My physics is rusty; need help.?

Question

Can anyone tell me how much energy it would take, measured in kilowatts, to bring an 80,000 truck to a complete stop from 55 miles per hour? Tire and wind resistance is nominal...

Thanks

Answer 1

I'll assume you mean 80000 lbs. (36287.39 kg)

55 mph = 24.59 m/s

Kinetic Energy (KE ) = ½mv²

==> ½(36287.39)(24.59)² = 10970913.58 J = 3.047 kWh (kilowatt hour)

A "J" is a Joule (energy).

You can't get JUST kilowatts. Energy (Joules) and power (Watts, kW, etc) are related by time. Power is the amount of energy consumed per unit time, so energy is the power multiplied by that unit of time.

Hope this helps.

Answer 2

80000 what?

In how much time you have to bring the truck to complete rest?

Answer 3

spiritro...

I think you're confused about Energy and Power (even with your 3rd entry)...

Energy is never measured in kilowatts!!!.  It is measured in Joules.

If you're asking us to find the answer in Joules, it's possible.  But in kilowatts... not possible.

Answer 4

It depends on how quickly you need to stop it.  (Or stopping distance).

EDIT:   The energy is the same, but power relates to time.

Power = Energy/Time.

Answer 5

No, Physics doesn't get rusty, Chemistry does.

So how much energy does it take to stop a truck with a certain amount of kinetic energy? Such energy can be calculated from its mass and its speed.  1/2 x mass x speed x speed

In this case you need to be careful with units. Pounds, miles, hours, kilowatts - watta mess.