Energy and force

[匿名]

2009-07-01 7:31 pm

(*Things are given: the kinetic energy of the car at a speed 15m/s is 120 kJ. )

1. How does doubling the speed of the car affect its kinetic energy?

2. To stop the car rapidly, work must be done by the brakes. If the brakes can produce a maximum braking force of 5000N. Calculate the shortest distance needed to stop the car when it is travelling at 15m/s.

3. How would the braking distance be affected if the speed of the car were doubled but the braking force remained the same? Give a reason for your anwer.

回答 (1)

Prof. Physics

2009-07-01 8:31 pm

✔ 最佳答案

1. Kinetic energy of an object is given by 1/2 mv2.

So, doubling the speed of the car will lead to an increase of its kinetic energy four times as before. The final kinetic energy = 4 X 120 kJ = 480 kJ

2. By K.E. = 1/2 mv2

120 000 = 1/2 m(15)2

Mass of the car, m = 1070 kg

By Newton's 2nd law of motion,

F = ma

5000 = 1070a

Deceleration, a = 4.69 ms-2

By equation of motion v2 = u2 - 2as

0 = 152 - 2(4.69)s

Stopping distance, s = 24 m

3. For the braking remained the same, the acceleration of the car unchanged.

By v2 = u2 - 2as

s = u2 / 2a

So, if the speed of the car is doubled, then the stopping distance will become four times as before.

Alternating answer:

Work done by the braking force = Fs = 5000s

In the view of conservation of energy

Work done by the braking force = K.E. lost by the car

5000s = 1/2 mv2

s = mv2 / 10 000

So, if the speed of the car is doubled, then the stopping distance would become four times as before.

參考: Physics king

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