What is the speed of the flatcar after the sack lands on it?

Question

A 500kg sack of coal is dropped on a 2000kg railroad flatcar which was initially moving at 3.0 m/s. After the sack lands on the flatcar the speed of the flatcar is...

A. 0.6 m/s

B. 1.2 m/s

C. 1.8 m/s

D. 2.4 m/s

E. 3.6 m/s

Answer 1

Hi Bella L.,

Momentum is conserved (always!) so going from 2000kg at 3.0 m/s to 2500 kg at the same momentum (the sack of coal has 0 initial momentum), that drops the velocity to 3.0*(2000/2500) ----- you do the math!

Note that technically the car would slow down, EVEN IF the coal caught on fire and totally burned. Just as long as it reaches a common velocity with the flatcar body first.

--Cheers, -- Mr. d.