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Estimating Decelerating Forces (OCR GCSE Physics A (Gateway))

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Estimating Decelerating Forces

Higher Tier Only

  • The work done by the brakes when a vehicle slows to a halt is given by the following equation:

Estimating Decelerating Forces, downloadable IGCSE & GCSE Physics revision notes

This equation shows the work done (energy transferred) by the brakes on the wheels of a car

  • This equation shows that:

    • The work done is the transfer of kinetic energy

    • The braking distance is proportional to the speed squared (if the speed is doubled, the distance increases 4 times)

  • We can use this equation to estimate the decelerating forces required for a typical vehicle moving at everyday speeds

Worked Example

At 18 m s-1 (40 mph) the braking distance of a typical car of mass 1500 kg is about 24 m.

Use this information to estimate the braking force for a typical car. 

Answer:

Step 1: List the known quantities

  • Mass, m=1500 kg

  • Braking distance, s=24 m

  • Speed, v=18 m/s

Step 2: State the relevant equation

Braking space force space cross times space braking space distance equals 1 half space mv squared

Step 3: Rearrange for the braking force

Braking space force equals fraction numerator 1 half space mv squared over denominator braking space distance end fraction

Step 4: Substitute the values into the equation

Braking space force equals fraction numerator 1 half cross times 1500 cross times open parentheses 18 close parentheses squared over denominator 24 end fraction equals 10 space 125 space straight N

Examiner Tips and Tricks

You should be able to deduce from the equation that the braking distance is proportional to the vehicle's speed2. Note, this actually doesn't apply at very high speeds because the brakes get hot and become less effective. This reduces the braking force, causing the braking distance to increase even further.

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    Ashika

    Author: Ashika

    Expertise: Physics Project Lead

    Ashika graduated with a first-class Physics degree from Manchester University and, having worked as a software engineer, focused on Physics education, creating engaging content to help students across all levels. Now an experienced GCSE and A Level Physics and Maths tutor, Ashika helps to grow and improve our Physics resources.