Physics - As Fast As You Can


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  • Created by: Elliza
  • Created on: 31-05-11 14:49

Forces and Acceleration

The overall force on an object is the resultant force.

The resultant force determines whether the object accelerates, decelerates or stays at a steady speed.

Example: A car of mass of 1750kg has an engine which provides a driving force of 5200N. At 70mph the drag force acting on the car is 5150N. Find it's acceleration

  • a) when first setting off from rest
  • b) at 70mph.

Steps: 1) Draw a force diagram for both cases. (no need to show vertical                            forces) 

   2) Work out the resultant force in each case, and apply "F = m x a" using the force diagram. (Remember backwards FAM)


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Reaction Forces

1) Whatever force you give an object, it will give a force back. For example, if you push a shopping trolley, it will push back against you, just as hard. 

2) When you stop pushing, so does the trolley.

3) Remember that things move because the two forces (action + reaction) are acting on different objects.

For example, two skaters are pushing their hands together. As Skater A pushes on Skater B, she feels an equal and opposite force from Skater B's hand.

As they both feel the same sized force, they accelerate away from each other.

4) Another example - when you go swimming, you push back against the water with your arms and legs and the water pushes you with an equal sized force in the opposite direction.

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Stopping Distances

Stopping distance is effected by two things:

1) Thinking Distance (The distance the car travels in the time between the driver noticing the hazard and applying the brakes).

- affected by two main things:

  • How fast you're going
  • How dopey you are - alcohol, tiredness etc.

2) Braking Distance (The distance the car travels during it's deceleration whilst the brakes are being applied

- affected by four main things:

  • How fast you're going
  • How heavily loaded the vehicle is
  • How good your brakes are 
  • How good the grip is - both on the road and the tyres

Weather conditions also affect both of these. :) 

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Momentum and Car Safety

Momentum = Mass x Velocity

1) The greater the mass and velocity of an object, the greater it's momentum

2) Momentum, like velocity, is a vector quantity, which means it has both a size and direction

Force causes change in Momentum

A larger force means a faster change of momentum, therefore a greater acceleration

Cars are designed to slow people down over a longer time when they have a crash. Eg - crumple zones, airbags and seatbelts.

There has been a massive decline in the number of road and car accidents. This is most likely because of new car safety features

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