The mechanical advantage of the screwdriver that is being described above is equal to 75N. This means that for every 30N that is applied on the screwdriver, this simple machine would in turn apply 75N of force to the lid of the can.
Frictional Force is the answer..
Hope it helps
The mass of ball a is twice the mass of ball b:

This means that the initial potential energy of ball a (

) is twice the potential energy of ball b (

):

When the two balls reach the ground, the potential energy of each ball has converted into kinetic energy (since now their altitude is h=0), because the total mechanical energy of each ball must be conserved. Therefore:


and so the kinetic energy of ball a must be twice the kinetic energy of ball b: