How is force applied? Is resistance or load necessary for force to exist?

  • Thread starter phoesix
  • Start date
  • #1
phoesix
2
1
I don't know if this is a dumb question, but I just can't seem to wrap my head around it.

I can imagine and feel when I'm pressing against a wall, that there's a force. Especially when the wall is not moving, I can continually increase the strength of my push and feel a greater resistance from the wall.

Same thing with hydraulic presses. When it's crushing something very hard, you can see the force increasing on the gauge.

My confusion involves scenarios when there's little to no resistance involved. Hypothetically, say in the wall example, I can push with 100Nm of force with my arm muscles. With no wall, can I also "push" against empty space in front of me with 100Nm of force? (Let's ignore air resistance or anything like that)

Or in the hydraulic press, say that it's pressing with 100Nm of force against an object that hasn't broken yet. But the second it breaks, you can see on the gauge that the force decreases suddenly. Why isn't the press pressing with 100Nm of force after the object breaks just like it was before it broke?

Or in a gear mesh example, I know that when the tooth of a driving gear applies a force of 100Nm on the tooth of the driven gear, the driven gear also applies 100Nm of force back. But in my mind, assuming there's no friction while rotating, if there's no counter resistance on the driven gear and the driven gear is able to just freely rotate with no resistance or load, I don't see how the driving gear can apply 100Nm of force. If the driving gear wants to increase force, I feel like it will only end up spinning faster, while the force is still nonexistent. Once there's some load on the driven gear, then maybe that's when the driving gear can apply 100Nm of force.

It seems like whenever there's no resistance, whether it's a wall that will immediately collapse when I push it, or an object that breaks under a hydraulic press, or a gear that has no load on it, if you try to apply force in these situations, the force somehow "melts" away because nothing opposes what's being applied. If I try to apply 100Nm of force on one end of a seesaw while the other end has nothing, I feel like I won't be able to apply 100Nm of force in the first place.

In one final scenario I was imagining, I'm poking the empty air in front of me (again ignore actual air molecules creating friction). The way I think of this is, if I'm moving my finger forward slowly, I'm applying only a little bit of force. So then to try to increase force, I feel like I have to lunge my finger forward faster. (Kind of like in the gear example above, the gears just spin faster with no load)

I'm trying to contrast this to a situation where my finger can poke a stationary object, because it's very easy to see and feel force. I can just keep pressing harder with my finger to increase the force or let go of my muscle strain to decrease the force. But in the above situation where it's just the air, I can't seem to understand where the force is and how it's regulated, or if there even is any force to begin with. So is force only possible when there is something to oppose? Does an object that has kinetic energy but does not collide or press against anything as it's moving in a straight line not have any force?

I'm not sure if any of this makes any sense, but please let me know how I can clarify my confusion any further. Thanks in advance!
 

Answers and Replies

  • #2
A.T.
Science Advisor
11,668
2,961
So is force only possible when there is something to oppose?
See Newton's 3rd Law.

Does an object that has kinetic energy but does not collide or press against anything as it's moving in a straight line not have any force?
See Newton's 2nd Law.
 
  • #3
Lnewqban
Homework Helper
Gold Member
2,634
1,426
Welcome, phoesix :cool:
That is a very good question, which deserves a better answer, but here goes mine.
It ssems that a mechanical force is hard to define.
It only manifests when there is interaction between at least two bodies.

The case of the hydraulic press: What you see is the pressure variations of the fluid that exerts pressure on a piston.
There is no reason for that pressure to increase until the moment mechanical compression of an object begins.

Because the object is trapped on the bench, the mechanical energy of the motor of the press becomes deformation and heating of the internal molecular structure of that body.
For creating more of that, the pressure of the fluid and the force of the piston must increase, even if deformation of the shape of the object is very small.

Once the limit of the internal resistance of the object is reached, it will get permanently deformed (if of plastic nature) or will break apart (if brittle material).
Again, there is no reason for that pressure to increase after the moment mechanical compression of that object ends, even if the piston keeps moving.

The case of an object going trough air: Molecules are few and far apart and that object will need to disturb many of those before a noticeable resistance is manifested.
That can be achieved by increasing the area of the object or its speed or both simultaneously.
 
  • #4
33,696
11,283
Summary:: Philosophical question of force?

With no wall, can I also "push" against empty space in front of me with 100Nm of force?
No, that would violate Newton’s 3rd law. Every force on one object is paired with an equal and opposite force on another object. Two such objects are said to be interacting and the forces represent the physical effect of that interaction.

Most materials deform when a force is applied to them, but the amount of deformation for a given force varies. Since contact forces have short range and fall of very quickly, that means that in a contact interaction the weaker material deforms at a low force which suddenly reduces the contact force.
 
  • #5
phoesix
2
1
Thanks so much to everyone who took the time to answer my question. I appreciate it a lot.

Would it be right of me to say, then, that when 2 gears are meshed together with no load and no rotational friction, and if I attempt to press down on one end of the 1st gear with 100N of force, what actually happens is that my attempt will fail, no force makes it through and the 2 gears just spin freely? Is it right to think that I might as well think there's no gears in the first place, and it's akin to attempting to "press" air?

Here's a section of a video about gears that relates a little to what I'm thinking about:


When the 1lb weight is attached to the red gear without any counterweight on the blue gear, the gears spin freely. So am I supposed to think that there's no force of 1lb being "felt" by the blue gear?

How about in the case of an internal combustion engine where the crankshaft is being spun by the pistons. Say I attach a gear to the end of the crankshaft and mesh a 2nd gear to it with no load, so that it's just spinning freely. Is it the case that even if the engine is making the crankshaft spin really fast and with a lot of torque, because there's no load on the gears, there's no force? I'm trying to contrast this to another scenario where the gears indeed are ultimately connected to the tires against the ground, which provide the resistive force against the engine's effort.

This is again the type of scenario where I can imagine "feeling" the force of the engine since there's a load. But remove the load and allow the engine to spin the gears freely, is it correct then that there's no force from the engine despite the fact that it's spinning fast and pushing hard?
 
Last edited:
  • #6
A.T.
Science Advisor
11,668
2,961
Would it be right of me to say, then, that when 2 gears are meshed together with no load and no rotational friction, and if I attempt to press down on one end of the 1st gear with 100N of force, what actually happens is that my attempt will fail, no force makes it through and the 2 gears just spin freely?
You need a force/torque to accelerate the gears. If you also assume they have no rotational inertia or that they spin at constant rate, then indeed no force is applied.
 
  • #7
Lnewqban
Homework Helper
Gold Member
2,634
1,426
Thanks so much to everyone who took the time to answer my question. I appreciate it a lot.
...
How about in the case of an internal combustion engine where the crankshaft is being spun by the pistons. Say I attach a gear to the end of the crankshaft and mesh a 2nd gear to it with no load, so that it's just spinning freely. Is it the case that even if the engine is making the crankshaft spin really fast and with a lot of torque, because there's no load on the gears, there's no force? I'm trying to contrast this to another scenario where the gears indeed are ultimately connected to the tires against the ground, which provide the resistive force against the engine's effort.

This is again the type of scenario where I can imagine "feeling" the force of the engine since there's a load. But remove the load and allow the engine to spin the gears freely, is it correct then that there's no force from the engine despite the fact that it's spinning fast and pushing hard?
You are welcome :smile:
In the case of the engine, there is the potential to "feel" instantaneous force or torque as soon as a resistance to the rotation is imposed.
Any machine that has heavy rotating masses also has rotational inertia.

That means that any load or resistance that naturally tends to slow down or stop those rotating parts, will encounter the resistive torque that is generated during the rotating deceleration of those parts.
The magnitude of that resistive torque equals the product of the moments of inertia and the angular decelerations of the chain of all those internal parts.

Please, see:
http://hyperphysics.phy-astr.gsu.edu/hbase/hframe.html
 
  • #8
wrobel
Science Advisor
Insights Author
997
859
Here's a section of a video about gears that relates a little to what I'm thinking about:
looks like he equalizes torques about different axes
 

Suggested for: How is force applied? Is resistance or load necessary for force to exist?

Replies
9
Views
472
Replies
6
Views
381
  • Last Post
Replies
5
Views
367
Replies
4
Views
548
Replies
10
Views
255
Replies
2
Views
669
Replies
43
Views
3K
Replies
11
Views
526
Replies
4
Views
484
Replies
1
Views
128
Top