# Acceleration of car and power

1. Homework Statement

A car o fmass 1250 kg accelerates from 0 to 100 km/h in 4 s . The average power of the car is ?

2. Homework Equations

3. The Attempt at a Solution

Though it looks simple , but i don see my mistake.

The average power is the change in kinetic energy over time ,

(1/2 x 1250 x 100^2)/(4)

is my working correct ? But that doesn't give the answer which is 121 kW

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Re: mechaniscs

For the formula to yield an answer in watts, you need to convert km/h into m/s. Divide the answer you get by 1000 to get an answer in kW.

Re: mechaniscs

You know the change in velocity and the change in time so you can calculate the acceleration. Knowing the acceleration and time you can calculate the distance the car travels. Knowing the mass of the car and its acceleration you can calculate the force on the car. Knowing the force and distance you can calculate the work done. Power equals work/ change in time.

Cyosis
Homework Helper
Re: mechaniscs

RTW69 said:
You know the change in velocity and the change in time so you can calculate the acceleration. Knowing the acceleration and time you can calculate the distance the car travels. Knowing the mass of the car and its acceleration you can calculate the force on the car. Knowing the force and distance you can calculate the work done. Power equals work/ change in time.
This over complicates things quite a bit. The work is given by the difference of kinetic energy. thereddevils only mistake is the usage of the wrong units.