# Motion Under Gravity

The height s (in feet) at the time t (in seconds) of a silver dollar dropped from the top of the Washington monument is

s=-16t+555

a.) Find the average velocity on the interval [2,3]

b.) FInd the instantaneous velocity when t=2 and when t=3

c.) how long will it take the dollar to hit the ground?

d.) Find the velocity of the dollar when it hits the ground.

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#### Solution Preview

Sol.

In the expression of s the exponent of time is one hence the body is moving with uniform velocity.

s=-16t+555 ............(1)

gives velocity as a function of time t as

v = ds/dt = -16 + 0 = - 16 feet/second (independent of t) ...

#### Solution Summary

The displacement of a coin dropped under gravity is given as a function of time and average velocity, instantaneous velocity and the time to reach ground is calculated.