We often think that naive conceptions of force arise, in part, to living in a frictionful world. And while I think that’s true, I also think it’s just at important to recognize that we basically never ever push or pull things with anything approximating a constant force. Our world is full of impulsive forces, forces that rise and fall in intensity. That would be true with or without friction.
There are lots of cases that are super impulsive like hitting a baseball, kicking a ball, or slamming a door, but even seemingly more constant pushes are not. Pushing a shopping cart, you push and the cart gets away from you, lessening the force you exert on it. It may get away from you enough that you lose contact, but more likely you’ve learned to lessen the force in just such a way as to push only hard enough to get the cart up to a speed, the speed you are comfortable, and then you just exert enough force to maintain a speed (against friction).
It makes me think about how foreign the concept of “constant” force really is. We often like to say that a force is simply a push or a pull, but I’d argue that a constant force is not anything like the pushes and pulls we experience in our lives. It makes me curious to spend more time helping students explore the notion of constant force by learning about just how impossible it is to accomplish. And to realize that the situations textbooks commit ask about are so weird as to almost be absurd.

1. April 6, 2017 3:59 am

This is cool. I’m trying to think of whether there is any real world example of a constant force. The one I think I’ve come up with is a rocket shooting out rocket fuel. But I don’t know enough about how rockets work in the real world to know for sure whether that is actually an example of constant force.

• April 6, 2017 11:45 am

I googled thrust vs time graphs and it doesn’t seem very constant. Although some graphs show a period of relative constancy. All examples I can think of are static… just holding an object. It’s like I want to ask’ “this spring scale has to exert a certain amount force, and that force stays constant as long as I hold the object still.” What would happen if I were to exert that same amount of constant force on the object horizontally? Is that even possible? What would I have to do in order to keep that force amount constant?

2. April 6, 2017 4:04 am

I have some lawn that your students can come over and mow any time that is convenient, all part of a good education. Just kidding. I tore out my lawn as soon as we moved in. Constant forces are too much like thinking -> they’re a lot of work.

3. April 7, 2017 2:01 am

What about moving from impulsive forces to constant forces by extending the length of time the force is applied for? In this way, constant forces just become a limiting case of the familiar impulsive forces. That’s tends to be what we mean by constant force anyways – boxes pulled across the floor aren’t going to be pulled forever.

The most obvious examples of constant forces I could think of are gravity and friction. You could really unpack the idea of free-fall as a way to explore constant forces.

• April 7, 2017 11:10 am

That path through from impulsive to constant makes sense. I’m thinking like, “like wow, how can we ever figure out what’s happening with forces when everything is changing so fast” How could we “stretch” out that process? Constant force investigations are motivated by a need to “stretch” out time? Not sure. Gravity and friction are great examples, except they are “pushes and pulls” that we exert. Students don’t necessarily think of either as “forces”… they think of the them as tendencies. Gravity is tendency of things to fall and friction is tendency to come to a stop.