Linear Inertia: Resistance to Change in Linear Motion

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Author: Janice VanCleave

Seat belts are put in cars to restrain people if the vehicles have to stop suddenly. A person in a car is going as fast as the car is, but when the car quickly decelerates (reduces its velocity in a given time), the person—who is not attached to the car—continues forward at the original velocity. This tendency of an object to continue in motion unless acted on by a force is called inertia. Inertia also describes the tendency of objects to continue to be stationary unless acted on by a force.

In this project, you will perform an investigation similar to one designed by Galileo Galilei (1564—1642) to determine the effect of net forces of varying amounts on the motion of an object. You also will design an inertia balance and use it to determine the mass (amount of matter) of objects.

Getting Started

Purpose: To show how a force affects the state of motion of an object.


  • pen
  • meterstick
  • 4-by-28-inch (10-by-70-cm) piece of poster board
  • scissors
  • protractor
  • transparent tape
  • marble


  1. Construct a double-ramp using the following steps:
    • Use the pen and measuring stick to draw two lines down the poster board 3 cm from each side.
    • Mark each 1 em down the center of the poster board strip.
    • Fold up the edges of the strip at the lines you drew in step 1 to make outside rims.
    • At the 15-cm and 25-cm marks, cut diagonal slits in the rim on both sides (see Figure 8.1).
    • Bend the strip at each slit to form an incline of 60° at each end of the strip. Use tape to secure the overlapped edges of the rims.
  2. Linear Inertia: Resistance to Change in Linear Motion

  3. Set the double-ramp structure on a table. Use tape to secure the top of ramp A to a vertical structure, such as a wall. Tape the middle of the structure to the table. Ramp B should be stable enough to stand unsupported, but if necessary, use a book to support it.
  4. Linear Inertia: Resistance to Change in Linear Motion

  5. Hold the marble at the second centimeter mark from the top of ramp A (13 cm—from the base of ramp A).
  6. Release the marble and allow it to roll down ramp A.
  7. Observe the height the marble reaches on ramp B. Using the marks on the ramp, measure the distance the marble travels from the base of ramp B to this height. Record the measurement in a Distance Data table like Table 8.l.
  8. Repeat steps 7 through 9 four times and average the results.


The distance will vary depending on the size of the marble used and the surface of the ramp. The author's marble traveled an average 11 cm up ramp B.


The ramp structure you built is similar to the one designed by Galileo. Galileo discovered that when the surfaces of the ramps (inclined planes) are very smooth, a ball will roll down one ramp and rise to almost the same height on the opposite ramp. The author's ramp had more friction than did Galileo's structure, as indicated by the ball being released at 13 cm above the surface and rising an average of only 11 cm on the opposite ramp. Once the ball was released, the force of gravity pulled the ball down. When it reached the bottom of ramp A, the ball had reached its maximum velocity, Vmax and would have continued to move at this velocity if no forces acted on it. The tendency of an object to remain at rest or to resist any change in its state of motion unless acted on by an outside force is called inertia. In this investigation there were forces acting on the ball, including friction between the ball and the poster board ramp. As the ball rolled up the ramp, gravity pulled down on it, also causing deceleration (a decrease in velocity per unit of time). Deceleration is also called negative acceleration.

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