Calculate Velocity of Dropped Rescue Raft

[physicsgurl12]

i feel like a moron. :(

Homework Statement

Tim is flying a helicopter when he drops a rescue raft. After the raft has fallen 2.151 s:
What is the rafts velocity?

Homework Equations

a=v/t
v=d/t

The Attempt at a Solution

9.81m/s= v/2.151
v=21.101m/s
this should be really basic right. i think I am just using the wrong equation. help.

 

[HallsofIvy]

If the question is what is the velocity after the raft has fallen for 2.151 s, that is correct.

However, the phrase "has fallen 2.141 s" is strange. Are you sure it is not "has fallen 2.141 m"?

 

[tiny-tim]

i feel like a moron. :(

will i do?

Tim is flying a helicopter when he drops a rescue raft.

huh … these things happen

After the raft has fallen 2.151 s:
What is the rafts velocity?

Homework Equations

a=v/t
v=d/t

The Attempt at a Solution

9.81m/s= v/2.151
v=21.101m/s

looks ok to me … v_f - v_i = at

(except too many significant figures … try 21.10 m/s )

 

[physicsgurl12]

hmmm i tryd 21.10 and it still says its wrong. and yes I am sure its seconds.

 

[tiny-tim]

was the helicopter stationary (did you give us the whole exact question)?

have you tried minus 21.10 ?

 

[physicsgurl12]

haha it was negative. thanks

 

[physicsgurl12]

part 2 says also how far has the raft fallen during this time. i did v=d/t but it said that was wrong.

 

[ideasrule]

That's because the raft's speed isn't constant; it changes with time as v=gt. To calculate how far the raft falls, dy/dt=-gt, so y=-1/2*g*t^2 (an equation you've probably seen before).

 

[physicsgurl12]

o yeah. thank youuuu :)