Does a constant force = Constant Power?

  • Context: Undergrad 
  • Thread starter Thread starter torquemada
  • Start date Start date
  • Tags Tags
    Constant Force Power
Click For Summary

Discussion Overview

The discussion revolves around the relationship between constant force and power, particularly whether a constant force results in constant power over time. Participants explore concepts related to work, power, and the effects of acceleration and displacement in various contexts.

Discussion Character

  • Exploratory
  • Technical explanation
  • Debate/contested
  • Conceptual clarification

Main Points Raised

  • One participant questions if power increases with time when a constant force is applied, considering displacement over time.
  • Another participant states that power is calculated as force times speed, implying that as speed increases, power also increases.
  • There is a discussion about whether the increase in speed is due to constant acceleration from a constant force.
  • Participants debate the nature of power, with one suggesting it is a characteristic of the force itself, while others argue that power depends on the motion of the object being acted upon.
  • It is noted that power is a measure of the rate at which a force does work, and that a force alone does not imply power without displacement.
  • Examples are provided, such as pushing against a wall where no work is done, and a scenario where a constant force results in constant velocity due to opposing friction, leading to constant power.

Areas of Agreement / Disagreement

Participants express differing views on the relationship between force and power, with some asserting that power is dependent on the motion of the object, while others suggest it can be a characteristic of the force itself. The discussion remains unresolved regarding the implications of constant force on power.

Contextual Notes

Participants highlight that the relationship between force, speed, and power can vary based on the specific conditions of the system, including the presence of opposing forces like friction. There are also mentions of different frames of reference affecting the observed power.

torquemada
Messages
108
Reaction score
0
Hey I'm trying to teach myself physics using H&R. Unfortunately I didn't take trig physics in high school so every concept is new to me. But I have taken a year of calc (up to but not including Multivariable) so i understand most of what I read (at least superficially). But i have to ruminate over things before I understand them (who doesn't i guess). I'm trying to understand if a force is constant and there is a displacement over time, then will the power increase with time or will the power be constant. In other words, will the scalar quantity u get by multiplying displacement with force always turn into the same value once its divided by time elapsed? Or does the scalar value increase faster than time passes, thus yielding an increase in power over time even with a constant force? thanks for your help
 
Physics news on Phys.org
Power is work/time = force*distance/time = force*speed. So as the speed increases, so does the power.
 
Doc Al said:
Power is work/time = force*distance/time = force*speed. So as the speed increases, so does the power.

Ok thanks! :)

And the speed increase would be caused by the constant acceleration of a constant force yes?
 
torquemada said:
And the speed increase would be caused by the constant acceleration of a constant force yes?
Right. (Presuming that the force in question is the only force or the net force.)
 
Ok so the idea of power is a bit confusing. If an object is moving at a certain speed and a force is accelerating it, then you multiply those two quantities to get power. But that is the power of what? The power of the system? Or the power of the force acting on the system? Or something else?
 
ok so i did more reading and research and it seems the power is a characteristic of the force itself. So if a force is constant then that force's power, it's ability to do X work in a given time interval, would increase.
 
A force does not have a single specific power associated with it. Force times velocity is power. So a force may generate any power (including none) depending on the motion of what it is acting on. It can even vary depending on the frame of reference from which you observe it!
 
Power is not a characteristic of only the force, if you had 2 equal and opposite forces acting on an object at rest, for example, no power would be done by either force.
 
torquemada said:
Ok so the idea of power is a bit confusing. If an object is moving at a certain speed and a force is accelerating it, then you multiply those two quantities to get power. But that is the power of what? The power of the system? Or the power of the force acting on the system? Or something else?
The power is just a measure of the rate at which that force is doing work on the object, thus transferring mechanical energy to the object.
torquemada said:
ok so i did more reading and research and it seems the power is a characteristic of the force itself.
As others have explained, just having a force implies nothing about power without knowing it's displacement. For that force to do work on something, there must be a displacement in the direction of the force. At any point, the instantaneous power will be force*speed (more precisely, P = \vec{F}\cdot\vec{v}).
So if a force is constant then that force's power, it's ability to do X work in a given time interval, would increase.
That's true only because the object accelerates (which we stipulated earlier). But you can certainly have situations where a force does no work at all, or where it's power is constant.
 
  • #10
Doc Al said:
But you can certainly have situations where a force does no work at all, or where it's power is constant.

Pushing against the wall I've read is an example of the former. What would be an example of the latter? Would one example be where a constant horizontal force across a surface yields a constant velocity due to an equal force of kinetic friction from the floor?
 
  • #11
torquemada said:
Pushing against the wall I've read is an example of the former.
Good.
What would be an example of the latter? Would one example be where a constant horizontal force across a surface yields a constant velocity due to an equal force of kinetic friction from the floor?
Yes. In that case, the work done ends up as 'heat' (thermal energy) due to work against friction, instead of increasing the speed (and kinetic energy) of the object.
 
  • #12
Ok - it's starting to coalesce in my mind. Thanks for all your help guys.
 

Similar threads

High School How Does Constant Power Affect Rocket Acceleration in Space?
  • · Replies 13 ·
Replies
13
Views
940
Undergrad Mechanical power generated by a force F
  • · Replies 41 ·
2
Replies
41
Views
4K
High School Why is speed constant when centripetal acceleration is constant?
  • · Replies 24 ·
Replies
24
Views
4K
High School Why is my experimental spring constant an order of magnitude off?
  • · Replies 4 ·
Replies
4
Views
2K
High School Does acceleration affect impact energy vs constant velocity?
  • · Replies 46 ·
2
Replies
46
Views
4K
Graduate Who Exerts Gravity Power? Force x Velocity Dot Product
  • · Replies 19 ·
Replies
19
Views
2K
Undergrad Question Regarding Force Being Equal, Even Moving Upwards (if V=0)?
  • · Replies 2 ·
Replies
2
Views
1K
Undergrad Acceleration as a Function of Time Using Constant Power
  • · Replies 27 ·
Replies
27
Views
4K
Undergrad The Plane in the Wind Puzzle: Does a Constant Wind Affect the Round Trip Time?
  • · Replies 5 ·
Replies
5
Views
3K
Undergrad Car acceleration if resistance forces don't exist
  • · Replies 95 ·
4
Replies
95
Views
7K