Teaching Physics with the Physics Suite

Edward F. Redish

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Jumping into a haystack

You are visiting a friend's farm one fine summer's day and wander out to the barn. You notice that a haystack has recently been built just outside the barn. The barn has a second story door into which the hay will be hauled into the barn by a crane. You decide it would be a neat idea to jump out of the second story door onto the haystack. But you know that if you jump out of the second story door onto the ground, that you are likely to break a leg. Knowing lots of physics, you decide to estimate whether the haystack is good enough to break your fall.

You estimate the height of the haystack to be 1 meter. You press down on top of the stack and discover that to compress the stack by 25 cm, you have to exert a force of about 50 N. The barn door is 6 meters above the ground. Solve the problem by breaking it into pieces as follows:

  1. Model the haystack by a spring. What is its spring constant?
  2. Is the haystack tall enough to bring your speed to zero? (Estimate using conservation of energy.)
If you do come to a stop before you hit the ground, what will the average force exerted on you be?

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Page last modified October 22, 2007: P&E18