# Homework Help: Need help with physics

1. Jan 13, 2008

### Dr physics

need help with physics plz!!!

heres a list of question im having difficulty with.

1)define clearly the principle of conservation of momentum and apply this to solve the following problem:
A man weighing 730N stands in the middle of a frozen pound of a radius 5.9m. he is unable to get to the other side becuase of lack of friction between his shoes and the ice. to overcome this problem, he throws his 1.2kg book horizontally towards the north shore at a speed of 5m/s. how long does it take him to reach the south shore?

2) a 40g ball travelling to the right , at 30 cm/s collides, in a stright line, with 80g ball that is at rest. if the collision is perectily elastic, what is the velocity of each ball after collision?

plz could you help me with this im completly a newbie to physics, so please could you help.

thank you very much

2. Jan 13, 2008

### Tom Mattson

Staff Emeritus
What have you done so far?

3. Jan 13, 2008

### malawi_glenn

if you are a newbie in physics, then this is called introductory physics. So first of all this is aksed in wrong forum.

Second you must show an attempt to a solution and the relevant equations that you can use, according to the forum rules.

4. Jan 13, 2008

### Dr physics

for the 1st question i know to use this formula

(before).............(after)
(m1+m2) * V = m1V1 + m2 * V2

5. Jan 13, 2008

### Tom Mattson

Staff Emeritus
Right, and what is V if the guy is just standing there with his book?

6. Jan 13, 2008

### Dr physics

v = 0

7. Jan 13, 2008

### Dr physics

o no ur offline , sry abpout that my comp was slow

8. Jan 14, 2008

### Dr physics

9. Jan 14, 2008

### Tom Mattson

Staff Emeritus
OK, so you know that the left side of the equation is zero. Now there's only one unknown. Can you plug in everything you know, and solve for it? Take a shot at it, and let's see what you come up with.

10. Jan 14, 2008

### Dr physics

(m1+m2) * V
inital = m1 (750N) * V1 (0) = 0

final = m1 (750N) V1 (0) + m2 (1.2kg) * V2 (5m/s)
= 0 + 6.25 = 6.25 seconds

hope this is right

11. Jan 14, 2008

### Tom Mattson

Staff Emeritus
None of what you wrote makes any sense. Let m1 be the mass of the man, let m2 be the mass of the book (both of these should be in kg). So v2 is the speed with which the man throws the book. All you have to do is substitute those values into the momentum equation, and solve for v1.

Try again?

12. Jan 14, 2008

### Dr physics

(before).............(after) 750N = 76.53kg
m1V1 + m2 * V2

76.53 * 0 + 1.2 * 5 = 6

13. Jan 14, 2008

### Dr physics

then to work out how long it took do u do this: -

d = v × t = t = v / d

= t = 6 / 5.9m = 1.02 seconds?

14. Jan 14, 2008

### Dr physics

wait have i worked out v1 wrong

is it ment to be: -

v1 = m1 - m2 * V2

15. Jan 14, 2008

### Dr physics

im really confused

16. Jan 14, 2008

### Tom Mattson

Staff Emeritus
This is quite sloppy. 750N does not "equal" 76.53kg. They don't have the same units, so they certainly cannot be equal to each other.

Why are you plugging in 0 for V1? The whole point of the problem is for you to solve for V1. We don't know what it is yet, but it obvioulsy isn't zero. Conservation of momentum tells us that the forward momentum of the book is going to be equal in magnitude to the backward momentum of the man.

17. Jan 14, 2008

### Dr physics

but i thought u use the formula M = F / A whereby "A" is constant 9.8

18. Jan 14, 2008

### Dr physics

is this write from what you i said previously, as this way i am working out the value of V1?

19. Jan 14, 2008

### Tom Mattson

Staff Emeritus
Yes, you are supposed to use that formula. And "750N = 76.53kg" is clearly not the same thing as that.

20. Jan 14, 2008

### Tom Mattson

Staff Emeritus
No, that is not right. You need to check your algebra.

21. Jan 14, 2008

### Dr physics

v1 = m1 - m2 / V2

22. Jan 14, 2008

### Tom Mattson

Staff Emeritus
Again, no. If you start with $m_1V_1+m_2V_2=0$, and you want to solve for $V_1$, you need to do 2 steps of algebra. I really can't say any more than that without doing the problem for you (which I'm not going to do), so try posting the 2 steps so I can see exactly what you're doing wrong.

23. Jan 14, 2008

### Dr physics

v1=m2v2/m1

24. Jan 14, 2008