# Finding the final velocity of a car after 8 seconds with energy [closed]

I'm having trouble on finding how to solve this question that I received.

A 1500kg race car's engine can produce 150,000 Watts of power. Assuming the race car is able to convert 75% of the energy into kinetic energy, how fast could the car go in 8 seconds?

I understand that I have the mass, work (with efficiency), initial velocity, and time. I tried using the Energy formula (Ei +/- W = Ef). Using that I found the final velocity but how was I supposed to incorporate 8 seconds into it?

• Remember that power (Watts) has units of Energy / Time. This means that the amount of energy added to the race car depends on the amount of time the engine is running. Multiply the power by the time to get the amount of energy added to the car. Apr 15, 2021 at 4:14

Power is simply energy/time. Since 75% energy is converted to kinetic energy: $$E = 150000W \times 8s \times 75/100 = 900000J$$ The formula for Kinetic energy is $$1/2mv^2$$. So equating the two expressions obtained: $$1/2mv^2 = 900000J; v^2 = 1800000J/1500kg = 1200m^2/s^2$$ $$v^2 = 1200m^2/s^2; v ≈ 34.64m/s$$ Good day!