# Conservation of momentum in one direction (add or subtract?)

• Chris Jarvis
In summary, the conversation discusses a formula for calculating momentum in one direction and when to add or subtract values. An example is provided for a football collision and a physics experiment. The formula is also shown for both scenarios. The conversation concludes with the question being answered and the issue being resolved.
Chris Jarvis

## Homework Statement

Hey everyone! I'm new to these forums so I apologize in advance if I do something wrong! I just had a quick question about the momentum in one direction equation m_1v_1o +/- m_2v_2o = m_1v_1f +/- m_2v_2f; I'm just not sure when I'm supposed to add these values and when I'm supposed to subtract them. I thought it had to do with the objects vector, but there is conflicting questions in my lesson (I think at least). I'm doing an online course so I don't really have a teacher to ask lol
For example:
Football player 1 with a mass of 110kg is moving at 8.0m/s when he crashes into football player 2 with a mass of 105kg, moving at 12 m/s [N]. During the tackle football player 1 holds onto football player 2, giving them the same velocity after 0.30s.
a) Find the final velocity of each player after collision

In a Physics experiment, a 1.2kg cart1 is moving at 3.2m/s [R], when it collides with a 1.8kg cart 2 which is moving at 5.8m/s[L]. After the collision cart1 is moving at 7.6m/s[L].Find the velocity for cart2 after the collision.

## Homework Equations

m_1v_1o + m_2v_2o= (m_1+m_2)v_f
m_1v_1o - m_2v_2o = m_1v_1f + m_2v_2f

## The Attempt at a Solution

So for the football one my book shows;
Let [N] be positive
P_to=P_tf
m_1v_1o + m_2v_2o= (m_1+m_2)v_f
(110kg)(-8.0m/s)+(105kg)(12m/s)= (110kg +105kg)v_f
v_f=1.77m/s[N]

For the Physics experiment one:
P_1o-P_2o=P_1f+P_2f
Let [R] be positive;
m_1v_1o - m_2v_2o = m_1v_1f + m_2v_2f
(1.2kg)(3.2m/s)-(1.8kg)(5.8m/s)=-(1.2kg)(7.6m/s)+(1.8kg)v_2f
-6.6kgm/s= -9.12kgm/s + (1.8kg)v_2f
v_2f= 1.4m/s [R]
So Is it because the two forces are combining in the first that you add? and in the second one they bounce away so you subtract?
Sorry if this is a really simple question, I haven't done physics in almost 8 years!

Is there a reason most of your text is struck out? Did you still have a question?

Chris Jarvis
Put (+) everywhere and use vector direction to see the correct sign.

Chris Jarvis
Hey sorry everyone I didn't respond I went on vacation and dint have internet but i figured it out thank you both!

## 1. What is conservation of momentum in one direction?

Conservation of momentum in one direction is a fundamental principle in physics that states that the total momentum of a system will remain constant in the absence of external forces in that particular direction.

## 2. Why is it important to consider conservation of momentum in one direction?

Conservation of momentum in one direction is important because it allows us to predict the motion of objects in a given system. It also helps us understand how forces act on objects and how they affect their motion.

## 3. How do we add or subtract momentum in one direction?

Momentum can be added or subtracted in one direction through interactions between objects. When objects collide, momentum can be transferred from one object to another, resulting in a change in motion.

## 4. Does conservation of momentum only apply to linear motion?

No, conservation of momentum can apply to any type of motion, including rotational motion. In rotational motion, the moment of inertia is used instead of mass to calculate momentum, but the principle remains the same.

## 5. Can momentum be created or destroyed in one direction?

No, according to the law of conservation of momentum, momentum cannot be created or destroyed in one direction. It can only be transferred between objects or converted into other forms, such as heat or sound energy.

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