I to find the distance of the freefalling objects when one hits the ground.

[Purpplehaze]

Homework Statement

The two objects are dropped from 60 m height. One is dropped having an initial velocity of 20m/s and another with 15 m/s. What is the distance of the objects when the first one hits the ground if there is no air resistance.

Homework Equations

The Attempt at a Solution

 

[Doc Al]

What have you done so far? What kinematic equations describe the position of an object as a function of time?

 

[Purpplehaze]

D= iV * T + a*t^2/2 I think that's the formula to be used.

 

[Doc Al]

D= iV * T + a*t^2/2 I think that's the formula to be used.

Good. So how can you apply this to solve for the time it takes for the 20 m/s object to reach the ground?

 

[Saitama]

D= iV * T + a*t^2/2 I think that's the formula to be used.

Plugin the values for first object and calculate t.

 

[Purpplehaze]

Would this work? 2(D-iV)/a = t

 

[Saitama]

Would this work? 2(D-iV)/a = t

No, how did you get this?

 

[Doc Al]

Would this work? 2(D-iV)/a = t

No.

Done right, you'll end up with a quadratic equation.

 

[Purpplehaze]

Then how can I isolate t?

 

[Saitama]

Then how can I isolate t?

Show us the equation you get after you plugin the values.

For solving quadratic equations: http://en.wikipedia.org/wiki/Quadratic_equation

 

[Purpplehaze]

60=20t + 4.9t^2

 

[Saitama]

60=20t + 4.9t^2

Are you allowed to use g=10 m/s^2? That would make calculations a lot easier.

 

[HallsofIvy]

You have two different objects, with two different initial velocities, each involving $t^2$. Once you put the initial velocities in, you will have two different quadratic equations. What are those equations?

 

[Doc Al]

60=20t + 4.9t^2

Good. Now you can put it into standard form and solve it using the quadratic formula (or using whatever method you like).

 

[Purpplehaze]

I got 2 as T for the first one.

 

[Doc Al]

I got 2 as T for the first one.

Good!

Now see if you can figure out where the other object (the one thrown with the slower speed) is at that time. Use the same formula, but with different data.

 

[Purpplehaze]

Ok I got the answer, the distance is 10.
Thanks a lot.