Here is a nice video from Veritasium.com that asks random people about the motion of falling objects with different masses. A classic question and a question that is very commonly misunderstood.
There are lots of things to say about this video, but here is my favorite part. In discussing which ball has a greater gravitational force on it, one person says
"I believe it should be more on this [the heavier ball] simply for the fact that this is heavier and just intuition tells me it should be more. But after learning physics we learn that it is actually the same."
Perfect. So, he had the right idea but then he took a physics class and got the wrong idea.
Here are some other thoughts about this video and the question about falling objects.
- This question isn't as great as it used to be. When you ask people about it, most will say that the balls hit at the same time. They THINK the heavier one hits first, but they have been asked the question before so that they know the answer.
- There might be a couple of reasons why they say the gravitational force is the same. I have seen that students will confuse the gravitational field (g) with the gravitational force. Yes, g is the same for all these objects.
- The other reason they say the force is the same is that they are trying to make their ideas of force and motion match up with the answer that they hit the ground at the same time.
- One of the people in the interview expressed another common idea. The idea that the gravitational forces are different while you hold them, but when you let go and they are falling the forces become the same.
So, why do the balls hit at the same time? I think the interviewer in the video does an ok job. He says that the heavier ball has a greater force, but also a greater inertia. Really the key is that the mass that is related to force and change in motion (inertial mass) is the same as gravitational mass (the mass in the gravitational force).
I think this video shows something else. It shows that learning these ideas are difficult. All the people in the interviews seem well educated, but they get this stuff wrong. The interviewer tells them the answer, but I doubt that will have a lasting effect. The problem is that people's common-sense ideas do not match up with the experimental evidence. How do you fix this? Time. Lots of time and experiments.
The Physics and Everyday Thinking curriculum for education majors does a pretty good job dealing with just this situation. It does it by having students do the following experiments:
- Explore force and motion. Push things on a horizontal track. Collect evidence that force CHANGES the speed and that no force means a constant speed. (this is actually a whole bunch of experiments)
- Change the mass of a cart and see that greater mass an object has the less change in speed it has (the word acceleration is avoided as much as possible).
- Increasing the force on an object (in this case the force is from a fan-cart) increases the rate at which the speed changes.
- Dropping things increase in speed the whole time they drop.
- Holding objects of different mass to see that the gravitational force is different.
- Dropping objects of different mass to see that they fall with the same acceleration
- Here is the key - playing with a simulator where they have two cars (horizontally moving) that have different mass. They have to adjust the forces so that the cars move with the same acceleration. This simulator lets them see what would happen if they have different mass, but the same force.
Even after all of those experiments (and with me not explicitly telling students the 'answer') some students still answer questions like the people in this video. Like I said, change is difficult.
Bonus Endorsements: I like these Veritasium videos. They are short, well edited and interesting to watch. Maybe they have some short comings (in terms of teaching something) but they are still enjoyable. I would also like restate my opinion of Physics and Everyday Thinking. If you are a college-level physics faculty and you have a course for education majors, you should use this curriculum.