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ZingPath: Velocity Vectors

Relative Motion

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Velocity Vectors

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Lesson Focus

Relative Motion


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You will observe the relative motion of moving things by altering their speed as well as the position of the observer.

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Now You Know

After completing this tutorial, you will be able to complete the following:

  • Explain that actual velocity is the velocity of an object relative to the ground.
  • Define relative velocity as a measurement of velocity of an object with respect to a second object that may be moving.
  • Explain that a moving observer will view a non-moving object as moving in the opposite direction.
  • Explain that an observer on a moving object observes an object moving in the opposite direction as moving at a higher speed in the direction of the object’s velocity.
  • Calculate the relative velocity of an object using the formula: Relative Velocity = Actual Velocity of the Object – Actual Velocity of the Observer.

Everything You'll Have Covered

All motions are relative to some frame of reference. The motion of one body relative to another is relative motion. Stating that a body is at rest means that it is being described with respect to a frame of reference that is moving together with the body. For example, a rock on the ground may appear to be at rest, but that is only because the observer is also on the surface of the Earth. The Earth and the rock are actually in constant motion in their orbit around the Sun.

An object is in motion when it is continuously changing its position relative to a reference point and as observed by a person or detection device. For example, you can see that a car is moving with respect to the ground. The distance the object goes in a period of time is its speed. If the speed of an object is in a specific direction, it is called velocity. Velocity is a vector quantity that has both magnitude and direction, such as 5 km/hour South. The change in velocity over a period of time is the acceleration of the object. It is important to note that neither speed nor velocity are absolute; they are dependent on the observer. All motion is relative to the observer or to some fixed object. When you see a car drive by, it is moving with respect to you. If you are in a car that is going at the same speed, the other car will not be moving with respect to you. But both cars are moving with respect to the ground.

When studying motion, it is important to indicate your point of reference because there are situations where the speed or velocity may be with respect to another object or an observer. For example, in an accident, one car hits another car at 75 miles per hour (mph), but there was very little damage. This is due to the fact that the cars were traveling in the same direction and the second car's actual speed was 74 mph, therefore its relative speed was only 1 mph with respect to the second car when they collided. Relative velocity is the velocity of the object after deducting the velocity of the observer.

Tutorial Details

Approximate Time 20 Minutes
Pre-requisite Concepts Students should be familiar with motion, solving for unknowns, speed, using formulas, vectors, and velocity.
Course Physics
Type of Tutorial Concept Development
Key Vocabulary actual speed, actual velocity, kinematics