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# Tag Archives: initial speed

## The problem of a rock thrown vertically up

**A rock is thrown vertically upward from the ground with an initial speed 15m/s.**

**a. how high does it go b. how much time is required for the rock to reach its maximum height? c. what is the rock’s height at t=2.00s?**

*(Posted by Merhawi)*

**Answer:**

(a)

u=15m/s

a=-10m/s^{2}

v=0 m/s (at the max height)

S=?

v^{2}-u^{2}=2aS

S=v^{2}-u^{2}/2a

=225/20

**=11.25 m**

(b) From the above case

using v=u+at

t=v-u/a=**1.5s**

(c) Use S = ut + 1/2 at^{2}

put t=2s, u = 15m/s, a=-10m/s^{2}

S=30 – 20 = **10 m**

*(If you use g = 9.8 m/s ^{2 }The answers will be slightly different)*

## A numerical from Projectile motion, friction and more

**Hale asked:**

One side of the roof of a building slopes up at 33.5°. A roofer kicks a round, flat rock that has been thrown onto the roof by a neighborhood child. The rock slides straight up the incline with an initial speed of 15.0 m/s. The coefficient of kinetic friction between the rock and the roof is 0.430. The rock slides 10.0 m up the roof to its peak. It crosses the ridge and goes into free fall, following a parabolic trajectory above the far side of the roof, with negligible air resistance. Determine the maximum height the rock reaches above the point where it was kicked.

**(The question is left for visitors to answer.)**