Answers

The energy at the top of the slope is gravitational potential energy. When something is lifted up in a gravitational field it gains energy.

The formula is E_gp = mgh (where m = mass, g = acceleration due to gravity, and h is the height raised)

The energy at the bottom of the slope is kinetic energy, as the car is of course moving.

The formula is E_k = 1/2 mv²

Setting them equal to each other (since the final kinetic energy came from the initial potential energy):

mgh = 1/2 mv² ...and the masses cancel out

Putting 0.3 in for h, and rearranging the equation, you can then solve for v:

v = √ (2gh)

You should get the answer 2.4 m/s

We measured it as about 1.8 m/s, so where has the rest gone?

Could we improve the experiment to get more convincing results?

Whenever we do practical work we should think about the quality of our measurements. The more carefully we take measurements, the more reliable the results. Good equipment helps a lot too of course!

Final conclusion?

Theoretically the gradient of the slope makes no difference to how fast the car will be going at the bottom, it's all about energy!