Recoil velocity on frictionless surface

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Homework Help Overview

The discussion revolves around estimating the recoil velocity of a child standing on frictionless ice after throwing a snowball. The subject area includes concepts of momentum and kinetic energy in the context of physics problems involving motion and collisions.

Discussion Character

  • Exploratory, Conceptual clarification, Mathematical reasoning

Approaches and Questions Raised

  • The original poster attempts to apply both momentum and kinetic energy equations to find the final velocity but expresses uncertainty about which equation is appropriate for the situation. Other participants inquire about the details of the calculations and the conditions under which kinetic energy is conserved.

Discussion Status

Participants are actively engaging in clarifying the application of the equations, with some noting the importance of recognizing when kinetic energy conservation applies. There is a recognition of the complexities involved in the energy dynamics of the scenario, particularly regarding the role of external forces and energy input from the child.

Contextual Notes

There is a mention of the need for detailed work to be shown, indicating that the original poster may have overlooked certain calculations. The discussion also touches on the implications of inelastic versus elastic collisions in the context of the problem.

Valenti
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Homework Statement


A child stands on frictionless ice and throws a snowball. Estimate the recoil velocity of the child.

Homework Equations


m1v1i + m2v2i =m1v1 +m2v2f
1/2mv21i + 1/2 mv22i = 1/2 mv21f + 1/2mv22f

The Attempt at a Solution


After choosing estimates for weight of snowball, speed of child throwing snowball and weight of child I plugged it into the momentum and kinetic energy laws and solved for the final velocity in both of them, but I am not sure which equation is supposed to be used and in what situations
 
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Show your work in detail.
 
gneill said:
Show your work in detail.
Thanks, checked my work again and noticed i forgot to sq root for Vf in the KE formula, since it was negative only the momentum equation works. What type of questions would the KE formula be used though?
 
Kinetic energy is conserved when no external forces act on the system, or when there are no other avenues for energy loss or gain from the system. In perfectly elastic collisions kinetic energy is conserved. In inelastic collisions kinetic energy is not conserved.

A person throwing an object is equivalent to a "reverse" inelastic collision, where two bodies that were once connected are separated by some energy being injected: the person throwing the snowball is providing energy via their muscles, which is not accounted for by the simple conservation of KE formula applied to the individual masses.
 

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