# Homework Help: Help please! velocity

1. Oct 11, 2007

### klm

i thought i figured it out, but i didnt

A rocket-powered hockey puck starts from the origin and moves on a horizontal frictionless table. In which direction is the puck moving at t=2s? (B) how far from the origin is the puck at t=5

for the first part i just thought i needed to look at t=2 s on both graphs, i guessed that vx=15 and vy=30 then i just found the inverse tan to find the direction to be 63.4 degrees. but that is incorrect

Last edited: Oct 11, 2007
2. Oct 11, 2007

### PhanthomJay

If you look at the slope of the first graph, by determining the vx/t ratio at 5 seconds, you can get exactly what vx is at 2 seconds.

3. Oct 11, 2007

### klm

i'm sorry i do not understand what you mean? because i think that the slope is equal to (40-30)/(5-4) = 10 , but vx does not equal 10 at t=2s

4. Oct 11, 2007

### klm

bump please. i am online so if anyone is willing to help and talk me through it that would be great

5. Oct 11, 2007

### learningphysics

slope is (40-0)/(5-0) = 8.

So what is vy at t = 2?

6. Oct 11, 2007

### klm

vy=30 ?

7. Oct 11, 2007

### klm

so does that mean vx=16 and vy=30. so you just do inverse tan (30/16)= 61.9 and that is the direction?

8. Oct 11, 2007

### klm

9. Oct 11, 2007

### learningphysics

Yes, that's correct.

10. Oct 11, 2007

### klm

thank you very much. can you please help me out with the 2nd part as well? how far from the origin is the puck at t=5

11. Oct 11, 2007

### learningphysics

area under the v-t graph gives displacement... area under the vx-t graph gives horizontal displacement. area under the vy-t graph gives vertical displacement.

12. Oct 11, 2007

### klm

okay so do you do for vx .5(5x40)=100 and vy (5x30)=150
then do you take the magnitude?

13. Oct 11, 2007

### learningphysics

yes, exactly.

14. Oct 11, 2007

### klm

so square root ( 100^2 + 150^2) = 180.28 m ?

15. Oct 11, 2007

### learningphysics

yes.

16. Oct 11, 2007

### klm

^when i try to put that in, it says it is incorrect.

17. Oct 11, 2007

### learningphysics

oops sorry... it's 180.28cm = 1.8028m

18. Oct 11, 2007

### klm

oohhh thank you so much! i wouldn't have caught that! thank you for your help!

19. Oct 11, 2007

no prob.