Direct Measurement Videos > Video Library > Cart push-off

Cart push-off

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This is a series of videos showing students on low-friction carts. Students are initially stationary, and then push off of each other, sending each cart moving in opposite directions. Students can measure the speed of each cart after the push off, and then calculate the momentum of each cart, and of the system. Various versions of this video allow students to explore different combinations of masses and velocities to develop their understanding of the law of conservation of linear momentum.

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Instructor Note

When teaching with this video, direct students to the student video library, which provides student access to all videos without links to instructor materials and solutions.

Cart push-off 1: Two students of similar masses push off. All masses are given.

Cart push-off 1 dmv player
Cart push-off 1 download (Quicktime Video 3.4MB May25 14)

Teaching Materials


There are several approaches one could take in analyzing this video. First, we could use the concept of the center of mass of a system. We note that in the absence of a net external force on the objects in the system, the center of mass will not accelerate. In this case, that means that if the friction between the wheels and floor is small enough to ignore, the center of mass of the system will remain motionless.

A second approach is to consider the momentum of the system of objects. Before the push off, the total momentum of the system is zero. Again we assume that the friction of the floor against the cart wheels is small enough to ignore. This means that the total momentum of the system after the push off should also be zero.

Finally, we can analyze each cart independently using the impulse-momentum theorem and Newton's Third Law. Each cart receives an impulse from the other cart. Newton's Third Law tells us that the magnitude of the force each student exerts on the other is the same. We can see from the video that the duration of the push on each cart is the same. This means that the impulses on each cart are equal to each other. If both carts received the same impulse, they must also have experienced the same change in momentum.