The frame of a cart has a mass of 45 kg, and the cart rolls on four railroad car wheels, each
with a mass of 22 kg. The cart is being ridden at a speed of 5.0 m/s by a 75 kg man. What is the
total kinetic energy? (Model the railroad car wheels as solid uniform disks of radius 20 cm.)
A) 3200 J
B) 1900 J
C) 2100 J
D) 2200 J

Here's how I tried to solve it:

KE = (.5)mv^2
KE = (.5)(45+4(22)+75)(5)^2
KE = 2600J

That's not one of the choices.... Is it a mistake in the book?

Question

The frame of a cart has a mass of 45 kg, and the cart rolls on four railroad car wheels, each
with a mass of 22 kg. The cart is being ridden at a speed of 5.0 m/s by a 75 kg man. What is the
total kinetic energy? (Model the railroad car wheels as solid uniform disks of radius 20 cm.)
A) 3200 J
B) 1900 J
C) 2100 J
D) 2200 J

Here's how I tried to solve it:

KE = (.5)mv^2
KE = (.5)(45+4(22)+75)(5)^2
KE = 2600J

That's not one of the choices.... Is it a mistake in the book?

anonymous · Answer

well :)

anonymous · Answer

Find anything wrong in the equation?

anonymous · Answer

Thou hast disregarded the energy due to the rotation of the wheels. You should get the right answer once you add in T=0.5*(4 wheels)*(Wheel's moment of inertia)*(angular velocity of wheel)^2. See http://en.wikipedia.org/wiki/Rotational_energy.

anonymous · Answer

Thank you. 

Our teacher never taught us rotational energy, so I didn't think he would ask us a question on that.