Teaching Force Concept Problem

In summary, the conversation discusses the concept of equal and opposite forces in Newton's third law and how it applies to a specific scenario with a car and truck. The main point to emphasize is that the action and reaction forces act on different objects and can result in different accelerations depending on the mass of the object. It is important to analyze all the forces acting on each individual body in order to understand its acceleration.
  • #1
Dahaka
3
0
I am what is called an LA (Learning Assistant) for an introductory physics class and I'm a freshman. One problem in which I have in convincing the students is how forces are equal and opposite. For instance:
A large truck breaks down out on the road and receives a push back into the town by a small compact car as shown in the figure below.

While the car, still pushing the truck, is speeding up to get to cruising speed:
A. the amount of force with which the car pushes on the truck is equal to that with which the truck pushes back on the car.
B. the amount of force with which the car pushes on the truck is smaller than that with which the truck pushes back on the car.
C. the amount of force with which the car pushes on the truck is greater than that with which the truck pushes back on the car.

How do I explain the answer and concept to them? Every time I bring this up, they argue that Newton's Law of equal and opposite forces is violated because if they were to be equal and opposite, then the truck would not move.
 
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  • #2
The equal and opposite force pairs in Newton's 3rd law act on different bodies. Thus they don't cancel out or produce equilibrium.
 
  • #3
Doc Al said:
The equal and opposite force pairs in Newton's 3rd law act on different bodies. Thus they don't cancel out or produce equilibrium.

yep :approve:
 
  • #4
Sorry for the late reply, I've been really busy, so if I were to explain this to a student, would I say that they are equal forces but produce different accelerations due to different masses?
 
  • #5
1. You should first make the student understand that the two forces act on DIFFERENT bodies.

2. If he complains that the sum of the two forces is zero, and hence that "the acceleration is zero", question him on WHICH acceleration he's talking about.
The acceleration of the SYSTEM comprised of the two bodies is, indeed, 0, but it does not at all follow from this that the bodies themselves have accelerations equal to zero.

3. Then tell them that although the magnitude of each force is equal, the magnitudes of their accelerations will differ if they have different masses.
 
  • #6
arildno said:
1. You should first make the student understand that the two forces act on DIFFERENT bodies.

2. If he complains that the sum of the two forces is zero, and hence that "the acceleration is zero", question him on WHICH acceleration he's talking about.
The acceleration of the SYSTEM comprised of the two bodies is, indeed, 0, but it does not at all follow from this that the bodies themselves have accelerations equal to zero.
Just a correction. If by system here we mean the car and truck, their acce;eration is not necessarily zero. They could be moving at constant velocity or they could have a nonzero acceleration. The net force on each object separately does not have to be zero.
3. Then tell them that although the magnitude of each force is equal, the magnitudes of their accelerations will differ if they have different masses.
This is true in general but in this particular example, assuming that the car and truck always stay in contact, the two objects must have the same acceleration.
 
  • #7
Dahaka14 said:
Sorry for the late reply, I've been really busy, so if I were to explain this to a student, would I say that they are equal forces but produce different accelerations due to different masses?

The contact force between the two is not the only force acting on them so it's incorrect to say what you said. If the contact force was the only force, then you would be correct. To have such a case, you can imagine an astronaut very far from any massive object like a planet or a star, pushing against a second astronaut. In that case the only force betwene them is the contact force (if we neglect the force of gravity between them). They feel the same force but have different accelerations if they have different masses.

Another example is an apple in free fall. It pulls on the Earth with the same force that the Earth pulls on the apple but the two objects have different accelerations.


Going back to your problem, as the others said, the key point to emphasize in the third law is that the action-reaction forces act on different objects. The best way to explain the problem is to draw a free body diagram of the car an done for the truck and to show all the forces, pointing out the action-reaction pair. And then to emphasize that the acceleration of each object is given by the net force on that object divided by its mass.
You should show two cases: one in which the car and truck move at constant velocity and one in which they have an acceleration. That should clarify things to them.
 
  • #8
Another thing to point out is that while it's certainly true (via Newton's 3rd law) that the force exerted by the car on the truck must be equal and opposite to the force that the truck exerts on the car, those forces are not the only forces acting on those bodies. Don't forget the force of the ground on the car!

To figure out how a body accelerates one uses Newton's 2nd law, not the third law. And one must consider all the forces acting on the body.

It will also be instructive to analyze forces on several systems:
(a) the car
(b) the truck
(c) the "car + truck" considered as a single body

Only in case (c) do those 3rd law forces "cancel out", since they are internal to the system and thus act on the same "body".
 
  • #9
kdv said:
Just a correction. If by system here we mean the car and truck, their acce;eration is not necessarily zero. They could be moving at constant velocity or they could have a nonzero acceleration. The net force on each object separately does not have to be zero.

This is true in general but in this particular example, assuming that the car and truck always stay in contact, the two objects must have the same acceleration.
You're right, I didn't bother to read the example at hand, and assumed it was a typical collision type issue. I readf his post 4, rather than the OP. Shame on me.
 
  • #10
Dahaka said:
I am what is called an LA (Learning Assistant) for an introductory physics class and I'm a freshman. One problem in which I have in convincing the students is how forces are equal and opposite. For instance:
A large truck breaks down out on the road and receives a push back into the town by a small compact car as shown in the figure below.

While the car, still pushing the truck, is speeding up to get to cruising speed:
A. the amount of force with which the car pushes on the truck is equal to that with which the truck pushes back on the car.
B. the amount of force with which the car pushes on the truck is smaller than that with which the truck pushes back on the car.
C. the amount of force with which the car pushes on the truck is greater than that with which the truck pushes back on the car.

How do I explain the answer and concept to them? Every time I bring this up, they argue that Newton's Law of equal and opposite forces is violated because if they were to be equal and opposite, then the truck would not move.

You can do a simple demonstration to get them thinking. First, show them that a spring scale shows how much force is being exerted (set a 100N weight on the floor and have someone pull up slowly and watch the spring scale stretch and stretch and, just past 100N, voila!). Then set 2 students on something like furniture dollys, free to move with some friction. Give one of them two spring scales - one attached to the wall the other attached to a spring scale held by the other student. Make sure all 3 spring scales are basically colinear. Have the one with two spring scales try to pull the other guy to him and read all 3 scales. Then repeat, except omit the spring scale attached to the wall.

This takes a little practice to get the friction right and to get the guy in the middle to stay still. Plus, you may have to do a little explaining about the friction. But, it might turn on some light bulbs.
You can even take it a little farther by putting the guy in the middle on a board and the gut on the end on a piece of carpet so the frictions are dramatically different.
 
  • #11
Thanks guys.
 

Related to Teaching Force Concept Problem

1. What is the force concept problem?

The force concept problem is a common teaching tool used in introductory physics classes to assess students' understanding of fundamental concepts related to forces, such as Newton's laws of motion and the concept of inertia.

2. How is the force concept problem used in teaching?

The force concept problem is typically presented as a multiple-choice question or a short answer question, and students are asked to explain their reasoning behind their chosen answer. This allows teachers to identify common misconceptions and address them in class.

3. What are the benefits of using the force concept problem in teaching?

The force concept problem helps students develop critical thinking skills and gain a deeper understanding of fundamental concepts in physics. It also allows teachers to assess students' understanding and adjust their teaching accordingly.

4. Are there any drawbacks to using the force concept problem in teaching?

One potential drawback is that students may become overly focused on finding the "correct" answer rather than understanding the underlying concept. This can result in a superficial understanding of the material. It's important for teachers to emphasize the importance of understanding the reasoning behind the answer, rather than just the answer itself.

5. How can teachers effectively use the force concept problem in their lessons?

To effectively use the force concept problem, teachers should provide students with a variety of problem-solving strategies and encourage them to explain their reasoning. Teachers should also use the results of the problem to guide their instruction and address any common misconceptions that arise.

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