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A ball is thrown in the air @ 30m/s...

by rdmfresno
Tags: 30m or s, ball, thrown
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rdmfresno
#1
Jul5-09, 11:17 PM
P: 11
1. The problem statement, all variables and given/known data
A ball is thrown straight up with an initial speed of 30m/s,
(a) Show that the time it takes to reach the top of its trajectory will be 3 seconds.
(b) Show that it will reach a height of 45m (neglecting air resistance).

2. Relevant equations
Earth's gravity = 9.8 meter/secondē (Professor said to just round it to 10).
Equation unknown.
d=1/2(g)tē ?

3. The attempt at a solution
(a) Played around with Subtracting negative -9.8m/sē (is it considered negative since the ball gets thrown up? - Instead of it coming down?).

(b) 45m = 1/2(9.8m/sē)tē - I figure I need to know "t" before I could solve this.
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LowlyPion
#2
Jul5-09, 11:28 PM
HW Helper
P: 5,343
Quote Quote by rdmfresno View Post
1. The problem statement, all variables and given/known data
A ball is thrown straight up with an initial speed of 30m/s,
(a) Show that the time it takes to reach the top of its trajectory will be 3 seconds.
(b) Show that it will reach a height of 45m (neglecting air resistance).

2. Relevant equations
Earth's gravity = 9.8 meter/secondē (Professor said to just round it to 10).
Equation unknown.

3. The attempt at a solution
(a) Played around with Subtracting negative -9.8 (is it considered negative since the ball gets thrown up? - Instead of it coming down?). I have some free-fall equations and solutions but I couldn't find the relation between them and 'throwing up in the air.'
If the speed is lowered by 10m/sē a second, how would this look in a equation?

(b) No idea where to start
Consider what the definition is of acceleration. It is the rate of change of velocity.

If something starts out at 30 m/s and slows 10 m/s every second, it will reach 0 m/s in how many seconds?

Here are some formulas for you:
http://www.physicsforums.com/showpos...63&postcount=2

These lessons might be helpful:
http://www.glenbrook.k12.il.us/gbssc...ors/u3l2c.html
rdmfresno
#3
Jul5-09, 11:39 PM
P: 11
Thanks a bunch. Will get started on the lesson.. then try the problem again.


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