Momentum Vectors: Adding or Subtracting in Isolated Systems

  • Thread starter boris16
  • Start date
  • Tags
    Vectors
In summary, the conversation discusses the difference between adding and subtracting velocity vectors and momentums. While it makes sense to add or subtract velocity vectors, it does not hold true for momentums in a system with multiple objects. The concept of momentum is often confused with energy, which is a separate quantity. In a collision between two objects, momentum is conserved while energy can be transferred to other forms.
  • #1
boris16
46
0
hi

If a person on a train moving at speed 100 km per hour is moving in opossite direction with speed of 10 km per hour, then person's speed is 90 km per hour. Adding or substracting velocity vectors makes perfect sense. But adding or substracting momentums doesn't, if it means adding or substracting two momentums that each belong to two DIFFERENT objects in an isolated system.


If in isolated system two balls are approaching each other from opposite directions ( friction is negligible ), one on the left with momentum of 160 and the one on the right with momentum 100, then since momentum is vector quantity the resultant momentum will be 60 and its direction will be to the left.

I know the two balls together represent a system, but at the end of the day they are still two distinctive balls each with its own momentum, so to me substracting the two momentums belonging to DIFFERENT objects is like ... I don't know ... apples and oranges.

I know that when they collide their individual momentums will change but total momentum will stay the same!
So is perhaps the main reason why we add or substract momentums of different objects ( like in the above example ) because system has net momentum of 60 even in a case where objects at collision experience impulse that equals the smaller of the two momentums ( that would be momentum of the ball going left -> M=100), the remaining momentum will still be 60?

thank you
 
Physics news on Phys.org
  • #2
It depends what you call the system. The positive and negative sign is a crude way of indicating the direction of the vector. Summation of the vectors gives the net or resultant vector. Finding the magnitude would be something completely different.
 
  • #3
boris16 said:
Adding or substracting velocity vectors makes perfect sense. But adding or substracting momentums doesn't,
This suggests that you don't really know what momentum is. If a system consists of two objects, one with momentum x and the other with momentum y, then the total momentum is x+y. Simple as that. I suspect that you are confusing momentum with energy. If y=-x, then the two objects can collide and as a result be at rest (say they are really sticky). Momentum is conserved. But the energy is a different matter; energy is not a vector, so the two objects had the same energy of motion to start with, and this energy must re-appear somewhere in the system. For example, the two objects stuck together could end at a higher temperature.
 

1. What is momentum?

Momentum is a property of moving objects that describes their quantity of motion. It is calculated by multiplying an object's mass by its velocity.

2. What is a momentum vector?

A momentum vector is a graphical representation of momentum that includes both magnitude and direction. It is typically represented by an arrow, with the length of the arrow representing the magnitude of the momentum and the direction of the arrow indicating the direction of motion.

3. How do you add momentum vectors in an isolated system?

In an isolated system, the total momentum remains constant and can only be transferred between objects. To add momentum vectors, you must first determine the direction and magnitude of each vector. Then, using vector addition, you can add the vectors together to find the resultant momentum vector.

4. Can momentum vectors be subtracted in an isolated system?

Yes, momentum vectors can be subtracted in an isolated system. This is done using vector subtraction, where the direction and magnitude of the resulting vector are determined by subtracting the individual vectors from each other.

5. Why is it important to consider momentum in isolated systems?

In isolated systems, the total momentum remains constant, which means that the momentum of one object can affect the momentum of another object. By considering momentum, we can better understand and predict the motion of objects in isolated systems, such as collisions or explosions.

Similar threads

Replies
13
Views
791
  • Introductory Physics Homework Help
Replies
1
Views
921
  • Introductory Physics Homework Help
Replies
11
Views
3K
Replies
4
Views
464
  • Introductory Physics Homework Help
Replies
12
Views
4K
  • Introductory Physics Homework Help
Replies
24
Views
2K
  • Introductory Physics Homework Help
Replies
10
Views
948
  • Introductory Physics Homework Help
Replies
2
Views
3K
  • Mechanics
2
Replies
53
Views
2K
  • Introductory Physics Homework Help
Replies
6
Views
979
Back
Top