Stopping Distances (AQA GCSE Physics Combined Science): Revision Notes
📚 Revision Notes
5.6.5 Estimating Decelerating Forces
Deceleration
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Deceleration is a reduction in speed or negative acceleration. When a car slows down, a decelerating force is applied. This is often caused by friction or air resistance.
The magnitude of the decelerating force can be estimated using the following formula:
where:
- F is the decelerating force (in Newtons),
- m is the mass of the object (in kilogrammes),
- Δv is the change in velocity (in metres per second),
- Δt is the time over which the deceleration happens (in seconds).
Factors Influencing Deceleration
- Mass of the vehicle: Heavier vehicles require more force to decelerate over the same time period.
- Speed of the vehicle: The faster the vehicle is moving, the greater the decelerating force required to stop it in a given time.
- Road conditions: Slippery surfaces, such as ice, reduce the available friction, making it harder to decelerate.
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Example: Estimating Decelerating Force
Imagine a car of mass 1,200 kg travelling at 30 m/s comes to a stop in 5 seconds. The decelerating force can be calculated as:
Thus, a force of 7,200 N is required to bring the car to a stop in 5 seconds.
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Key Points
- The decelerating force depends on both the mass of the vehicle and how quickly it needs to stop.
- Braking distance and stopping time increase with speed due to the greater decelerating forces required.
- Friction between the tyres and the road surface plays a crucial role in generating the necessary decelerating force.