Steps to Solving Mechanics Problems (7 min)

Free Body Diagrams and 3rd Law Force Pairs (14 min)

https://media.oregonstate.edu/media/t/0_l0dxbt04

Summary

The objective is introduce Newton's laws and apply the fundamentals of a force analysis. Specifically we focus on drawing free-body diagrams to facilitate translating the 2nd law to the mathematical representation. Connections to kinematics are also made.

Atomistic Goals

Students will be able to...

1. Demonstrate the fact that a force is inherently an interaction between two objects or two systems.
2. Explain Newton's 1st law as it relates to inertia.
3. Explain Newton's 1st law and how it relates to the 2nd law.
4. Define static and dynamic equilibrium.
5. Explain Newton's 2nd Law.
6. Explain Newton's 3rd Law force pairs.
7. Define the point particle model
8. Define a system(s) boundary and adhere their analysis to that boundary(s).
9. Identify the type of forces interacting with your system and the direction they are applied.
10. Differentiate between contact and non-contact forces.
11. Demonstrate that the normal force is always perpendicular to the surface
12. Draw a free-body-diagram (FBD) for the system(s).
13. Demonstrate the ability to draw a properly scaled FBD.
14. Draw the coordinate system that reduces the complexity of the vector analysis next to the FBD.
15. Apply geometry to determine appropriate angles for the given coordinate system.
16. Find the components of a force in the chosen coordinate system.
17. Differentiate between A force and a NET force.
18. Apply Newton's 2nd law in the mathematical representation.
19. Differentiate between static and dynamic equilibrium.
20. Differentiate between weight and apparent weight.
21. Synthesize a force and kinematics analysis via the acceleration.
22. Demonstrate that the net force points in the same direction as the acceleration
23. Demonstrate the fact that the net force can be in the opposite direction of the motion.

Khan Academy's introduction to Newton's Second Law in a 2 part video sequence:

https://www.youtube.com/watch?v=ou9YMWlJgkE

https://www.youtube.com/watch?v=24vtg9Ehr0Q

Doc Schuster's introduction to Newton's Second Law part 1:

https://www.youtube.com/watch?v=_Z7qivqbSBI

Doc Schuster's introduction to Newton's Second Law part 2:

https://www.youtube.com/watch?v=_Z7qivqbSBI

Here is a really great video by Dr. Davies from Gordon State College that relates a physics problem, Newton's Second Law, and FBD:

https://www.youtube.com/watch?v=NDiQ363uBj8

Other Resources

This link will take you to the repository of other content related resources.

This PHet simulation allow you to view changing magnitudes of the force vectors in a FBD;

This is a nice tool that will help you understand how to construct free body diagrams

Here is a simple interactive to help you understand how to properly arrange force vectors,

For additional simulations on this subject, visit the simulations repository.

Low Friction Atowood Machine demo,

https://www.youtube.com/watch?v=4ovhEkSIqV0

Pasco dunamics cart and track: Forces, Mass, and Acceleration connection to Newton's 2nd law,

https://www.youtube.com/watch?v=aDLYpJoSRhM

For additional demos involving this subject, visit the demo repository

Oh no, we haven't been able to write up a history overview for this topic. If you'd like to contribute, contact the director of BoxSand, KC Walsh (walshke@oregonstate.edu).

Oh no, we haven't been able to post any fun stuff for this topic yet. If you have any fun physics videos or webpages for this topic, send them to the director of BoxSand, KC Walsh (walshke@oregonstate.edu).

Here is a concise explanation of Newton's Second Law:

Resource Repository

This link will take you to the repository of other content on this topic.

As we get more forces on our free-body diagrams, it becomes more and more important to think about the physical situation. Will the object move in the y-direction? No? Then all the forces in the y MUST cancel out. If they don’t it means we accelerate in that direction per F=ma.

Normal force will always point directly away from the object generating it, at a 90-degree angle from the surface of the object creating the external force.

• Don't be tricked into thinking that if the net force is 0, the object isn't moving, it could still be moving but not accelerating (i.e. moving with a constant velocity).
• Often accelerating objects will move in directions that they aren't accelerating in. This is because if they have an initial velocity, they will move in the inital velocity's direction immediately, but over time will forced into moving in the direction of the acceleration. Given enough time under the same constant force, an object will eventually move in the direction of its acceleration.
• The Difference Between Mass and Weight Veritasium video:

https://www.youtube.com/watch?v=_Z0X0yE8Ioc

• Generally, it most advisable to think about tilting your coordinate system so one access points in the direction of acceleration if there is one. This results in one direction’s sum of forces being equal to mass times that direction’s acceleration, while the other direction’s sum of forces will equal 0. If there is no acceleration, it is often best to tilt to make your axes align so you have to decompose the fewest vectors, in other words, tilt so that most or all of your arrows are sitting on the two axes.
• Everytime you sum forces, it will usually result in one of 3 situations: 1) it equal 0, meaning a=0 and the object is not moving in that direction. 2) It will equal mass times acceleration where acceleration is non-zero and the object IS moveing. Or 3) it will equal another force (this is more for later). This critically about what is happening in the physical situation after you sum your forces and make the correct choice of what to set them equal to (and a sum of forces is only useful if you set it equal to SOMETHING, and to do that, you ned to make a physical assumption).

Multiple Representations is the concept that a physical phenomena can be expressed in different ways. 

Physical

Mathematical

Graphical

Descriptive

Experimental