Posted by Madison on Wednesday, October 8, 2008 at 11:42pm.
hi guys, i had the following question but i keep on getting it wrong, should i use 9.8 or 9.8 m/s2 for the acceleration? im confused and would really appreciate some help.
A student throws a set of keys vertically upward to her sorority sister, who is in a window 3.70 m above. The keys are caught 1.60 s later by the sister's outstretched hand.
(a) With what initial velocity were the keys thrown?
m/s upward
(b) What was the velocity of the keys just before they were caught?
m/s is it upwards or downwards?
thanks to anyone who can help me

physics and math  bobpursley, Thursday, October 9, 2008 at 5:45am
the sign mean direction. Gravity is downward, so if you have + as the upward direction, then gravity is downward and negative.
vf^2=vi^2+ 2acceleration*height
when the keys are caught, one wants vf to be zero.
0=vi^2+ 2 (9.8)(+3.70)
not the plus signs upward on the height, but negative for gravity.
Solve for vi.
Just before they were caught? They were at the top, going up, near zero.

physics and math  drwls, Thursday, October 9, 2008 at 5:58am
g = 9.8 m/s^2 if the positive direction fot the vertical axis is defined as up.
g = +9.8 m/s^2 if the positive direction fot the vertical axis is defined as down.
You can do it either way.
(a) Assume that the keys are caught while moving up. If Y is measured upwards from the lower sister's hand,
Y = V*1.6  (1/2) g *(1.6)^2 = 3.7 m
1.6 V = 3.7 + 12.5
Solve for V, the initial velocity.
V = +10.1 m/s
(b) When caught, v = V  gt
v = 5.6 m/s
The keys are headed back down when caught.
Answer This Question
Related Questions
 Chemistry  I would really appreciate some help with this thermochemistry ...
 Chemistry  The second half of the equation is how i had done on the exam but ...
 Physics  A curve of radius 59.9 m is banked so that a car of mass 1.4 Mg ...
 Chemistry  Help me set up this problem. I have been looking at a few examples ...
 Physics  Hi, I had some questions about Physics and Newton’s Three Laws of ...
 Physics  Hi, I had some questions about Physics and Newton’s Three Laws of ...
 BPC110  Hello, I am trying to use APA format. I did some short answers and ...
 PHYSICS  Could u help with physics hw! i keep trying but i keep getting the ...
 Thank Jiskha tutors  I really appreciate all that you tutors do for us, I know ...
 Physics  I really don't get why I am getting this question wrong? Please can ...
More Related Questions