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Louisa88 · May 7, 2010, 10:51 AM

A ball is thrown from the top of a tall building upward at an angle of 30 degrees to the horizontal. Assume the height of the building including the point of release of the ball is 45m.

1. Find the horizontal and vertical components of the initial velocity of the ball.
2. Determine how long it will take the ball to reach the ground.
3. Determine the velocity of the ball just before is strikes the ground.

lJ. · May 8, 2010, 05:37 AM

There are different equations you can use.
W=Fd
W=mgh [PE= mgh too, since W and PE equal each other].
[one if for vertical and one for horizontal].
If angles are involved you may have to use sine, etc.
Also, since acceleration is not given and something is being thrown in the air, a=9.8 m/s [a of gravity].
To find velocity there are different equation you could use too. You should try googling about this topic as well for help.

THIS SHOULD BE ABLE TO HELP YOU!
Physics Homework Help: Free Fall Problems
It uses different numbers but seems like a similar problem.

Unknown008 · May 8, 2010, 11:07 AM

Actually, it is very difficult to work through this problem using energy equations, that is W = Fd or W = mgh, unless you are given the initial energy provided to the object. However the link that 'l' posted does contain the appropriate formulae to use.

First off, try to make a sketch for such problems, they are very handy. Here, you need to break the motion of the ball into two components.

Let v be the initial velocity of the ball.

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From this diagram, you'll see that the vertical component of the ball is given by v sin 30 and the horizontal component of the ball is given by v cos 30.

Along the horizontal, you have no force, hence, what does the Newton's first law say? Along the vertical, you have a constant force due to gravity acting, and causing the ball to accelerate downwards at a rate of 9.81 m/s^2 (3 sig fig).

For the second part, using $s = ut + \frac12 at^2$ is the best choice. However, since you didn't give any information about the initial velocity, we cannot go further.
s is the displacement.
u is the initial velocity.
a is the acceleration which affects the velocity u.
t is the time.

For the third part, you'll need to use the formula: $v^2 = u^2 + 2as$
where v is the final velocity affected by the acceleration.

You can use the formula $v = u+ at$ but it is generally not advised because if the time you got earlier is wrong, then this part will be wrong too.