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Linear Motion

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More Lessons for High School Physics

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A series of free Online High School Physics Video Lessons.

In this lesson, we will learn

  • what is relative motion
  • what is displacement
  • how to calculate average speed
  • how to instantaneous speed

Relativity in Motion

The motion of any object is only meaningful when given relative to something else. Relativity in motion assures us that the laws of physics don't vary depending on how much inertia an object has. This is important because everything has some amount of inertia. Standing on the surface of the earth, we feel that we are not in motion, but because the earth is moving, so are we. If we measure the speed of a ball thrown atop a train, we can either measure the speed of the ball with respect to the train's motion or with respect to the motion of the earth.
The laws of physics are not affected by varying amounts of inertia.
A brief introduction to relative motion and relative velocity for high school honors physics students.


Displacement is the change in a position vector. It is not the same as distance, which is a scalar measurement. The net vector of multiple displacement vectors if found according to the rules of vector addition.
What is the difference between distance and displacement

Average Speed

Speed refers to the rate at which an object travels. Unlike velocity, speed is a scalar quantity so direction does not matter. Average speed is calculated by dividing distance by time (e.g. miles/hour).
Example of calculating speed and velocity

Speed - Instantaneous Speed

Speed refers to the rate at which an object travels. Unlike velocity, speed is a scalar quantity so direction does not matter. Instantaneous speed is the speed of an object in motion at a specific point in time. This is determined similarly to average speed, but we narrow the period of time so that it approaches zero. If an object has a standard speed over a period of time, its average and instantaneous speeds may be the same.

Understanding the differences between speed and instantaneous speed.

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