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Time and velocity

A stone falls down a well vertically. Ignore friction.

it falls 60m , initial speed = 5m/s

acceleration must be -9.8m/s/s

If i use s = ut +1/2at^2 I can get an answer but it comes out as

-60 = 5t - 4.9t^2

I'm fairly sure you shouldn't have to use quadratic formula for a short question. What's wrong?
(edited 7 years ago)
Reply 1
Avatar for alow
alow
19
What's the issue with using the quadratic formula?

You've messed up a sign btw.
Reply 2
Avatar for Feynboy
Feynboy
OP
10
Original post by alow
What's the issue with using the quadratic formula?

You've messed up a sign btw.


got the mistake.My point was is there a different way
(edited 7 years ago)
Reply 3
Avatar for alow
alow
19
Original post by Feynboy
which one?


If you're saying the fall is in the -ve direction, what is your starting velocity?
Reply 4
Avatar for GUMI
GUMI
19
The quadratic form would be
0 = - 4.9t^2 + 5t + 60
(edited 7 years ago)
Original post by Feynboy
A stone falls down a well vertically. Ignore friction.

it falls 60m , initial speed = 5m/s/s

acceleration must be -9.8m/s/s

If i use s = ut +1/2at^2 I can get an answer but it comes out as

-60 = 5t - 4.9t^2

I'm fairly sure you shouldn't have to use quadratic formula for a short question. What's wrong?


Acceleration is positive if it's falling down. The equation should be:-
4.9t^2 + 5t-60=0.
Reply 6
Avatar for alow
alow
19
Original post by sabahshahed294
Acceleration is positive if it's falling down. The equation should be:-
4.9t^2 + 5t-60=0.


Not if you take upwards as +ve. As long as you're consistent it can be either way.
Original post by alow
Not if you take upwards as +ve. As long as you're consistent it can be either way.


You mean the displacement right? Because if a=g here then a is positive.
Reply 8
Avatar for alow
alow
19
Original post by sabahshahed294
You mean the displacement right? Because if a=g here then a is positive.


I mean taking upwards as being +ve displacement, like OP has done.

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