Can Forces Be Transferred?

Suppose you toss a ball in the air. After leaving your hand, it rises and slows down as it rises. Why? Does it slow down because some force runs out as it rises? Is the force of the hand transferred to the ball? Are forces transferred at all?

Students might have a discussion something like this:

• "Are forces transferred? Let me just Google that on my phone."
• "Forces are transferred through objects - look, it says it right here on this webpage. So, yes. Forces are transferred."
• "This also says that when a string is pulling on something, the force is transferred through the string"
• "Even more - here it says when you push on an object, the force is transferred through the object to the other atoms on the other side of the object. Clearly, forces are transferred."
• "Forces have to be transferred. Suppose you push a cart and let go. It keeps moving. Later, it might hit something and exert a force. How could it exert a force if it didn't have a force with it so forces have to be transferred?"

If I summarized common ideas about forces, I might get the following two answers:

Before you pushed a cart, it wasn't moving. After you push it, it moves. What does that mean? It must mean that pushing a cart makes it move. Since the cart is still moving after you push it, there must be a force transferred to the cart. Yes, forces are transferred.

Suppose you push a cart and it moves. Now suppose that the this cart is moving along a track and then collides with another cart. Since that target cart will move in the collision, the pushed cart has to exert a force on it. This means that the cart carries with it a force. And where did that force come from? Yes, it can from the hand that pushed it. Forces are transferred.

But are these answers correct? How can students figure out if forces are transferred? Isn't it the instructor's job to tell them if forces are transferred or not?

In this case, I am looking at students in a conceptual physics course (we use Physics and Everyday Thinking curriculum - it's awesome). The basic idea is that students work in groups and perform some simple experiments to collect evidence. From this evidence, they build ideas about how forces relate to motion. The big idea is that they (the students) collect evidence and base their ideas on evidence, ideas are not based on the authority of a textbook or the instructor.

What kind of experiments do students do to help them build an idea about forces? Well, the first experiment is sort of simple. They just pull on a spring scale so that they can get a feel for forces (since the spring scale measures forces). They are also given a definition of forces: an interaction involving pushes or pulls. Yes, that's not very detailed, but they can get a better idea from the next experiment.

There is actually a way you can push a cart and measure the force on that cart. A force probe connects to your computer and records the forces. I can push the cart and record both the force and the cart's speed. This is what that data looks like.

This force probe doesn't give the best data - but still you can see that when a hand pushes on the cart, the cart increases in speed. After the push from the hand, there is no force on the cart. In the actual course, students do this with a simulator instead of with real carts. This will give a much better picture of the relationship between forces and changes in speed.

After that experiment, there are quite a few more so that students can see:

• Forces cause objects to increase or decrease in speed (or for sideways forces the object changes directions).
• Friction is a force that slows things down.
• When more than one force acts on an object only the net force matters.
• Greater forces means greater changes in speeds (greater rate of a change of speed).
• If the object has a larger mass, it will have a lower rate of change of speed than a lower mass object with the same force.

Along with a summary document (that goes with the curriculum), students should have plenty of evidence regarding forces and interactions.

Dear Students, Please Stop Googling

Here is my hope. I hope that The Google picks up this post as an answer to the question "are forces transferred?" It's not such a crazy wish. Go ahead and search for "why do astronauts float in space?" See what I mean?

But what's wrong with Google? There are a couple of problems. First, Google doesn't always give you what you want. Oh, it looks like it's what you want, but it's not. Google will give you an answer, but is it even the correct answer? In the cases of transferring forces, Google is sort of correct but it isn't actually what the students are looking for. Whatever the answer Google gives, a human could interpret it correctly. Of course that is the real role of a human, to figure out the right answers. Not just to read the right answers.

The second problem with searching for an answer is that students miss the point. The point is not to research and find things that have already been figured out. Instead the purpose of the class is to practice the process of building ideas and supporting ideas with evidence. The curriculum uses ideas that are already established (like forces and motion) because it would be almost impossible to base a course around new ideas - although that would be super cool.

Searching for answers is not the answer to physics classes. Of course, I've said this before.

The Real Answer: Forces Aren't Real

Forces can't be transferred. Why? Because forces aren't real. Oh, sure. You can push on a wall and that's real. If you drop a ball, it will speed up as it falls. That's real too. However, this method of describing interactions with a force is just how we describe these things. Humans made it up. It's like the word "red". That's not real, it's just a word we use to describe light of a particular wavelength. Is it wrong to call shorter wavelengths "red"? Yes. It's wrong because we already defined the word "red" to means something else. The same is true for forces.

Here is my best definition of "force":

Force: When two objects interact, we say that this interaction is a force that either pushes or pulls the two objects. Forces are always an interaction between two objects such that each object pushes (or pulls) on the other object with the same force. When a net force is applied to an object, it changes the vector momentum of that object.

Perhaps this definition shows something that students have trouble with. A force is a way to describe an interaction. Often, students will think of a force as a property of an object instead of an interaction. An object can have properties like mass, color, velocity and momentum. It might seem natural to add "force" as one of those objects. However, we define force to be a property of the interaction instead.

Is anything transferred in an interaction? Well, as far as human definitions go, yes. We can say that energy is transferred. We can also say that momentum is transferred (if you look at a big enough system). But in the end, that doesn't really matter. What matters is that the students practice supporting their answers with evidence. Sometimes there are nice experiments that they can use but sometimes they have to base their answers on "because it's defined that way."