# B Help with rolling cars -- science project

1. Apr 3, 2016

### Praline

I'm sure this is elementary for this forum, but can someone help me figure out if my son's science fair project results are correct? He rolled his toy car, with and without weight added, down hill and measured the time to get to the bottom of the hill and the distance traveled once the car hit the flat part of the track. He kept bumping into the track and messing it up so I'm not sure if his results are accurate. He has both cars traveling down hill in the same amount of time and the heavier car traveling farther. Does that sound right? How would you explain it in elementary age terms? Thank you in advance.

2. Apr 3, 2016

### drvrm

his both cars are travelling taking same amount of time it means the cars had same accelerations
so if net force experienced by cars were say F(L) and F(H) were such that

F(L)/M(L) = F(H)/M(H) = acceleration
so their velocities after traversing the ramp -touching the flat surface must be same-
but you are saying that they travelled different distances on flat surface......
there is a catch - on coming down their kinetic energy must be different the heavier one having larger kinetic energy and momentum than the lighter one ;
therfore the heavy one gets to stop after traversing larger distance (taking the frictional forces to be same.)
this is just my guess- may be some one can enrich us.

3. Apr 3, 2016

### Praline

Thank you! That is so helpful!!

4. Apr 3, 2016

### CWatters

See video of the Apollo astronaught dropping a feather and hammer on the moon. Both hit the ground at the same time but the hammer has more KE.