Trying to understand power/work

  • Context: Undergrad 
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Discussion Overview

The discussion revolves around the concept of power and work in the context of mechanical systems, particularly focusing on how power relates to the ability to lift or move masses over time. Participants explore the implications of power in both vertical and horizontal movements, questioning how power affects acceleration and speed in different scenarios.

Discussion Character

  • Exploratory
  • Technical explanation
  • Debate/contested

Main Points Raised

  • One participant notes that a forklift or winch with "twice" the power can lift twice the mass in the same time or the same mass in half the time, but questions how this applies to horizontal movement.
  • Another participant suggests that tractors do not pull at the same power regardless of speed, implying that if they did, velocity would increase quadratically.
  • It is mentioned that tractors typically pull against a constant load, and that power equals force times speed, indicating that a tractor with double the power could pull the same load at twice the speed.
  • A later reply introduces a scenario comparing two cars with different engine powers, questioning if a car with double the power can achieve a faster acceleration without a greater force.

Areas of Agreement / Disagreement

Participants express confusion and uncertainty regarding the relationship between power, force, and acceleration. There is no consensus on how power affects acceleration in the context of the examples provided.

Contextual Notes

Participants highlight the complexity of power in relation to kinetic energy and acceleration, noting that the relationship is not straightforward and depends on various factors such as load and speed. There are unresolved assumptions regarding the effects of friction and other forces in real-world scenarios.

Who May Find This Useful

This discussion may be useful for individuals interested in understanding the principles of power and work in mechanical systems, particularly in the context of vehicles and machinery.

southernson
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you see a lot of reference to the fact that a forklift or winch or something with "twice" the power of a another one can lift twice the mass in the same time, or the same mass in half the time, etc. This makes sense.

Power in the horizontal direction makes less sense to me. If a tractor has twice the power it can similarly do twice the work in the same time, but can it pull the same mass from point A to B in half the time? If you look at power as KEf-KEi/t, it would appear that it can't because velocity is squared in the KE equation (KE = 1/2mv^2).

Can someone help me clarify this?
 
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southernson said:
you see a lot of reference to the fact that a forklift or winch or something with "twice" the power of a another one can lift twice the mass in the same time, or the same mass in half the time, etc. This makes sense.

Power in the horizontal direction makes less sense to me. If a tractor has twice the power it can similarly do twice the work in the same time, but can it pull the same mass from point A to B in half the time? If you look at power as KEf-KEi/t, it would appear that it can't because velocity is squared in the KE equation (KE = 1/2mv^2).

Can someone help me clarify this?

Well, tractors don't pull at the same power regardless of speed. But if they they did then I guess the velocity of the tractor would increase quadratically (assuming no friction).
 
Tractors are normally pulling against a constant load, not acceleration. Power equals force times speed, so a tractor with double the power will be able to pull the same load (tension force) at twice the speed.

In the case of acceleration with a constant mass, since the change in energy is relative to 1/2 m V^2, then twice the power only increases the rate of acceleration by SQRT(2) (with the same mass), and it takes 4 times the power to double the rate of acceleration (or think of 4 times the power as twice as much force at twice as much speed with constant acceleration).
 
Sorry guys, I'm still confused. Let's use a different example. Say just a car with an engine of power P and another with a power of 2P. If the first can go 0 to 60m/s in 20 seconds can the second to it in 10 seconds? If so, this can only happen by the second engine causing a faster acceleartion. How is that going to happen without a greater force?
 

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