# Homework Help: Sliding Mass

1. Dec 16, 2008

### hodgepodge

1. The problem statement, all variables and given/known data

A 15 kg mass is sliding at 30 m/s when it encounters a slope (smooth=no friction) and descends 60 m. It encounters a horizontal rough stretch (friction = 80N in opp. direction). Calculate the velocity at x=0 (rough stretch starts here), x=50, x=100, x=200. Calculate where it comes to a stop.

MASS
_______
h =60m \
............\friction = 80N for horizontal line only (indicated by ~)
.............\~~~~~~~~~~
..............x=0 x= 50.....

2. Dec 16, 2008

### Hootenanny

Staff Emeritus
What are your thoughts on the problem?

3. Dec 16, 2008

### hodgepodge

i do not know how to do the sliding part to get the initial speed, but i think from x=0 i would use w=K final- K initial, which would be (friction x m x g)x= 1/2 x mass x final velocity -1/2 x mass x initial velocity

also, when i calculate the velocity for x=50, do i use the final velocity for x=0 for the initial velocity for x=50, or do i use the velocity from when it came down the slope for initial velocity for x=50

Last edited: Dec 16, 2008
4. Dec 16, 2008

### Hootenanny

Staff Emeritus
Let's stick to the first question initially. Can you think of some quantity that will be conserved whilst the block is sliding down the slope?

5. Dec 16, 2008

### hodgepodge

i guess energy would be conserved

this would be easrier but i have a lack of formulas

6. Dec 16, 2008

### Hootenanny

Staff Emeritus
That would be correct. Hopefully, the formulas will come to you as we talk though it.

What two forms of energy are involved in the block sliding down the slope?

7. Dec 16, 2008

### hodgepodge

kinetic and potential

8. Dec 16, 2008

### Hootenanny

Staff Emeritus
Good. So you know that the sum of the potential and kinetic energy at the top and the bottom of the slope must be equal.

What is the value of the total energy at the top of the slope?

9. Dec 16, 2008

### hodgepodge

Total energy = PE + KE
TE = mgh+1/2 x m x v squared
TE= 9000+6750
TE=15750

i used g=10

10. Dec 16, 2008

### hodgepodge

now wat?

11. Dec 16, 2008

### hodgepodge

can anybody help with this?

12. Dec 16, 2008

### glueball8

As you know energy is conserved.

$$E_{total\ before}=E_{total\ after}$$

$$Eg1+Ek1+Et=Eg2+Ek2$$

$$mgh_{1}+\frac{1}{2}mv_{1}^2-F_{f} \triangle d = mgh_{2} +\frac{1}{2}mv_{2}^2$$

always make the lowest point have a gravitional energy of zero

13. Dec 16, 2008

### hodgepodge

wat is Ff change in d?

14. Dec 16, 2008

### glueball8

Ff (80N) is the force in friction and Change in d is x

15. Dec 16, 2008

### hodgepodge

thank you so much bright wang

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