Physics Demo Number: 030

Approximate Run Time:7 min

Friction and Third Law as the Decider of Tugs-of-War Contests

Demo Description

A student seated on a 'stool with wheels' cannot win a tug-of-war contest because he does not have enough friction force from the floor.

Scientific Principles

  • Newton's 3rd Law of Motion

Equipment

  • Cart with wheels

  • Folding Step Stool to Sit Upon

  • Rope

Equipment Location

  • Cart and Seat live on [F-1-4]

  • Rope is in Kit(030) on [F-1-3]

Instructions

The photo shows a cart which has 4 very low rotational friction bearings as wheels.

One seats a strong student on the small step stool with the alignment of stool and cart as in the picture. The student is required to keep his feet on the cart at all times.

If the student takes one end of the yellow rope and tugs on it while a weaker student tugs on the other end of the rope (with the rope perpendicular to the axles of the cart) there is no contest as to the weak student winning the tug-of-war, thanks to the stronger friction between the floor and feet of the weaker student.

On the other hand if the stronger student is allowed to turn the cart through ninety degrees and then turn the stool ninety degrees, so that the rope now is parallel to the cart axles, the weaker student has no chance to win the tug-of-war thanks to the friction between the cart wheels and the floor being larger than the friction between the weaker student's shoes and the floor.
  
Writeup created by David A. Burba
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