The Misconception: In order for there to be motion there must be a force continually acting on the object.

Examples:
  • An elevator is moving at a constant speed upwards. People might say there was some kind of net force causing that motion.
  • After sledding down a hill the sled is gliding across the flat snow (neglecting friction) at a constant velocity. Again, it might seem to someone that force would act on the sled to keep it moving.
  • Sliding a book across a frictionless table with a constant velocity-the same idea applies here as well.

Some Explanation
This misconception can be cleared up with a clear understanding and application of Newton’s Laws. In addressing this misconception the first and second law will be emphasized. The first law being what we all probably have memorized by heart: “An object in motion stays in motion unless acted on by an unbalanced force.” And the second law being F=ma.
In each case above, there is constant velocity. This implies there is no change in velocity which means there is no acceleration. Since F=ma is dependent on acceleration, there is no force. And in all of these cases, the object is already moving, so it will continue moving because there is no net force. Although the force of gravity is always there, the normal force cancels out this force, causing the net force to be zero and the object to have a constant velocity.
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This leads to the question:
Why Does this Misconception Exist?
In many cases, the misconception still exists because the idea of motion without force is very unintuitive. It would seem that if something is moving, there must be something causing it, but as shown, this is not always the case. It appears that people don’t always know the difference between a net force and a regular force. In order to get an idea about how prominent this misconception is, a survey was conducted:
Say you’re on vacation and are heading to your hotel room. You walk into the elevator and push the button to go to the 3rd floor, you feel a slight jerk as it moves upward (acceleration). And then you notice nothing (constant velocity). You then arrive to the 3rd floor and feel another slight jerk as it comes to a stop (deceleration).

Below are four different stages of your ride: when you first got on the elevator, when it first accelerated, when it was at a constant velocity and when it decelerated and arrived at your floor. Using the pictures below, choose one of the options below that correctly represents whether the elevator had a NET force (forces that don't cancel out) acting on it or didn’t in each part of the scenario. Hint: F=ma. No cheating!!


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When I started getting results from this survey, I was pleasantly surprised. 13 out of 19 (about 70%) of participants were able to answer correctly. So this got me thinking about some possible reasons why the results turned out this way:
  1. They cheated and googled the answer.
  2. They have a background in physics.
  3. Since it is such a small group, it is not as accurate.
  4. Most people have had this idea seared into their education and they just “know” the right answer, but don’t necessarily understand.
  5. The way I phrased and explained the question made it easier for them to figure out the right answer.
  6. A combination of 3 and 4.

If I could do it over, I would have an answer text box so participants could give an explanation of why they put the answer they did. However, I can make some inferences. Since I specifically said not to cheat, I would hope most people didn’t and I am pretty sure a majority of the participants do not have a very strong background in physics. This pretty much rules out the first two reasons. Number three seems to be very possible since people hear about constant velocity and that there aren’t any net forces involved. But they probably don’t fully understand. Reason four appears to be very possible as well since I clearly explained what a net force was. I think the most probable, was probably reason 5. People came into the survey with a little bit of background, and then I reinforced what they had heard. Either way, I realized one of the main reasons motion is associated with force, is because people don’t always understand the difference between a net force and a regular force. The way the question is worded or how the concept taught is very important.

Solutions
Some ways to help others not become infected with this misconception:

Pictures: http://www.physicsclassroom.com/Class/newtlaws/u2l3b.cfm