Spring and car collision I am working on designing an experiment for a class, and wanted some feedback. We will be rolling a cart down a "frictionless" track to hit a spring and measuring how far it will bounce back. We will be adding weight to it in order to see if it bounces farther back. I expected it to bounce farther back with increasing from 100g to 1kg but it does not. Can someone explain what I'm overlooking? Is it the spring system?
 A: Assuming no friction, perfect Hookean spring, then energy is conserved.
When the cart comes down the incline, its gravitational potential energy is converted to kinetic energy,
So when the cart hits the spring, it has some kinetic energy $K$. The spring then is compressed and the $K$ is converted to potential spring energy $U$, so that $K=U$.
During the bounce-back, the spring potential energy is converted back to the cart's kinetic energy $K$.
This is then converted to cart gravitational potential energy and the cart ends up exactly where it started from, regardless of the cart's mass.
Your experiment's result is correct.
A: I expected it to bounce farther back with increasing from 100g to 1kg but it does not.
Is this because you think that the spring-cart system will be more efficient (smaller percentage loss due to friction of mechanical energy to heat) when it the cart is more massive?
In an ideal system the reduction of gravitational potential energy as the cart rolls down the slope $(mgh)$,
will equal the gain in kinetic energy of the cart $(\frac 12 mv^2)$,
which in turn will equal the spring potential energy when the cart stops during rebound $(\frac 12 kx^2)$,
which in turn will equal the gain in kinetic energy as the cart starts moving up the slope  $(\frac 12 mv^2)$,
which in turn will equal the gain in gravitational potential energy when the cart stops at the top of its motion $(mgh)$.
This sequence illustrates that if the system was $100\%$ efficient in its mechanical energy transfers then the cart would return to the same height as that at the start of its motion irrespective of the mass of the cart, a result which you found by experiment.
