Solve Conservation of Momentum Problems | Physics Homework Help

[Dr physics]

need help with physics please!

heres a list of question I am 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 traveling 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?


please could you help me with this I am completely a newbie to physics, so please could you help.

thank you very much

 

[quantumdude]

What have you done so far?

 

[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.

 

[Dr physics]

sry didnt new to thread.

for the 1st question i know to use this formula

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

 

[quantumdude]

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

 

[Dr physics]

v = 0

 

[Dr physics]

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

 

[Dr physics]

please reply

 

[quantumdude]

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.

 

[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

 

[quantumdude]

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?

 

[Dr physics]

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

76.53 * 0 + 1.2 * 5 = 6

 

[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?

 

[Dr physics]

wait have i worked out v1 wrong

is it ment to be: -

v1 = m1 - m2 * V2

 

[Dr physics]

im really confused

 

[quantumdude]

750N = 76.53kg

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.

m1V1 + m2 * V2

76.53 * 0 + 1.2 * 5 = 6

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.

 

[Dr physics]

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

 

[Dr physics]

wait have i worked out v1 wrong

is it ment to be: -

v1 = m1 - m2 * V2

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

 

[quantumdude]

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

 

[quantumdude]

wait have i worked out v1 wrong

is it ment to be: -

v1 = m1 - m2 * V2

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

 

[Dr physics]

v1 = m1 - m2 / V2

 

[quantumdude]

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.

 

[Dr physics]

v1=m2v2/m1

 

[lasershadow]

Edit: Accidental post

 

[quantumdude]

v1=m2v2/m1

You're missing a minus sign, aren't you?

 

[Dr physics]

v1=m2-v2/m1

 

[Dr physics]

so is that right for the 2 steps in doing algebra. first i rearraged it, then change the signs

 

[Dr physics]

v1 = m2 - V2 / m1

m2 = 1.2kg
V2 = 5 m/s
m1 = 76.53

v1 = 1.2 - 5 / 76.53

V1 = -3.8 / 76.53

V1 = -0.049654

V1 doesn't look right does it? is it suppose to be 76.53 - 1.2 / 5?

 

[quantumdude]

v1=m2-v2/m1

No. Let me ask you something: Have you ever taken a course in algebra? If so, then how would you solve the equation 2x+1=4?

 

[Dr physics]

2x+1 = 4

1-4 = 3

3/2 = 1.5

x = 1.5

2 * 1.5 + 1 = 4

 

[quantumdude]

OK, now try to do the same thing with your previous equation. It really is no different.

$m_1V_1+m_2V_2=0$

Can you show the following two steps?

1.) Subtract $m_2V_2$ from both sides.
2.) Divide by $m_1$.

Then you will have solved for $V_1$. But be careful about the negative sign that appears after step 1.

 

[Dr physics]

M1 + m2V2 = 0
M1 + M2V2 - M2V2 = 0 -M2V2
M1 / M1 = M2V2 / M1

 

[quantumdude]

M1 + m2V2 = 0

That is not the equation. Where did V1 go??

 

[Dr physics]

o sry,

M1V1 + m2V2 = 0
M1V1 + M2V2 - M2V2 = 0 -M2V2
M1V1 / M1V1 = M2V2 / M1V1

 

[quantumdude]

o sry,

M1V1 + m2V2 = 0

OK, that's the equation.

M1V1 + M2V2 - M2V2 = 0 -M2V2

Yes.

M1V1 / M1V1 = M2V2 / M1V1

No. First of all, what happened to that negative sign that I cautioned you to be careful about? You do understand that 0-M2V2=-M2V2, right? And second, why are you dividing by M1V1? You're supposed to be solving for V1. That's why I said in step 2, "divide by $m_1$"!