# Homework Help: Physics problem

1. Oct 28, 2005

### Physicsdummy_101

A brick is thrown upward from the top of a builidng at an angle 25 degrees to the horizontal and with an initial speed of 15m/s. If the brick was in the air for three seconds how tall is the building

2. Oct 28, 2005

### Galileo

So, what have you done so far?

3. Oct 28, 2005

### ritz

giv this a try!

this probs is a projectile motion probs where the object is launched at an angle from horizontal surface. i wud rather u to c this probs in the first half of its motion. when the total time taken for the object to travel up to the height of the buildin and then come back to the position that it started from is 3 sec.....it has ta take 1.5 sec to reach to the maximum height of the buildin where the y component of the velocity has to be 0. now when u divide the prob and solve it imaginin the first half....its much less complicated!
the initial y-velocity will be- 15 sin25= 6.3 m/s
to find the max height...we will use the formula- y= ut + .5 at^2
6.3(1.5)+.5 (9.8)(2.25)
=1.6 m
i will feel much better if the answer come out rite....if not we will try solvin it in other ways!
take care

4. Oct 28, 2005

### Staff: Mentor

ritz:
(1) Please do not provide complete solutions. It's a violation of the PF Guidelines: https://www.physicsforums.com/showthread.php?t=5374
(2) Your solution assumes the projectile lands on the roof of the building! You won't find the height of the building with that assumption.

5. Oct 29, 2005

### ritz

doc.....i havnt assumed that the ball lands on the buildin....just that it crosses the buildin! so when u consider just one part of the problem....where the ball moves up and reaches the highest point...u cud solve the probs in much easier way....try urself! i m sure it will make much more sense.....
and thnx lettin me know the rules.....appreciate that!
cheers!

6. Oct 30, 2005

### Staff: Mentor

Seems to me like you did. You said this:
But that 3 sec total time must be the time for the brick to go from rooftop to highest point and then down to the ground, else the problem won't make sense. (The time it takes to get to the highest point is not 1.5 sec.)

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