Constant Force and Constant Motion

Constant Force and Constant Motion

Constant Force and Constant Motion

I see this problem all the time. The idea that if you apply a constant force on an object, it should move at a constant speed. Really, this is what Aristotle would say, right?

There are two course that have non-science majors: Physics for Education Majors (using the Physics and Everyday Thinking curriculum) and Physical Science (using Learning Physical Science). I assume you know that I love both of these curriculums. In them, students have the opportunity to build ideas based on the evidence they collect. In terms of force and motion, they see several things like:

  • Using a motion detector to see that the speed of a cart increases when you push it.
  • Using a fan on a cart (with a motion detector) to see that with a constant force, the cart continues to increase in speed.
  • With a simulator, the students see that constant force produces an increase in speed.
  • The simulator also shows with no friction, an object will move at a constant speed.

At some point during the collection of this data, the following question (or something similar) will come up:

What do you think will happen to an object with a constant force?

At this point, the students technically already have enough evidence to see that a constant force will mean the object will move with a continuously increasing speed (if the force is in the same direction as the motion of the object). However, these ideas don't usually stick. They still have this battle inside their head. A battle between Aristotle and Newton sitting on their shoulders. This is normal.

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Here is the interesting part. It seems that every semester when this discussion comes up, someone says this:

"Well, I think that a constant force will make the thing go at a constant speed. It just makes sense. Look at your car. You push on the gas pedal with a constant force and the car goes at a constant speed."

For some reason, the students think of pushing the gas pedal with a constant force as the same as pushing the CAR with a constant force. Perhaps this is because the gas pedal is part of the car. Maybe they are just trying to bring one of their own experiences into the discussion. Really, I am all too sure why it keeps coming up.

Of course, in a way these students are correct. For a car traveling at highway speeds, there is a constant force (from the friction between the tires and the road) pushing on the car. But for this case, they are not pushing the car with their foot - well, unless they are Fred Flinstone with a foot powered car.

If this gas pedal force problem comes up so often, what do I do about? Mostly, nothing. Usually, someone in the class will realize that they aren't pushing a car with their foot. Also, I have never had a class where someone didn't point back a previous experiment like this:

"I get what you are saying about the gas pedal. But, don't you remember the cart with the fan on it? We saw the fan pushed with a constant force, but the cart just kept getting faster and faster."

I think these students see this gas pedal idea is just an attempt to cling to their old ideas. It is painfully obvious that the new data says it isn't true. But like I said, change is difficult.