Speed is the distance an object travels over a period of time. Speed is written as a ratio. In physics, scientists typically use a ratio of meters per second (m/s). In your car, you probably use a ratio of miles per hour to measure speed.


Velocity is a measure of speed moved in a particular direction. If a measurement of speed was in meters per second, a measurement of velocity might be in meters per second north.


Acceleration is a measure of how much velocity is increasing. It is written as a ratio of distance divided by time squared. Usually meters per second squared (m/s2) is used


Momentum is the mass of an object times the velocity. A large object has more momentum than a small one moving at the same velocity.


Collisions are a transfer of momentum. When objects in motion collide, a change in momentum of one object is met by a change in momentum of the other object.


Force is the amount of push or pull an object has when it interacts with another object. The formula for calculating force is mass times acceleration (F=ma).

Newton’s Laws of Motion

Physicist Isaac Newton defined three laws for motion. 1. Objects in motion stay in motion unless an external force is applied. This is also called the law of inertia. If you roll a ball on the ground, it will eventually stop rolling because gravity is pulling it down. In space, without gravity, a moving object continues to move forever. 2. Force equals mass times acceleration. 3. Every action has an equal and opposite reaction.


All physical bodies attract each other. The more mass a body has, the more gravity it may have. The Earth is a large enough body that it has gravity. Objects on Earth have weight and fall to the Earth because of gravity. All falling objects on Earth accelerate toward the ground at the same rate, 9.8 m/s2. Physicist Galileo demonstrated that all objects fall at the same rate by dropping two balls of different mass from the Leaning Tower of Pisa in Italy.

Mass and Weight

Mass and weight are tow measures that are often confused. Mass is a measure of how much matter makes up an object, while weight is a measure of how much force gravity is putting the object. On Earth, mass is relative to weight: an object has a certain mass and weight. If that object is taken to space and free of gravity, it would have the same mass, but it would lack weight.


Work is measured when a force acts on a body and moves, or displaces, it. Work is measured in joules. The formula for work is force times displacement (W=fd). If a baseball player throws a ball, he exerts force on it and he moves it, so a measurable amount of work has been done. If a person pushes against a brick wall and does not move it, they have done no work, even if they get tired.

Simple Machines

Simple machines are machines that allow work to be done more easily by changing or redirecting force. There are six classic simple machines: The lever, wheel and axle, pulley, inclined plane, screw and wedge.

Mechanical Advantage

Mechanical advantage is a measure of the amount force is increased by using a tool, such as one of the simple machines.



Speedo Photo:  User Powerbooktrance/Wikimedia;  Scale Photo:  M.M. Minderhoud; Pitcher Photo:  Richmond Times-Dispatch