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Science on a Skateboard - Applications of Newton's Third Law

This page authored by Joel D. Donna, University of Minnesota
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This material is replicated on a number of sites as part of the SERC Pedagogic Service Project


This is a discrepant event demonstration that can be used to help students begin to understand some applications of Newton's Third law of motion. In this activity, students are asked to predict what would happen if you were sitting on a low friction cart and threw a heavy medicine ball off the back. Students will apply knowledge learned from this demonstration to help them understand basic rocket propulsion.

Learning Goals

  • Students will be able to apply their basic understanding of Newton's third and second law in a new situation
  • Students will be able to apply understanding of this concept towards basic rocket propulsion

Context for Use

Educational level: Conceptual Physics Course
Setting: Lecture
Time required: 10 minutes
Special equipment: Low Friction Student Cart, Medicine Ball, Sandbags
Pre-requisite knowledge: Newton's Third, Second, and First Laws of motion, Friction

Description and Teaching Materials

This activity works well after having students "discover" Newton's Third law of motion by doing a tug of war activity with a pair of spring scales or force probes. Make sure that it is reinforced that for each force, the force is equal and opposite.

Get students into groups of two or three and give them this series of question. See attached questions. They should first think individually about their answers for approximately one minute and record what they think will happen and why it will happen. Then they will work with another student or two to discuss their answers and work to come to a common answer. These groups will share their predictions with the larger class. Probe student answers for greater detail if they are vague.

Have a student sit down a stable and sturdy low friction cart and throw the medicine ball. Have students record observations. Ask students to explain to each other what happened and why think it happened. Guide students through questions.

Student Questions (Microsoft Word 43kB Jul30 07)

As an extension, have students throw a number of smaller sandbags.

After the demonstration, use Newton's Third Law in conjunction with Newton's Second Law to explain.


Then use the attached PowerPoint to discuss rocket propulsion.

Newton's Thirds Extras PowerPoint (PowerPoint 185kB Jul30 07)

Teaching Notes and Tips

The transition from the skateboard to rockets gets a bit tricky. Follow up with a water or balloon rocket demonstration. High definition video footage of a NASA shuttle launch can help to tie these demonstrations to real engineering applications.

Make sure the students are seated safely on the cart before they throw the objects from the cart.


Answers given through the activity can help guide the instructor to if the students have understood Newton's laws of motion and can apply them in a new, complex situation.

Have students complete a ticket to leave in which they write the answer to an assessment question on a piece of paper and hand it to you as you leave.