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Learning outcomes

After studying this unit you should know:

• All objects, irrespective of their mass, experience the same acceleration g when falling freely under the influence of gravity at the same point on the Earth. The weight of an object is the force F g due to gravity acting on the object, and for an object with mass m the weight is given by F g=mg.

• If the height of an object of mass m changes by Δh, the change in gravitational energy is ΔE g=mgΔh.

• If gravity is the only force acting on an object, the sum of kinetic energy and gravitational energy is constant. Increases in kinetic energy are balanced by decreases in gravitational energy, and vice versa.

• There are various forms of potential energy, all of which depend on the position of an object rather than on its motion. The potential energy of an object increases as it moves in the opposite direction to that of the force acting on it. Strain energy depends on the extension or compression of an object.