Forces in Motion

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Braking Distance 2

We will examine deeper into what goes on during braking distance, in terms of energy.

When a driver pushes the brake pedal, brake pads are pressed against the wheels. This increases which type of force between the wheels and the brake pads?

The contact between the brake pads and the wheels increases friction, causing energy to be transferred from the wheels to the brakes. Remember that work done is another way of describing a transfer of energy.

Energy can not be created nor destroyed. It can only be stored or transferred from one type of energy store to another. Which type of energy store do the wheels have before braking?

Have you every applied your bike brakes really hard and then felt the brake pads afterwards? What type of energy store does the kinetic energy in the wheels get transferred to in the brakes when work is done to slow down for example a car?

When the brakes are applied, kinetic energy is transferred from the wheels to the thermal energy store of the brakes.

Which car will need the greatest braking force to stop it?

Remember that work done is another way of describing a transfer of energy, and the formula is $Work\space done = force\times distance$ . To bring a moving car to rest, the work done has to transfer all of the kinetic energy of the wheels, so that must mean that the kinetic energy of the wheels is the same as the work done to bring the car to a stop. The kinetic energy must therefore be equal to the braking force multiplied by the distance it takes to bring the car to a stop.

If $kinetic \space energy = braking \space force \times braking \space distance$ , what is the correct formula for finding the braking distance of vehicles, travelling at different speeds?

A car which is travelling at $13\space m/s$ (30 miles per hour) has $59,200\space J$ of energy in its kinetic energy store. If a braking force of $2000\space N$ is applied, what is the braking distance?

If the same car is travelling at $27\space m/s$ (60 mile per hour), it has more kinetic energy. If the vehicle now has $255,200\space J$ of kinetic energy and the braking force remains at $2000\space N$ , what is the braking distance?

The faster a vehicle is travelling, the greater the braking force is needed to stop it within a certain distance.

When driving at high speeds, a driver must apply a larger braking force to stop a vehicle within a safe distance in an emergency. This causes a larger deceleration. Why might a large deceleration be dangerous?

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