Week 6 of February 8th

Week 6 of February 8th

Objectives:

Agenda:
1. Engineering a smooth ride powerpoint: Students will see how different materials, shapes, temperatures and other factors effect the vibration of skis.
2. Ski trip paperwork
3. Friction Powerpoint
4. Coefficient of Friction Worksheet (on thumbdrive)

5. Friction Clicker Questions use website below. http://tuhsphysics.ttsd.k12.or.us/Tutorial/NewIBPS/Friction/Friction.htm

6. Free Body Diagram WS (on thumbdrive)

Notes:

Free Body Diagrams:

Free Body Diagrams show the relative magnitude and direction of forces acting on an object. Force vectors are represented as arrows, pointing outward from (usually) or inward toward the center of mass of an object. Drawing free body diagrams is the first step in solving problems involving more than one force on an object.
Newton’s First Law states that if no net force acts on an object, it maintains its state of rest or its constant speed in a straight line. We will be looking at an object maintaining its state of rest.

Explanation – Newton’s First Law regarding an object maintaining its state of rest, means that if an object is placed at rest on a horizontal surface, with no external unbalanced forces acting upon it, then it will never move. A free body diagram help us to visually analyze the forces acting on an object and assess their effects.

In this case, there are three forces on the block. I imagine that everyone is ok with the gravitational force of the Earth pulling on block A - right? Here you see why the force of the plane pushing on the block is called the normal force. It is because that force is perpendicular to the surface (normal). There is another force between the block and the plane that is NOT normal. It is the friction force and it is parallel to the surface.
Adding Vectors on the inclined plane
Suppose you want to calculate the frictional force or something using the assumption that the forces all add up to zero vector. Here you can use a small trick. Since N and the friction force are perpendicular, you can put the x-y axis tilted so that those two forces are ONLY in x or y direction:

This would give the equation for the x-direction as (I am going to call the normal force N, friction F and the gravitational force G):

Note that these are not vectors, here the symbols stand for the magnitudes of the vectors. Also, I will leave it as a geometry exercise for you to show that the angle between the gravitational force and the y-axis is the same as the angle of the incline.
Identifying Forces
I understand it can be difficult to determine what forces are acting on an object. All forces that you will see can be in one of two groups:
Long range forces: These are forces between two objects where the objects do not have to touch (thus long range). Really, there are only two interactions you will see that do this. The gravitational interaction (between objects with mass) and the electromagnetic interaction between things with electric charges.
Contact forces: Secretly, there is no such thing as contact forces (see this post) but we will pretend for simplicity. Contact forces are from things that are touching that object. Examples: friction, normal force, tension from a rope, hand pushing on something, air resistance.
When you are identifying forces, first look for long range. In the first semester physics this will probably JUST be gravity. All the other forces on that object are from things touching it.
In your journey to create free body diagrams, I encourage you to properly label your forces. This will help you find forces that really shouldn’t be there.