Solving a Ball Momentum Physics Problem: Understanding Change in Momentum

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Homework Help Overview

The discussion revolves around a physics problem involving the momentum of a ball that falls and rebounds. The original poster presents the scenario of a ball with a mass of 200g, detailing its velocities before and after striking the ground, and seeks assistance in calculating the change in momentum, including its magnitude and direction.

Discussion Character

  • Exploratory, Assumption checking, Conceptual clarification

Approaches and Questions Raised

  • Participants discuss the calculation of change in momentum, with some emphasizing the importance of tracking signs to determine direction. There is also a mention of the relationship between change in momentum, mass, and change in velocity.

Discussion Status

The conversation includes various interpretations of how to calculate the change in momentum, with some participants providing guidance on the formula and the significance of signs. There is no explicit consensus on the final calculation, but productive dialogue is occurring regarding the underlying concepts.

Contextual Notes

Participants note the importance of unit consistency in momentum calculations and highlight that the formula for change in momentum applies only when mass remains constant. There is also a suggestion that the original poster may have overlooked a step in their calculations.

fabbo
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a ball of mass 200g falls to the gound and just before striking the ground its speed is 5m/s. It rebounds with a velocity of 4m/s. What is the magnitude and direction of the change in momentum.

I have the momentum before the rebound as -1kg/m/s and after as 0.8kg/m/s but I don't know how to work out the change or the direction? Can anyone help me do it?
 
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The change (in anything) is final value - initial value. Keep track of your signs and you'll be able to tell the direction.
 
Doc Al said:
The change (in anything) is final value - initial value. Keep track of your signs and you'll be able to tell the direction.

does this mean the change is 0.8 - - 1 giving 1.8kg/m/s?
 
Absolutely. And what does the sign tell you about the direction of the change in momentum?

Note added: Be careful in expressing your units. The units of momentum are kg-m/s (not kg/m/s).
 
Last edited:
Fabbo-the change in mommentum is the product of change in velocity and mass. don't jump to answer the change in mommentum without doing this step. this i think makes it easier for you to understand the change in mommentum. but remember even this that this isonly applicable when the body does not changes its mass.
earlier the velocity is 5units and then the velocity is 4 units in the opposite direction. thus they have opposite signs. rest do alone!
 
vaishakh said:
Fabbo-the change in mommentum is the product of change in velocity and mass. don't jump to answer the change in mommentum without doing this step.
What makes you think he skipped this step?
 

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