# AP Physics

posted by .

A 1050kg car has four 15kg wheels. When the car is moving, what fraction of the total kinetic energy of the car is due to the rotation of the wheels about their axles? Assume that the wheels have the same rotational inertia as uniform disks of the same mass and size. Also, why do you not need the radius of the wheels?

• AP Physics -

I will be happy to critique your thinking on this.

You do not need the radius, because you are dealing with fractions of energy. When you work out the KE, radius will be there, and the same radius term will be in the rotational energy part, and the radius terms divide out.

• AP Physics -

The equation I came up with to use was KE(rotational)/KE(total)=4(.5mr^2w^2)/(4(.5mr^2w^2)+(.5mr^2w^2)) and got an answer of 5.4%. My physics teacher gives us answers to problems in our book that are almost the same as our homework problems, but when I used this equation with the corresponding question, I did not get the right answer. Where am I going wrong?

• AP Physics -

I agree with your formula, except how you used it. The masses of the wheels are not the same as the mass of the car, and the portion of KE translational has to use the mass of the cars.

• AP Physics -

The formula for the moment of inertia is actually incorrect for you.

I = (1/2)mr^2

so

K = (1/2)((1/2)mr^2)(w^2)

## Similar Questions

1. ### physics

A 1000 kg car has four 20 kg wheels. When the car is moving, what fraction of its total kinetic energy is due to rotation of the wheels about their axles?
2. ### physics(help!!!!!!!)

A 1200kg car going 25m/s is brought to a stop using its brakes. Assuming that a total of approximately 20kg of iron in the brakes and wheels absorbs the heat produced by friction. What was the car’s original kinetic energy and after …
3. ### physics

a car is traveling with a speed of 15 m/s on a stratight horzontal highway. the wheels of the car have a radius of 50cm.if the car then speeds up with an acceleration of 2 m/s for 5s, find the number of revolution of the wheels during …
4. ### Physics

A bicycle has wheels of radius 0.32 m. Each wheel has a roatational inertia of 0.080 kg•m^2 about its axle. The total mass of the bicycle including the wheels and the rider is 79 kg. When coasting at constant speed, what fraction …
5. ### physics

An automobile traveling 80 mi/hr has wheels 31 inches in diameter. If the car is brought to a stop uniformly in 58 turns of the wheels, what is the magnitude of the angular acceleration of the wheels (in radians/second^2)?
6. ### hihhh

A car starts from rest and covers a distance of 22.0 m while accelerating at a constant rate of 2.6 m/s2. If the radius of the car wheels is 26.0 cm, what is the final angular speed (in rad/s) of the car wheels?
7. ### Physics

A car with a total mass of 1.48 103 kg has wheels of radius 33.5 cm and moment of inertia 0.810 kg · m2. (a) Find the car's translational kinetic energy when the car is moving at a speed of 30.0 m/s. (b) Find the car's rotational …
8. ### Physics

A 1000-kg car has four 10-kg wheels. What fraction of the total kinetic energy of the car is due to rotation of the wheels?
9. ### physic 210

A car is traveling with a velocity of 17 m/s on a straight horizontal highway. The wheels of the car have a radius of 48 cm. If the car then speeds up with an acceleration of 2 m/s² for 5 s, determine the number of revolutions of …
10. ### science

A car is moving at a speed of 72 km/hr. The radius of its wheels is 0.25m. If the wheels are stopped in 20 rotation by applying brakes, then angular retardation produced by the brakes is:

More Similar Questions