# Homework Help: Vectors and Trajectories, help with problems

1. Sep 18, 2006

### wakejosh

just a few similar problems.

38- A ball is rolled off a table with an initial speed of 0.24 m/s. A stop watch measures the ball's trajectory time from the table to the floor to be 0.3 s. What is the height of the table? (neglect air resistance)
a. 0.11m b. 0.22m c. 0.33m d. 0.44m

I think its D
For this I used a kinematics equation i found here: http://www.glenbrook.k12.il.us/gbssci/phys/Class/1DKin/U1L6b.html [Broken] to get values for the horizontal and vertical motions, I came up with 0.369 m so my guess would be C. is this wrong? Actually, Now that I look at this its a little tricky, and I think I should only be concerned with the motion in the y direction. If so I calculate y = 1/2(9.8)(.3^2) and I come up with y= 0.44 . So it looks like it may be D ??

16- A bridge that was 5 m long has been washed out by the rain several days ago. How fast must a car be going to successfully jump the stream? Although the road is level on both sides of the bridge, the road on the far side is 2m lower than the road on this side.
a. 5 m/s b. 8 m/s c. 13 m/s d. 25 m/s

I think it's B
I did practically the same for this: 2 = 1/2(9.8)(t^2) then i put V= 5m/0.64s and came up with 7.8 m/s so i guess it is B?

32- Wiley Coyote has missed the elusive road runner once again. This time, he runs off the edge of the cliff at 25 m/s horizontal velocity. If the canyon is 100 m deep, how far from the edge of the cliff does the coyote land?
a. 112 m b. 225 m c. 337 m d. 400 m

I think it's A
Again, very similar, I found the T the same way:

100m = 1/2 * 9.8 m/s * T^2 and solving for T i got 4.52 seconds. then i put this in:
25 m/s = X/4.52 s and ended up with 113 m so Im pretty sure it's A. am i right?

29- Superguy is flying at treetop level near Paris when he sees the Eiffel Tower elevator start to fall (the cable snapped). His X-ray vision tells him Lois LaTour is inside and needs help. If Superguy is 1.0 km away from the tower, and the elevator falls from a height of 240m, how long does Superguy have to save Lois, and what must be his average velocity? (Superguy needs only touch Lois before she hits the ground because he is magically immune to the laws of physics and anyone he touches can also instantly become immune.)
a 3 s, 333 m/s b. 5 s, 200 m/s c. 7 s, 143 m/s d. 9 s, 111 m/s

I think it's C. 7s, 143 m/s
same thing:
240 = 1/2(9.8)(t^2), and i get t= 6.99 s
then 1000m/6.99s I get 143 m/s

(my backpack was stolen so I dont have my book with me, and it really sucks :(

Last edited by a moderator: May 2, 2017
2. Sep 18, 2006

### wakejosh

could anyone check these for me? I'm not positive I'm doing them right.

3. Sep 18, 2006

### dmoravec

work looks good and the numbers seem to work out.

4. Sep 18, 2006

### wakejosh

I added two more problems if anyone could double check those for me as well. also,,, I think my answer was wrong in the first problem in the post so I changed it and only concerned myself with motion in the Y or falling direction.

5. Sep 18, 2006

### wakejosh

im still not sure about the first one (38), i cant decide which way is the right way to do it.

6. Sep 18, 2006

### wakejosh

bump.

please a simple yes/no is all im looking for. can someone please just double check my work and see if i did them right? also on the first problem im a little unsure.

7. Sep 18, 2006

### dmoravec

the correct way for 38 is to look at only the y motion. so the second way you did it is correct.

8. Sep 18, 2006

### wakejosh

great.. so all the rest look good im assuming?