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P5 T) Braking Force
P5 T) Braking Force
Pressing the brake in a car causes the brake pads to be pressed onto the moving wheel. The contact between the brake pads and the wheel causes friction. The friction causes energy to be transferred from the kinetic energy stores of the car to the thermal energy stores of the brakes and the surroundings; the temperature of the brakes and the surroundings increases. As energy is transferred, work is done.
In order for a slower and faster moving identical vehicle to stop after the same distance, the faster moving vehicle will need a greater braking force than the slower moving vehicle. This is because when the same vehicle is travelling at a faster speed, it will have more energy in its kinetic energy stores. This means that more energy needs to be transferred to stop the vehicle, thus meaning that a greater braking force is needed to stop the vehicle in the same distance.
A greater braking force for a vehicle means a greater deceleration rate for the vehicle. Large decelerations can lead to the brakes overheating as large quantities of energy are transferred from the kinetic energy stores of the car to the thermal energy stores of the brakes (and the surroundings). Overheating brakes do not work as effectively, which can result in the driver losing control of the vehicle.
In order for a slower and faster moving identical vehicle to stop after the same distance, the faster moving vehicle will need a greater braking force than the slower moving vehicle. This is because when the same vehicle is travelling at a faster speed, it will have more energy in its kinetic energy stores. This means that more energy needs to be transferred to stop the vehicle, thus meaning that a greater braking force is needed to stop the vehicle in the same distance.
A greater braking force for a vehicle means a greater deceleration rate for the vehicle. Large decelerations can lead to the brakes overheating as large quantities of energy are transferred from the kinetic energy stores of the car to the thermal energy stores of the brakes (and the surroundings). Overheating brakes do not work as effectively, which can result in the driver losing control of the vehicle.
Estimating the Braking Force
We are now going to estimate the braking force required when a car does an emergency stop; an emergency stop is when a driver of a vehicle applies the full braking force to stop as quickly as possible. This is an estimation question, so we can make up the figures that we are not given.
Question
A typical car is travelling at a typical speed along a road. The car does an emergency stop and the braking distance is 20 metres. Estimate the braking force.
We answer this question by completing two different calculations. The first calculation is to work out the acceleration (or deceleration) of the car. We can calculate the acceleration (or deceleration) by using the formula below with a as the subject.
We are now going to estimate the braking force required when a car does an emergency stop; an emergency stop is when a driver of a vehicle applies the full braking force to stop as quickly as possible. This is an estimation question, so we can make up the figures that we are not given.
Question
A typical car is travelling at a typical speed along a road. The car does an emergency stop and the braking distance is 20 metres. Estimate the braking force.
We answer this question by completing two different calculations. The first calculation is to work out the acceleration (or deceleration) of the car. We can calculate the acceleration (or deceleration) by using the formula below with a as the subject.
The question tells us that the car is travelling at a typical speed and then does an emergency stop. The typical speed for a car is around 25 m/s, which means that the initial velocity is 25 m/s (u is 25). The car does an emergency stop, which means that the car is stationary at the end; the final velocity of the car is 0 m/s (v is 0 m/s). s in the equation above is the distance and we are told in the question that the distance is 20 metres. We sub all of these values into the formula; u as 25, v as 0 and s as 20.
This tells us that the acceleration rate is -15.625 m/s2. A negative acceleration rate means that the car is decelerating, so the deceleration rate is 15.625 m/s2.
We now use Newton’s second law to find the (braking) force of the car. Newton’s second law is shown below.
We are not given the mass of the car, so we estimate it; a typical medium sized car is about 1,500 kg. In the previous calculation, we found that the acceleration rate is -15.625 m/s2 (we sub this into the formula as a positive value).
Therefore, an estimate for the braking force for our car is 23,437.5 N. This is an extremely large force.
There is often a wide range of acceptable answers when you are completing estimation questions.