forces in accidents

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Crumple zones are less rigid structures at the front and back of the car. They are crushed in impact, so that the force is not transferred directly to the ocupents (who are protected in a rigid steel cage.

Seat belts and air bags bring the driver gradually to rest, avoiding a sudden sharp impact with the windscreen or dashboard. The release or an air bag is triggered by an accelerometer, a tiny elctronic device that responds to the vehicle's sudden, large deacceleration in impact. An explosive charge is set off, exploding the bag outwards towards the driver.

Controlling impact forces

These safety features ensure that the occupants of the car are brought slowly to a halt. This increases the time taken to stop, leading to a smaller deacceleration. According to F=ma, this means that the impact force is reduced. A sudden stop which happens in shorter time results in bigger and more damaging impact forces

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Describe how the tilateration technique is used in GPS (GLOBAL POSITIONING SYSTEM IN cars

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  • GPS satellites act as "beacons" in space, sending out regular signals containing information about their indentity and time of transmission.
  • The receiving system can calculate its distance from each of the satellites.
  • Now the receiver knows its distance from each of three or more satellites, and so it can work out its own position.
    ( If the receiver knows what distance it is from three satellites, it must lie somewhere on a sphere of radius of each of the three distances.
  • In practice, three satllite signals will allow a receiver to determine its position to within a few metres. This can be made more precise using four satellites.
  • The GPS system uses a transmission of 10.23 MHz. However, the GPS satellite s orbit high above the Earth where the Earth's gravitational field is weaker than on the surface.In weaker gravity a clock will run faster. To compensate for this, the transmission frequency must be slightly below.
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Work done by a force- The product of the force and the distance moved by the force in the direction of travel.

The two factors involved are:

  • The size of the force F- the bigger the force, the greater the amount of work you do
  • The distance in the direction of travel.

Joule-  The amount of work done when a force of one newton moves a distance of 1 metre in the direction of the force. 


Work done = energy transferred

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Energy can't be created or des

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