# Homework Help: Inclined plane and elapsed time

1. Dec 7, 2008

### Sassenav22

1. The problem statement, all variables and given/known data
A block is given a initial speed of 5.0 m/s up the 20° plane. (a) How far up the plane will it go?
(b) how much much time elapses before it returns to its starting point?

2. Relevant equations
I think

V= u + at
s=ut t 1/2at^2
v=d/t

3. The attempt at a solution

(a) V= u + at
0= 5-9.8(sin20)t
t = 1.65s

s=ut+1/2at^2
=5(sin20)(1.65)+ 1/2(9.8)(1.65^2)
s= 16.16m

(b) v=d/t
t=d/v
=16.16/5
t= 3.2s

I know this is wrong but i can't figure it out can someone help me?

Last edited: Dec 7, 2008
2. Dec 7, 2008

### Staff: Mentor

Recheck your arithmetic. (Tip: Once you've solved for the time the block takes to rise, what's the answer to part b?)

The initial speed up the ramp is already parallel to the incline, so there's no need to multiply it by sin20. On the other hand, the acceleration along the incline is only a component of g, so you do need the sin20 there. Also, should the acceleration be + or -?

3. Dec 7, 2008

### Sassenav22

acceleration should be positive

4. Dec 7, 2008

### Sassenav22

So what your saying is that the sin20 is suppose to beby the 9.8?

5. Dec 7, 2008

### Staff: Mentor

No. The ramp slopes up, so acceleration is negative (down).
Yes, the acceleration of the block is g sin20.

6. Dec 7, 2008

### Sassenav22

Oh, Is the first part of (a) correct because I found the time so that i could find the distance, is that what is suppose to be done?

7. Dec 7, 2008

### Staff: Mentor

Yes, that's a perfectly fine way to solve the problem. You just made an error when you did the final arithmetic to solve for the time.

8. Dec 7, 2008

### Sassenav22

so its:

(a) V=u+at
0=5+9.8(sin20)t
t = 1.65s

s= ut+1/2at^2
= 5(1.65)+1/2(-9.8)(sin20)(1.65^2)
=3.69m

(b) v=d/t
5=3.69t
t=3.39/5
t=0.68s

9. Dec 7, 2008

### Staff: Mentor

The final answer is still wrong. Redo the calculation.

You'll need to redo this with the correct time.

No. Read my "tip" in post #2.

10. Dec 7, 2008

### Sassenav22

v=u+at
= 5-9.8(sin20)t
t= 1.64s

I still get the same answer

11. Dec 7, 2008

### Staff: Mentor

I'm curious how. t = 5/(9.8sin20) = ??