# Energy-Work Problem

1. Nov 23, 2013

### neededthings

1. The problem statement, all variables and given/known data

A cyclist is competing in a race and decides to pass some fellow cyclists who are getting tired. The initial speed of the cyclist is 21 km/h and he has 3.0 * 10^2 W of power. If the bicycle is able to convert 92% of input energy into kinetic energy, how fast will cyclist be travelling after 4.0 s. The combined mass of the cyclist and the bicycle is 78 kg.

The answer is 7.9 m/s. But I dont know how.

2. Relevant equations

p=e/t
Ek=1/2mv^2

3. The attempt at a solution
energy= Pt= 3x10^2 * 4 = 1.2 x10^3 J
Useful energy = 1.2 x10^3 *0.92 = 1.104 x10^3
KE= 1/2m( v)^2
root(1.104 x10^3 *2 /78) = change in v = +/-5.32 m/s (3 S.F.). He is accelerating hence +5.32m/s
21km/h = 21000m/60x60s= 5.83 m/s

2. Nov 23, 2013

### Kishlay

why are you putting the same post every time???

3. Nov 23, 2013

### Kishlay

go back to your previous post......!!!!
i have posted solution over there

4. Nov 23, 2013

### Kishlay

initial kinetic energy + useful energy given= final kinetic energy