Physics Demo Number: 017

Approximate Run Time: 5 min

Friction, Wheelies, Inertia, Torque, and Third Law

Demo Description

Use the 3 wheel cart minus Rocket Thrust to Illustrate Some Principles of Vehicle Motion

Scientific Principles

  • Newton's 3rd Law of Motion

  • Inertia

  • Torque

  • Frictional Effects in Vehicle Motion

Equipment

  • 3 wheel cart

Equipment Location

  • The cart lives on the floor in SC4228 opposite the [H] Columns.

Instructions

The photos show the relevant parts and force labels for using the rocket cart (without benefit of motor) to illustrate the above concepts.

The demonstration consists of sitting on the cart and propelling oneself  in the forward direction or in the rearward direction .

One brakes briskly to a halt in each case while firmly holding both handgrips.

The results are that one comes to a straight-line uneventful halt in the forward case, but one pops a wheelie (front wheel leaves floor) and the front end spins to the left in the rearward motion case.

(FI, HHB) is the label for the inertial force exerted on the handgrip by the hand in the direction shown (i.e. to the rear) during the course of the stop for the rearward motion case. (FF, BW) denotes the friction force exerted on the brake wheel (BW) during the course of the just- mentioned stop. (BP) shows the brake pedal, while (BS) points out the brake shoe.

((BS)’s lack of a mate (brake shoe) on the left rear wheel and the singlet front wheel are of course the source of some of the asymmetry in the carts forward and rearward braking motions.)

One may point out the general result that the normal force at the “forward axle” (relative to the direction of motion) increases during deceleration from constant speed. This fact can lead to a real-world example of mechanics in action (sometime during a four year residency at Vanderbilt one will probably see at least one rear end collision on West End in which the front end of the rear car will be hung under the rear end of the front car ).

The unbalanced torque of the frictional braking force on the right rear wheel will cause a rotation of the entire rocket cart system unless there is sufficient transverse friction force on the front wheel to cancel it. However friction forces have maxima limited by normal forces.

By noting the direction of friction forces exerted on driven wheels, one can connect the wheelie to the motions sometimes observed at real world drag races.
   
Writeup created by David A. Burba
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