Calculating the Distance of a Block from a Table After a Bullet Collision

[mikefitz]

A 0.01-kg bullet traveling horizontally at 400.1 m/s strikes a 4.2-kg block of wood sitting at the edge of a table. The bullet is lodged into the wood. If the table height is 1.3 m, how far from the table does the block hit the floor?

I have calculated the final velocity of the bullet and block to be:

.01(400.1)=.01(Vf)+4.2(Vf)

Vf= .9504 m/s

So, now I have the velocity of the bullet embedded into the block. The time in the air is 1.3/9.82 = .1325 seconds.

.1325s * .9504 m/s = .1259 meters

Why is this wrong? Thanks

 

[Doc Al]

So, now I have the velocity of the bullet embedded into the block. The time in the air is 1.3/9.82 = .1325 seconds.

How did you determine this? How long does it take for something to fall 1.3 meters?

 

[OlderDan]

Why is this wrong? Thanks

Your calculated time in air is wrong.

 

[mikefitz]

Your calculated time in air is wrong.

Sorry, I meant to type 1.3/9.81 = .1325 -- the 9.82 was a typo.

 

[OlderDan]

Sorry, I meant to type 1.3/9.81 = .1325 -- the 9.82 was a typo.

That is not the problem. You are not calculating time. It would help you to keep your units in your calculations. Distance divided by acceleration is not time.

 

[mikefitz]

Stupid me, I used this formula and found the correct answer: http://www.glenbrook.k12.il.us/gbssci/Phys/Class/1DKin/U1L6b4.gif

thanks!