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Lesson: Instantaneous Speed Physics • 9th Grade

In this lesson, we will learn how to determine the instantaneous speed of an object by using a tangent to find the slope at a point on the object's displacement–time graph.

Lesson Plan

Students will be able to

  • identify from straight-line displacement–time graphs what the speed of an object at a particular instant in time is (as opposed to over a time range),
  • correctly identify tangents to curves on displacement–time graphs,
  • use tangents to curves to find the speed of an object at an instant,
  • recognize that if the tangent is a horizontal line, then the speed is zero,
  • recognize that the greater the slope of the tangent, the greater the instantaneous speed at that point.

Lesson Presentation

Lesson Video

Video Thumbnail
12:21

Lesson Explainer

Lesson Playlist

Lesson Worksheet

Q1:

An object moves along a straight line. On the graph, the blue line shows the displacement, 𝑑, of the object from its starting position over time, 𝑡.

Which of the three dashed lines is a tangent to the blue line at 𝑡=4 s?

Which of the three dashed lines is a tangent to the blue line at 𝑡=1 s?

Which of the three dashed lines is a tangent to the blue line at 𝑡=16 s?

Q2:

An object moves along a straight line. On the graph, the blue line shows the displacement, 𝑑, of the object from its starting position over time, 𝑡.

Which of the three dashed lines is a tangent to the blue line at 𝑡=4 s?

Q3:

The graph shows the displacement of a motorcycle in a race along a straight track. At which of the labeled points on the graph does the motorcycle have the greatest speed?

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