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Exploration 13.2: Static Friction on a Horizontal Beam

Please wait for the animation to completely load.

You hold a piece of wood by pushing it horizontally against a wall as shown in the animation (position is given in meters). Restart.

  1. What forces act on the wood? Draw a free-body diagram for the wood showing the forces at their proper locations. Compare your diagram to the one shown in Animation 2.
  2. What is the force in the +y direction that counteracts the weight of the wood in this example? Note that this force is parallel to the surface of the wall and wood where they are in contact.
  3. In this case, do you know if the static frictional force of the wall on the wood is equal to its maximum value?

In Animation 3 you can adjust the magnitude of the push by clicking and dragging on the white circle at the tip of the vector representing the force of your hand on the wood. The maximum frictional force (shown as a red vector) adjusts accordingly. At the instant where the actual frictional force (black vector) equals the maximum frictional force (red vector), the beam will still be in equilibrium. In this case this is the least force that you can push the wood and have it remain in equilibrium. If you push it with less force, the meter stick will fall.

In Animation 4 you can adjust the magnitude of the push so that it is less than the minimum push required for the wood to remain in equilibrium. If the maximum possible static frictional force is less than the actual frictional force needed for equilibrium, the piece of wood will fall. In the animation, if you adjust fs max < fs, the resulting situation depicted in the animation is unphysical since the piece of wood will actually fall.

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Exploration authored by Aaron Titus.

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