Warren Wilson Physics.

Fan cart with two opposing fans.

Two fans (shown as the green attachments) are mounted on a low-friction cart. The thrust of the two fans are equal and opposite so that the net thrust is zero. The cart may be launched with an initial velocity as selected in the table.

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Large thrusts that cancel

V0 > 0

V0 = 0

V0 < 0

Smal thrusts that cancel

V0 > 0

V0 = 0

V0 < 0

The bottom arrow (dark red) is the initial velocity at the origin.


  1. The two fans cancel each other, so the net total force on the fancart is zero. If the cart is given an initial push (initial velocity) describe its subsequent motion.

  2. Try all six combinations in the above table. Two entries are not very interesting. Explain based on Newton II.

  3. Explain the results for initial velocity non-zero. Does the cart stop, continue with the constant initial velocity, or accelerate?

  4. Explain the results based on Newton II.

  5. Suppose a parachute is falling with constant velocity. What is the total force (gravity and friction) on the parachute when it is falling with constant velocity? Remember Newton II.

End of mid-week assignment. Please e-mail discussions to dcollins@warren-wilson.edu

Physlets written by Wolgang Christian and Mario Belloni of Davidson College.

Javascript by Donald Collins of Warren Wilson College.