# Constant acceleration question: Do I have enough info?

• Sep 18th 2010, 03:30 PM
Constant acceleration question: Do I have enough info?
A wrecking ball is hanging at rest from a crane when suddenly the cable breaks. The time it takes for the ball to fall halfway to the ground is 1.2s. Find the time it takes for the ball to fall from rest all the way to the ground.

??? HUH? The answer is 1.7 s, but I have no clue on this one??

HELP HELP
Vo = 0 m.s
a = 9.8 m.s.s
t 1/2 way is 1.2s.
I can't see what formula to give me ANY Other info???
• Sep 18th 2010, 03:48 PM
skeeter
Quote:

A wrecking ball is hanging at rest from a crane when suddenly the cable breaks. The time it takes for the ball to fall halfway to the ground is 1.2s. Find the time it takes for the ball to fall from rest all the way to the ground.

??? HUH? The answer is 1.7 s, but I have no clue on this one??

HELP HELP
Vo = 0 m.s
a = 9.8 m.s.s
t 1/2 way is 1.2s.
I can't see what formula to give me ANY Other info???

$\displaystyle \frac{1}{2}g(1.2)^2 = \frac{h}{2}$

solve for $\displaystyle h$, then use in this equation ...

$\displaystyle h = \frac{1}{2}gt^2$

... then solve for $\displaystyle t$
• Sep 18th 2010, 03:48 PM
awkward
Consider the fact that the distance traveled is proportional to the square of the time.

 Beaten to the punch by Skeeter ![/edit]
• Sep 18th 2010, 03:58 PM
What formula are you using?? We were only given 4 of them the main ones. V=Vo+at, d=1/2(V+Vo)t, d=Vot+1/2at^2, V^2=Vo^2+2ad.
That's all we were given. What is h=height? I understand g is my a above.
I am still a bit confused, not catching on to this.
If I subbed in the first eq. I get 14.112 is that correct? probably not.
• Sep 18th 2010, 04:07 PM
skeeter
Quote:

What formula are you using?? We were only given 4 of them the main ones. V=Vo+at, d=1/2(V+Vo)t, d=Vot+1/2at^2, V^2=Vo^2+2ad.
That's all we were given. What is h=height? yes, the ball's original height above the ground.

...
• Sep 18th 2010, 04:08 PM
mr fantastic
Quote:

What formula are you using?? We were only given 4 of them the main ones. V=Vo+at, d=1/2(V+Vo)t, d=Vot+1/2at^2, V^2=Vo^2+2ad.
That's all we were given. What is h=height? I understand g is my a above.
I am still a bit confused, not catching on to this.
If I subbed in the first eq. I get 14.112 is that correct? probably not.

Use the one in red:

d = h/2, Vo = 0, t = 1.2 seconds, a = 9.8 m/s^2.

Substitute and solve for h.

Then use it again:

d = h (found above), Vo = 0, a = 9.8m/s^2, t = ?
• Sep 18th 2010, 04:34 PM
h/2=0(1.2)+1/2(9.8)(1.2)^2
h/2=0 + 7.056
To get rid of the 2 I multiply each side by 2?? then I get 14.112 m??
Then use the quadratic equation as I have to solve for t as I can't add t plus t^2???
• Sep 18th 2010, 06:21 PM
sa-ri-ga-ma
h/2 = 1/2*g*t1*2....(1)

h = 1/2*g*t2^2...(2)

Divide (2) by (1)

2 = t2^2/t1^2

t1 is given. Find t2.
• Sep 18th 2010, 07:22 PM