# Which directions should be set to positive? Impulse/momentum

• Specter
In summary, in a physics lab, a 0.30 kg puck A moving at 5.0 m/s [W] collides with a 0.40 kg puck B at rest. After the collision, puck A moves off at 4.2 m/s [W 30 degrees N] and the final velocity of puck B can be determined by choosing a direction as positive and using the equation for conservation of linear momentum. The convention in two dimensions is "to the right" is positive and "up" is positive, and textbooks often follow this convention for simplicity.
Specter

## Homework Statement

In a physics lab, 0.30 kg puck A, moving at 5.0 m/s [W], undergoes a collision with 0.40 kg puck B, which is initially at rest. Puck A moves off at 4.2 m/s [W 30 degrees N]. Find the final velocity of puck B.

## The Attempt at a Solution

One thing I haven't understood is which directions to set positive. For example "Let north and east be positive". How do you know which direction to set positive? In this question, would I let north and west be positive? At the beginning of each example question in this unit, they set two directions to positive, but they don't explain why, or how to do it.

Thanks.

Last edited by a moderator:
You can choose any direction you wish as positive. The convention in two dimensions is "to the right" is positive and "up" is positive. Textbooks follow this convention to make things easier for beginners. With your specific problem, I would choose as positive the direction in which puck A is moving before the collision. The 30 degree angle is probably with respect to that direction.

esale and Specter
kuruman said:
You can choose any direction you wish as positive. The convention in two dimensions is "to the right" is positive and "up" is positive. Textbooks follow this convention to make things easier for beginners. With your specific problem, I would choose as positive the direction in which puck A is moving before the collision. The 30 degree angle is probably with respect to that direction.
Okay, thanks!

You can choose. Most coordinate systems are arbitrary/relative/subjective. What matters the most is that you are consistent. So, in this instance, if you pick W as positive, then E better be negative. Same goes for N and S.

Also, it helps to understand the [W 30 degrees N] here and then Cartesian system becomes arbitrary. It means you have a vector pointing out straight west, pin the origin down, then rotate the end of the vector 30 degrees toward N. After you understand this, make your point of contact the origin, and you can solve from there.

-E

Specter

## 1. What is the difference between impulse and momentum?

Impulse and momentum are both measures of an object's motion, but they refer to different aspects. Impulse is the change in an object's momentum over time, while momentum is a measure of an object's mass and velocity. In simpler terms, impulse refers to the force applied to an object, while momentum refers to the object's movement.

## 2. Why is it important to set directions to positive when calculating impulse and momentum?

Setting directions to positive is important because it allows for consistency in calculations. By setting a direction as positive, we establish a reference point from which all other directions are measured. This helps to avoid errors and makes it easier to interpret the results.

## 3. Can the direction of impulse and momentum be different?

Yes, the direction of impulse and momentum can be different. This is because impulse is a vector quantity, meaning it has both magnitude and direction. Momentum, on the other hand, is a vector quantity only in situations where the object's mass or velocity changes.

## 4. What happens when the direction of impulse and momentum is opposite?

If the direction of impulse and momentum are opposite, it means that the object experiences a change in velocity in the opposite direction of the applied force. This can happen, for example, in situations where an object bounces off a surface or when a force is applied in the opposite direction of an object's movement.

## 5. How does the direction of impulse affect the overall motion of an object?

The direction of impulse can have a significant impact on the overall motion of an object. If a force is applied in the same direction as an object's movement, it will increase the object's momentum and speed. On the other hand, if the force is applied in the opposite direction, it will decrease the object's momentum and slow it down. The direction of impulse is crucial in determining the direction and magnitude of an object's final velocity.

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