Here’s a quick experiment. Take an object–just about any object will work–and place it on a flat surface. Give it a shove. I don’t think anyone would be surprised to see the object speed up when shoved and slow to a stop afterward.
But it’s not the experiment that’s important. It’s the question:
Why did the block slow down and stop after it was pushed? What force(s) are acting on the block after it was pushed?
I’ve asked a similar question in class for about 10 years, using the excellent curriculum Physics and Everyday Thinking. Here are the most common responses (oh, and this is before collecting experimental evidence on forces and motion).
- Gravity makes the block slow down.
- The block slows down because the force of the push runs out.
- The block slows down because there are no longer any forces pushing it forward.
Of course there are other explanations, but those are the big three. Try this experiment and ask this question with friends and see what they say. Yes, some will discuss friction, but students rarely use friction correctly in this case. But those are ideas students offer before collecting evidence and building a model. Once they collect evidence and build a model, they realize one of several things:
- With low friction cars (on tracks), they give a car a push and see that it moves at a mostly constant speed after the push. The velocity is determined using a computer with a motion detector.
- If you exert a constant strength force on an object, it continues to increase in speed. For this experiment, the students attach small electric fans to the cars and see that they continue gaining speed (at least until the end of the track).
- When the constant strength force (the fan) pushes in a direction opposite of the motion of the car, the car slows.
- With a simulator, students see the forces on a car both while it is pushed and after it is pushed (with no friction).
- A simulator also reveals that in the case of zero friction, a car moves at a constant speed after being pushed.
After these experiments, most of the class has a better understanding of force and motion. Most students understand that after a block is shoved on a table, it slows down because there is a backward force (friction) acting on it.
Now for the fun part. One awesome thing about Physics and Everyday Thinking is that it also explores children’s ideas about force and motion. In this case, the college students watch a video of kids trying to explain why an object slows down after being pushed. Their ideas are almost exactly the same as those of the college students before collecting evidence. And here is the best question in the whole course:
Why is it that 5th graders have similar ideas as adults when it comes to forces and motion?
This is a great question because by now, many of the college students start to realize that there is a difference between being told the answer and building the answer (from experimental evidence) themselves. I like to ask a follow up question.
As a college student, have you “covered” forces and motion in a previous class or is this the first time?
This starts a very interesting discussion. Some students are not sure if this has been covered, but I’m pretty sure it has. In fact, you can look at the National Science Standards or your state’s version of this (I’m not sure why there is a difference). Either way, you should be able to see that this idea (what does a force do to the motion of an object) is included in multiple grade levels. So, almost certainly these students have seen this idea before–but they still get it wrong. Why? Here are some options.
- Perhaps they did not actually cover this idea. Oh sure, it’s in the science standards–but maybe they were absent that day. Maybe that was the day that it snowed so there was no class.
- What if this concept was taught in class (multiple times) but the instructor or textbook explained it incorrectly?
- Another option is that the idea was indeed covered, but it was done in a way that left the students with a lack of understanding.
The debate over which of these is more likely can be interesting. The best part is when most students realize that this class is different. They realize that if you just explain an idea, especially a complicated idea like force and motion, they might be able to repeat it back, but they won’t understand it. Real learning includes a mental struggle. As I’ve said before, “confusion is the sweat of learning.” And the best thing about this particular in-class activity is that students can learn about learning by their own experiences in the class–and that’s awesome.