Question

50 POINTS

Answer 1

vi is going in the positive direction (up). (That's my choice). a (acceleration) is going in the minus direction (down). The directions could be reversed.

Givens

vi = 160 ft/s

vf = 0 (the rocket stops at the maximum height.)

a = - 9.81 m/s

t = ????

Remark

YOu have 4 parameters between the givens and what you want to solve. Only 1 equation will relate those 4. Always always list your givens with these problems so you can pick the right equation.

Equation

a = (vf - vi)/t

Solve

- 32 = (0 - 160)/t Multiply both sides by t

-32 * t = - 160 Divide by -32

t = - 160/-32

t = 5

You will also need to solve for the height to answer part B

t = 5

vi = 160 m/s

a = - 32

d = ???

d = vi*t + 1/2 a t^2

d = 160*5 + 1/2 * - 32 * 5^2

d = 800 - 400

d = 400 feet

Part B

You are at the maximum height. vi is 0 this time because you are starting to descend.

vi = 0

a = 32 m/s^2

d = 400 feet

t = ??

formula

d = vi*t + 1/2 a t^2

400 = 0 + 1/2 * 32 * t^2

400 = 16 * t^2

400/16 = t^2

t^2 = 25

t = 5 sec

The free fall takes the same amount of time to come down as it did to go up. Sort of an amazing result.