Advertisements
Advertisements
Question
Calculate the work done by the brakes of a car of mass 1000 kg when its speed is reduced from 20 m/s to 10 m/s?
Advertisements
Solution
Mass of the car, (m) = 1000 kg
Initial velocity, (v1) = 20 m/s
Final velocity, (v2) = 10 m/s
So, initial kinetic energy can be calculated as,
`K.E = 1/2 mv^2`
Therefore ,
`(K.E)_1 = 1/2 (1000)(20)^2` J
= 200000 J
Similarly, final kinetic energy ,
`(K.E)_2 = 1/2 (1000)(10)^2` J
= 50000 J
So,
Work done by the brakes = Change in kinetic energy
Therefore work done by the brakes,
Work done by the brakes
= (K.E)2 – (K.E)1
= (50000 – 200000) J
= –150000 J
= -150 KJ
Negative sign shows the force applied by brakes is opposite to the direction of motion of the body.
APPEARS IN
RELATED QUESTIONS
What is power?
What is the work done by the force of gravity on a satellite moving round the earth? Justify your answer.
A person holds a bundle of hay over his head for 30 minutes and gets tired. Has he done some work or not? Justify your answer.
An electric heater is rated 1500 W. How much energy does it use in 10 hours?
A load of 100 kg is pulled up by 5 m. Calculate the work done. (g = 9.8 m/s2)
Name the physical quantity whose unit is watt.
The power output of an engine is 3kW. How much work does the engine do in 20 s?
A boy weighing 40 kg carries a box weighing 20 kg to the top of a building 15 m high in 25 seconds. Calculate the power. (g = 10 m/s2)
In loading a truck, a man lifts boxes of 100 N each through a height of 1.5 m.
If the man lifts 4 boxes per minute, at what power is he working?
(g = 10 m s−2)
An electric motor raises a load of 0.2 kg, at a constant speed, through a vertical distance of 3.0 m in 2 s. If the acceleration of free fall is 10 m/s2, the power in W developed by the motor in raising the load is :
