Slide 1
A raised ball has gravitational potential energy
A ball held above the ground has gravitational potential energy relative to a chosen reference level. Dropping it allows that energy to change form.
More to Read
Potential energy belongs to the ball–Earth arrangement. Raising the ball requires work against gravity, and the reference height can be chosen conveniently because changes in potential energy determine the motion.
Slide 2
Falling converts that energy into motion
As the ball falls, gravitational potential energy decreases while kinetic energy increases. Ignoring air resistance, their sum stays constant.
More to Read
Near Earth’s surface the decrease is approximately mass times gravitational acceleration times height. A 1-kilogram ball falling 2 metres releases about 19.6 joules, using 9.8 metres per second squared for gravity.
Slide 3
Doubling speed gives four times the kinetic energy
Kinetic energy depends on speed squared. Doubling a ball’s speed gives four times its kinetic energy if its mass is unchanged.
More to Read
Momentum depends linearly on speed, so two conservation questions can lead to different comparisons. Equal momentum does not guarantee equal kinetic energy when masses differ.
Slide 4
Impact stores some energy in deformation
During impact, the ball and floor deform. Some energy is temporarily stored elastically and can return the ball upward.
More to Read
The returned fraction depends on the materials and impact conditions. Vibrations, sound and internal heating receive some of the energy, which is why an ordinary ball usually rebounds to a lower height.
Slide 5
A lower bounce means energy went elsewhere
A lower bounce shows that less energy returned to the ball’s upward motion. Energy transferred elsewhere has not ceased to exist.
More to Read
To account for the entire event, enlarge the system to include the floor, air and internal motion of matter. A mechanical-energy balance for the ball alone is smaller than a complete energy balance.