Braking and momentum

Stopping distance

The total distance travelled during the time it takes for a car to stop in response to some emergency

Stopping distance = Thinking distance + Braking distance

the greater the speed the greater the stopping distance

Thinking distance

 the distance travelled in the time it takes the driver to react (reaction time) in metres (m)

Braking disance

the distance travelled under the braking force in metres (m)

Typical range value of a persons reaction time

0.2 to 0.9

How to measure human reaction time?

Person A holds a 30 cm ruler vertically, such that the bottom end of the ruler hovers over the top of the hand of Person B*

Person A should **release the ruler** **unexpectedly** * As soon as Person B sees the ruler move, they should close their hand, catching it * The ruler is marked at the point at which it was caught by Person B - i.e. in line with the top of their hand * This gives a measurement of the **distance** the ruler fell * The **greater** the distance, the **longer** the reaction time

What effects reaction time?

* tiredness
* drugs
* alcohol
* distractions may also

Factors effecting braking distance

Speed is the main one along with

Vehicle condition- e.g. worn tyres or poor brakes *

Road condition- wet or icy roads make it harder to decelerate

Vehicle mass- a heavy vehicle, such as a lorry, takes longer to stop

What happens when you break a car? (in terms of energy transfer and force)

* When a driver applies the brakes, there is a **frictional force** between the **brakes** and the **wheels** of the car

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* This frictional force does **work** on the brakes 
* i.e. it **transfers** **energy** from the car to the brakes

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* The **kinetic energy** of the car **decreases** and the **thermal energy** of the brakes **increases** -
* i.e. the brakes heat up

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* The car **decelerates** (slows down

The link between newtons second law and braking with speed

* The greater** the speed of a vehicle, the **greater** the braking force required to bring the vehicle to a **halt** for a given distance * Since the **braking force** would need to be **larger**, the **deceleration** of the vehicle will be **large** as well * This is due to the link between resultant force and acceleration as stated in **Newton's second law of motion** \

DAngers caused by large decelerations?

ead to the brakes **overheating** and / or **loss of control** of the vehicle

Why does the temperature of a break increase when the vehicle stops?

* %%Work is done by the **frictional** **force** between the **brakes** and the **wheel**.%%
* %%The brake temperature **increases** because there is a **transfer of energy** from the car's **kinetic energy** to the **thermal energy** of the brakes.%%

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* t's a common mistake to write about the friction between the wheels and the road. This does happen, but in this case, the wheels heat up the road!

Equation for calculating braking force

Braking force x braking distance = 0.5 x mass x velocity ^2

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**braking distance** is **proportional** to the vehicle's **speed^2**.

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( 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.)

What is the quation for momentum?

momentum = mass × velocity

p  = m v

* momentum, p, in kilograms meter per second, kg m/s mass, m,
* in kilograms,kg


* velocity, v, in metres per second, m/s

What is the conversion of momentum

In a closed system, the total momentum before an event is equal to the total momentum after the event.

**The total momentum before a collision = The total momentum after a collision**

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