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# Help with physics homework, I have no idea how to approach or solve.?

A clown is fired straight up into the air from a cannon at time t=0s. The cannon is placed on a retractable platform extending over the side of the cliff. The platform is pulled back after launch so the clown can go straight up and then back down all the way to the bottom of the cliff. The clown's motion is tracked with a radar gun that measures speed. After 5 seconds the radar gun shows the clown is traveling at 30m/s. Use g=10m/s^2

(i) What is the clown's initial speed when fired from the cannon?

(ii) What other time in the path would the clown have a speed of 30 m/s?

(iii) If the clown is traveling at 145 m/s when she reaches the ground, about how high is the platform from the bottom of the cliff?

### 2 Answers

- NCSLv 71 month ago
(i) The clown is traveling at 30 m/s, but is he traveling up or down? Assuming he's traveling down, then

v = v₀ + a*t

-30 m/s = v₀ - 10m/s²*5s

v₀ = 20 m/s

but then looking at part (ii) there is NO other time when the clown would be moving at 30 m/s.

So, he's headed UP:

30 m/s = v₀ - 10m/s²*5s

solves to v₀ = 80 m/s ◄

(ii) Same equation, different unknown:

-30 m/s = 80m/s - 10m/s²*t

t = 11 s ◄

(iii) There are several routes -- quickest is probably

v² = v₀² + 2ah

(145m/s)² = (80m/s)² - 2 * 10m/s² * Δh

Δh = -731 m

so the height was 731 m.

Hope this helps!

- oubaasLv 71 month ago
(i)

What is the clown's initial speed when fired from the cannon?

30 = Vo-g*t

Vo= 30+10*5 = 80 m/sec

(ii)

What other time in the path would the clown have a speed of 30 m/s?

-30 = 80-10*t-110 = -10t

t = 11 sec

(iii)

If the clown is traveling at 145 m/s when she reaches the ground, about how high is the platform from the bottom of the cliff?

145^2 = 80^2+20*h

h = (145^2-80^2)/20 = 730 m ( just 2 significant figures)