Week 12 will bring us forces at equilibrium, friction, and torque
Updated November 7th, 2021 This is your class landing page, you will want to come here every week to check for any class announcements. Make certain you read everything on this page and understand completely all the directions. You can access past weeks announcements in their respective Quarter pages.
This week we started Chapter 5 - Forces and Equilibrium, comprised of the following sections:
Section 5.1 - The Force Vector
Section 5.2 - Forces and Equilibrium
Section 5.3 - Friction
Section 5.4 - Torque and Rotational Equilibrium
We learned this week how to write free body diagrams, reflecting the force vectors acting on objects. Free body diagrams are great for graphically representing force vectors because we can easily indicate not only the relative magnitude of the vectors but direction as well.
You learned this week in the videos force vectors can be represented three different ways:
as a graph
a magnitude-angle pair (10N 30 degrees), and
as an x-y pair (8.7, 5.0) N (note the graphic below)
Section 5.1 - The Force Vector If you completed Skill & Practice 5C - Pythagorean Theorem, you should have realized force vectors can be analyzed and calculated by using the Pythagorean Theorem. If we know two sides of a right triangle (sides a and b), then we can calculate the hypotenuse; alternatively, if we know the hypotenuse and just one side (side a, for example), then we can calculate the unknown side, b.
Hopefully from this, you can see that every vector can be represented by x and y components, as the vector is the sum of those two components - this is really obvious whenever we do head-to-tail addition of the x and y components to determine what is called the resultant vector.
Forces at Equilibrium - Section 5.2 If you watched the videos as assigned on the Weekly Pacing Schedule and this Weebly page, you should know that whenever discussing forces that are at equilibrium we are usually observing their action on a body that is not undergoing an acceleration, such as a book sitting on a table. Think about the free-body diagram that you would draw for such an instance: you would draw a vector arrow going straight down representing the force due to gravity; and a vector arrow equal in magnitude, but opposite in direction pointing upward representing the normal force. The table on which the book sits can be thought of as a compression spring: the book is pressing against the table wherever it is in contact, and the table is in turn pressing back against the book (see figure at right).
An actual object hanging from a tension spring is another example of a force at equilibrium. In this case, the object is pulling on the spring and the spring is pulling back with an equal but opposite force. In fact, when a hanging spring scale weighs an object, the distance the spring stretches is proportional to the object’s weight. An object that is twice as heavy changes the spring’s length twice as much (figure at right).
This relationship can be expressed as an equation: F = -kx, where "F" is the force exerted by the spring, "k" is the spring constant (we'll discuss in class), and "x" is the displacement of the spring in meters. We can restate this as "the force (F) exerted by a spring is directly related to the spring constant (k) multiplied by the displacement (x) of the spring." There is a negative sign on the right-hand side of the equation because the force of the spring always acts in a direction opposite to the displacement. This is in fact, Hooke's Law.
What to expect in Week #12
This coming week, we'll complete Sections 5.3 and 5.4 from your text. Below is a summary of each.
Friction - Section 5.3 As you all observed in the lab you started Thursday (and will finish Tuesday) Friction is a force that resists the motion of objects or surfaces due to the microscopic hills and valleys that come into contact with each other (see image at right). Because friction exists in many different situations, it is classified into several types. Sliding friction is present when two objects or surfaces slide across each other. Static friction exists when forces are acting to cause an object to move but friction is keeping the object from moving.
Torque - Section 5.4 Torque is the rotational equivalent of force and is a measure of how much a force acting on an object causes the object to rotate. Torque causes objects to rotate or spin (as seen on canoe in figure at right).
Torque, like most other concepts of physics can be quantified: T = F x r, where "T" is torque, "F" is the force applied, and "r" is the distance between the axis of rotation, and the applied force, or lever arm. We will go into greater detail on torque next week.
Week 12 Resources/ Assets
Use the "Class Questions Forum" to ask any questions about assignments, labs, quizzes, due dates, etc.
Writing should be legible and tidy, answers fit within the space provided
When a problem asks you to explain, you need to explain
For mathematical calculations, you need to show your work
Your parents need to sign all of your chapter worksheets, signifying that you checked your answers against the Answer Keys and corrected your wrong answers.
Indicate how many you got wrong and circle that number, writing this next to your name
Check-off List of Things to Do:
Please make sure you do the following before classes: Tues, November 9th:
Turn in completed and graded Chapter 4 Homework(if you did not do this already)
Turn incompleted and gradedSkill & Practice 4A-D, E-G, I (if you did not do this already)
Turn inTest #2– Chapters 3 and 4, making sure it is signed by parent and placed in a sealed envelope with parent's signature across the seal (if you did not do this already)
Print out, read, and bring to classFriction Laboratory (we started on Thursday, will finish)