# Mechanical displacement

A hollow steel ball weighing 4 pounds (mass = 1/8 slugs) is suspended from a spring. This stretches the spring 1/6 feet. The ball is started in motion from the equilibrium position with a downward velocity of 8 feet per second. The air resistance (in pounds) of the moving ball numerically equals 4 times its velocity (in feet per second) .

Suppose that after t seconds the ball is y feet below its rest position. Find y in terms of t. (Note that the positive direction is down.)

Take as the gravitational acceleration 32 feet per second per second.

Saying that a mass "weighs" 1 kilogram is technically incorrect usage, but it is often used. What one really means is that it has 1 kilogram of mass and therefore weighs 9.8 newtons.

When using English units (lb, ft, etc.) you need to be a bit careful with equations involving mass. Pounds (lb) is a unit of force, not mass. Using mg=F and g=32 ft/sec^2 we see that if an object weights L pounds it will have a mass of L/32 lb*sec^2/ft (or slugs).

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#### Solution Summary

This is an example of a problem regarding mechanical displacement.