2. The gravitational potential energy remains the same, despite the added weight, due to the formula used to find gravitational potential energy (mgh) in this formula the gravitational potential energy is determining by multiplying the height of the object by its mass, and by gravity. Since gravity  is a constant the only changeable factors are height and mass, and as the mass is doubled the distance the car travelled was halved, generating the same gravitational potential energy.

3. the coefficient of friction is 715.6