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Deceleration is the rate at which a vehicle slows down when braking, measured in metres per second squared (m/s²); it is negative acceleration. Free notes.
Braking Distance is part of P6.1 Physics on the move in GCSE OCR Higher Combined Science Physics.
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GCSE OCR Higher
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Deceleration is the rate at which a vehicle slows down when braking, measured in metres per second squared (m/s²); it is negative acceleration. Free notes.
Use this lesson to stay inside P6.1 Physics on the move while connecting the detail back to the wider Topic P6: Global challenges topic.
Read the core explanations from Braking Distance before testing yourself from memory.
Picture this: you're driving at 30 mph when a child runs into the road. You slam on the brakes immediately. How far will your car travel before it stops? The answer could mean the difference between a close call and a tragedy.
Braking distance isn't just theory - it's the physics that keeps us alive on the roads. Every time you press the brake pedal, you're converting the kinetic energy of your moving car into heat energy through friction. The faster you're going, the more energy needs to be converted, and the longer it takes to stop.
Understanding braking distance helps explain why speed limits exist, why following distances matter, and why driving conditions can be so dangerous. The physics is straightforward, but the consequences are very real.
Don't confuse braking distance with stopping distance. Braking distance only counts from when the brakes are pressed. Stopping distance includes thinking distance too - the distance you travel while your brain processes the danger and moves your foot to the brake pedal.
Larger decelerations mean the car slows down more quickly, reducing braking distance. However, there's a limit - brake too hard and you'll skid, lose control, or cause the brakes to overheat and fail.
This is the energy that must be removed to stop a vehicle. Notice that kinetic energy depends on velocity squared - double the speed means four times the energy to remove, which explains why braking distances increase so dramatically with speed.
When you press the brake pedal, brake pads clamp onto the brake discs, creating friction. This friction force opposes the car's motion, causing deceleration. The work done by this friction force converts the car's kinetic energy into heat energy, which is why brakes get hot during heavy braking.
The relationship is elegant: Work Done by Friction = Kinetic Energy Lost. Since work done equals force × distance, we get: Braking Force × Braking Distance = ½mv². This explains why faster cars need much longer distances to stop - the energy to remove increases with the square of speed.
The greater the braking force, the greater the deceleration and the shorter the braking distance. However, there's a catch - apply too much force and the wheels lock up, causing skidding. On wet or icy roads, the maximum friction force is much lower, so even gentle braking can cause loss of control.
Large decelerations create serious dangers. The car might skid and lose steering control. The brakes can overheat and fade, becoming less effective. Passengers experience large forces that can cause injury, especially if they're not wearing seatbelts properly.
Speed is the biggest factor affecting braking distance. Because kinetic energy increases with the square of speed, braking distance increases dramatically as speed rises. A car travelling at 60 mph has four times the kinetic energy of the same car at 30 mph, so needs roughly four times the distance to stop.
Road conditions have a huge impact on braking distance. Wet roads reduce the friction between tyres and road surface, increasing braking distances by up to 50%. Icy conditions are even worse - friction can be reduced to just 10% of dry road values, making braking distances ten times longer.
Vehicle condition is crucial for safe braking. Worn brake pads reduce the maximum braking force available. Worn tyres have less grip, especially in wet conditions, leading to longer braking distances and increased risk of skidding. Poor tyre tread depth is particularly dangerous on wet roads where water can't be channelled away effectively.
The mass of the vehicle also affects braking distance, though not as much as you might expect. Heavier vehicles have more kinetic energy to remove, but they also have more weight pressing the tyres onto the road, creating more friction. These effects partly cancel out, though heavier vehicles generally need slightly longer to stop.
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Move to the related lessons or back to the topic page when you need the wider sequence before exam-style practice.
Deceleration is the rate at which a vehicle slows down when braking, measured in metres per second squared (m/s²); it is negative acceleration. Larger decelerations reduce braking distance, but braking too hard can cause skidding.
Speed has a big effect because kinetic energy = ½mv², so it depends on velocity squared. A car at 60 mph has four times the kinetic energy of the same car at 30 mph, so it needs roughly four times the distance to stop.
Cars skid when braking too hard because the wheels lock up when the braking force is too great for the friction between the tyres and the road. Skidding means the driver can lose steering control.
When a car brakes, brake pads clamp onto the brake discs, creating friction. The work done by friction converts the car's kinetic energy into heat energy, which is why brakes get hot: braking force × braking distance = ½mv².
Wet roads reduce the friction between the tyres and the road surface, increasing braking distances by up to 50%. Even gentle braking on wet roads can cause loss of control.
On icy roads, friction can be reduced to just 10% of dry road values, making braking distances ten times longer. The maximum friction force is much lower, so even gentle braking can cause loss of control.
Worn tyres have less grip, especially in wet conditions, leading to longer braking distances and a higher risk of skidding. Poor tread depth is particularly dangerous on wet roads because water cannot be channelled away.
Brakes get hot because the friction between the brake pads and brake discs does work that converts the car's kinetic energy into heat energy. Heavy braking can make the brakes overheat and fade, becoming less effective.
Braking force and braking distance are related by braking force × braking distance = ½mv², because the work done by friction equals the kinetic energy lost. A greater braking force gives a shorter braking distance.
A car at 60 mph needs roughly four times the braking distance of the same car at 30 mph. This is because kinetic energy (½mv²) depends on speed squared, so doubling speed gives four times the energy to remove.