Force
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In physics, a force is an action that can cause an object to change its velocity or its shape, or to resist other forces, or to cause changes of pressure in a fluid. In mechanics, force makes ideas li
Aristotle famously described a force as anything that causes an object to undergo "unnatural motion"
Sir Isaac Newton in 1689. His Principia presented his three laws of motion in geometrical language, whereas modern physics uses differential calculus and vectors.
Galileo Galilei was the first to point out the inherent contradictions contained in Aristotle's description of forces.
Images of a freely falling basketball taken with a stroboscope at 20 flashes per second. The distance units on the right are multiples of about 12 millimeters. The basketball starts at rest. At the time of the first flash (distance zero) it is released, after which the number of units fallen is equal to the square of the number of flashes.
Angular momentum
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Angular momentum is the rotational analog of linear momentum. It is an important physical quantity because it is a conserved quantity – the total angular momentum of an isolated system remains constan
This gyroscope remains upright while spinning owing to the conservation of its angular momentum.
A figure skater in a spin uses conservation of angular momentum – decreasing her moment of inertia by drawing in her arms and legs increases her rotational speed.