Solve Conservation of Momentum Problems | Physics Homework Help

[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 $m_1$"!

 

[Dr physics]

im sorry i knew i shouldn't have, made a silly mistake sry about that.

M1V1 + m2V2 = 0
M1V1 + M2V2 - M2V2 = 0 -M2V2 = -M2V2
M1V1 / M1V1 = -M2V2 / M1

 

[quantumdude]

im sorry i knew i shouldn't have, made a silly mistake sry about that.

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

Right.

M1V1 / M1V1 = -M2V2 / M1

Again, you're dividing by M1V1. This is not right. The last line should read:

M1V1/M1=-M2V2/M1

Then we obtain V1=-M2V2/M1. You can then plug in the numbers you were given to find V1.

 

[Dr physics]

M2 = 1.2kg
V2 = 5m/s
M1 = 750 N
V1 = ?

V1 = -M2V2/M1
V1 = -1.2 * -5 / 76.53 = 0.0784

is that correct?

 

[quantumdude]

M2 = 1.2kg
V2 = 5m/s
M1 = 750 N

As I have already explained to you, M1 is not 750N. It is 76.53 kg. But you did use the correct number below, so this mistake is minor.

V1 = ?

V1 = -M2V2/M1
V1 = -1.2 * -5 / 76.53 = 0.0784

is that correct?

No, it isn't. The answer should obviously be negative. Think about what you're doing. If the book moves in one direction, then obviously the man has to move in the other direction! Why did you insert that extra negative sign in your last step? It shouldn't be there.

 

[Dr physics]

V1 = ?

V1 = -m2v2/m1
V1 = -1.2 * 5 / 76.53
V1 = -6 / 76.53 = -0.0784

 

[Dr physics]

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 because 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?

to work out how long it take him to reach the south shore do you use this formula

d = vi + vf / 2 * t = t = vi + vf / 2 / d

t = -0.0784 + 5 / 2 / 5.9
= 4.922 / 2 / 5.9
= 5.9 / 2.461
t = 2.34 s

hope that's right

 

[quantumdude]

V1 = ?

V1 = -m2v2/m1
V1 = -1.2 * 5 / 76.53
V1 = -6 / 76.53 = -0.0784

Correct. And the units are m/s.

 

[quantumdude]

to work out how long it take him to reach the south shore do you use this formula

d = vi + vf / 2 * t = t = vi + vf / 2 / d

No, you don't use that formula. In that formula, vi and vf refer to the initial and final velocities of the same object under uniform acceleration. You are trying to apply it to two different objects. You need to stop guessing and start thinking if you are going to succeed at this.