5.3.16

Decelerations

Test yourself

Work Done when Braking

When we push the brake pedal, brake pads are pressed onto the wheels. This contact causes friction. This causes work to be done. The work done between the brakes and the wheels converts (changes) energy from kinetic energy in the wheels to thermal energy in the brakes. The temperature of the brakes then increases.

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Higher speed

  • The greater the speed of a vehicle, the greater the braking force needed to stop the vehicle before a certain distance.
    • This means that more work needs to be done on the brakes to stop the car.
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Higher mass

  • The greater the mass of the vehicle, the greater the braking force needed to stop the vehicle. This means that more work needs to be done on the brakes to stop the car.
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Higher grip

  • For the same work done, the stopping distance will decrease if the force (grip) between the road and the vehicle increases.
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Estimating forces

  • When a car comes to a stop, the work done by the brakes must equal the initial kinetic energy of the car.
  • Work done = initial kinetic energy.
  • F d = 12 m v2.
  • We can use this equation to estimate the force applied by the brakes.

Dangers of Large Decelerations

The greater the braking force, the greater the deceleration of the vehicle. Large decelerations can cause brakes to overheat and/or the car to skid. A larger deceleration will transfer more stopping force to passengers. This harms passengers. Compare these situations:

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Traffic lights

  • When a typical family car decelerates at a set of traffic lights, only a small force is exerted on (applied to) the passengers.
  • This is because the deceleration happens over a long period of time.
  • The force should not be enough to harm the passengers.
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Emergency stop

  • When a typical family car suddenly stops on the road to avoid a collision, a greater force is exerted on (applied to) the passengers.
  • This is because the deceleration happens over a shorter period of time.
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Crash

  • When a typical family car suddenly is stopped by a crash, an even greater force is applied to the passengers than the last example.
  • This is because the deceleration happens in even less time.
  • This could harm passengers.

Jump to other topics

1Energy

2Electricity

3Particle Model of Matter

4Atoms & Radiation

5Forces

5.1Basics of Motion

5.2Forces

5.3Effects of Forces

5.4Pressure

6Waves

7Magnetism

8Astrophysics

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