# Homework Help: Rest and motion kinematics

1. Aug 7, 2010

1. The problem statement, all variables and given/known data
A ball is dropped from a balloon going up at a speed of
7 m/s. If the balloon was at a height 60 m at the time
of dropping the ball, how long will the ball take in
reaching the ground ?

2. Relevant equations
u = –7 m/s, s = 60 m, a = g = 10 m/s^2

3. The attempt at a solution
i tried solving it by using D=ut+1/2at^2
n got stuck wit this 5t^2+7t-60=0
can any one tell me a simpler say to solve this equation?
1. The problem statement, all variables and given/known data

2. Relevant equations

3. The attempt at a solution

2. Aug 7, 2010

### Redbelly98

Staff Emeritus
You are nearly done with finding the solution. Remember that u is -7m/s, not +7m/s.

Are you familiar with the quadratic formula? Use that and you'll have t.

3. Aug 20, 2010

### dharavsolanki

DO NOT THINK THERE IS ALWAYS A SIMPLER WAY!

See, you first made the equations. Was there any complicated part involved over there? No! There was no imagination, no setting up, no interpretation required.

So the problem itself was very simple.

Now, AS A PART OF THE PROBLEM, you have to solve a quadratic equation.

Don't give this part too much importance in your mind. Tell yourself - yeh aa gaya toh answer mil gaya. So, mindlessly, solve the quadratic equation. THAT's IT.

Set up an equation and solve it - ismein kuchh aur simple ho hi nahin sakta!

4. Aug 20, 2010

### dharavsolanki

Keep posting more and more questions dude - we have a lot to work on! And whenever someone tells you something here - think over it and also make a note of it in your notebook or something.

At the end of a month or so - you should have the collective intelligence of all the contributors on physics forums. Sab ka experience tujhe muft main mil raha hai!

5. Aug 20, 2010

### dharavsolanki

ALSO - DO YOURSELF A FAVOUR:

* At the bottom of the page - you have "similar threads for post title", right? Read all of them one by one, and save a copy on your hard disc as a PDF (Use A PDF printing tool like CutePDF or something.).

Its a nice way to learn - listening to the doubts of others and understanding how they had been solved. Again, at the end of the month, you should have around 500 PDFs on your hard disc if you are regular here!