DAY 37
GOALS: SWBAT...
- Define "Apparent Weight"
- Solve problems using FNET
WARM-UP & HW CHECK (pg 101 #23; pg 113 # 64; Newton's Laws of Motion H/O)
Your name is Marge, your mass is 50kg, and you are standing on a bathroom scale in an elevator. Starting from rest, the elevator accelerates downward at an acceleration of 2.00m/s2.
1. Is the scale reading during acceleration higher, lower, or the same as when the elevator is at rest?
2. Draw an FBD for the system (include the scale and Marge only). Define positive & negative
3. Find the force on the scale during the acceleration by using FNET
***If you get stuck, look at example problem #2 on pg 99***
CLASSWORK
1. HW Review
Take 5min to review the solution to the "Newton's Law's of Motion Handout". Put any questions in the Struggle Zone
2. Newton's 2nd Law Practice
The goal here is to get more familiar with how F=ma works, specifically with how changing one variable's value changes the
answer. By the time you get to #2h, you should feel more comfortable, and be able to solve problems faster than when you
started with #2a.
In case you're absent, here's the --> LINK
HOMEWORK:
Apparent Weight
Notes:
Recall that weight is a force
Apparent means "clearly visible, or obvious". So, Apparent Weight is the weight that you feel (or "see") at any given time. Your book uses a scale to teach apparent weight, and it's a great tool. If you're standing on a scale, the weight (force) reading on the scale shows your apparent weight.
Some scenarios:
If you're accelerated upwards, your apparent weight increases (you feel heavier)
If you're accelerated downwards, your apparent weight decreases (you feel lighter)
If you're accelerated downwards enough, your apparent weight decreases so much that you will feel weightless. This doesn't mean that you have zero weight, instead, it means there are no contact forces pushing up on you
Watch the following video: https://www.youtube.com/watch?v=jgKvR59IvM0
Answer the questions listed below:
1. Can an object with mass even have a weight of zero? Prove it. (pics or it didn't happen)
2. At minute 3:27, is gravity absent or present?
3. Astronaut Don Thomas mentions that he's in a state of "free-fall" inside the airplane. Which way must he be accelerating? Which way must the plane be accelerating? What can you say about his acceleration and the planes? If you took a side view of the plane's flight path, what shape would it take?
Problem Solving Tips:
To determine the apparent weight, we need to consider Vertical Net Force and Acceleration
An object that accelerates vertically will always have to take into account the force of gravity. It is crucial in such problems to define a positive and a negative direction before finding your givens. After you choose positive and negative, draw your free body diagram and find your net force. Then solve the problem.
Video: https://www.youtube.com/watch?v=Cbp2Zh3RphU
Questions:
FYI, the narrator is using an approximation that the acceleration due to gravity is 10m/s2, not the 9.8m/s2 we use in class. It makes calculations easier...
1. Draw a FBD for the man in the elevator when the elevator is motionless
2. When the elevator is accelerating, which force describes how heavy you feel? The normal force, or the force from gravity?
3. Draw the FBD for the elevator when it's accelerating upwards.
4. In general, when an elevator is moving down, will you feel lighter?
5. Why is it that when the elevator is moving at a constant velocity down, you don't feel lighter?
6. Show the calculations for finding Fnet when the elevator is accelerating down at 4m/s2. Which way is Fnew acting, up or down? How does this affect his apparent weight?
Textbook problems: Newtons 2nd Law & Apparent Weight
pg 99-100 #19, 20, 22, 24, 26
Problems Solving Quiz Friday: Newton's 1st and 2nd.
Reading Quiz Friday: pg 102-103
Goals: SWBAT...
1. Learn to draw vector components using the tip-to-tail method.
2. Evaluate the sum of two or more vectors in two dimensions graphically
3. Determine the components of the vectors
4. Solve for the sum of two or more vectors algebraically by adding the components of the vectors.
WARM-UP: (10 min)
Voice is still lost, so you'll need to be independently focused today.
1. Vectors Revisited:
a. What does this symbolize? F
b. What about this?
c. What about this? F
d. What is the definition of a vector?
2. Vectors in Multiple Dimensions: Look at the pictures in the margin of page 122.
a. What vector are Ax and Ay components of?
b. Are Ax and Ay vectors?
3. Vectors in Multiple Dimensions: Look at the pictures in the margin of page 123.
a. What does Rx represent?
b. What is the vector sum of the small vectors A, B, and C?
4. Reading Quiz today on pg 119-120. Take 60-sec to review :-)
CLASSWORK: (for each CW activity, think about which goal you are meeting. Also, tell me how the activity helps you achieve that goal)
1. Reading Quiz
2. Read Example Problem #1 on pg 121. Note that the green picture only applies to the 135* triangle - the 90* triangle wasn't drawn by the authors.
a. Why is the solution for the 90* triangle so much simpler than the 135* triangle?
b. Use the Example as a template to solve the following problem:
"Find the magnitude of a 10-km displacement and a 15-km displacement when the angle between them is 90*. Repeat the problem when the angle between them is 120*"
3. Law of Cosines: Review pg 121 problem #3
4. Reading:
Read pg 122-123. Create four more reading quiz questions
5. Notes: Introductory Vector Addition Problem using Component Vectors. If you have questions, raise your hand and I'll pause the video
Check your notes here: http://www.flippingphysics.com/uploads/2/1/1/0/21103672/0053_lecture_notes_-_introductory_vector_addition_problem_using_component_vectors_also_0054.pdf
6. Practice Problems pg 125 5-7
7. Connect activities with goals
Still need help with Cardinal Directions?
Go here --> How to use Cardinal Directions with Vectors
HOMEWORK:
pg 121 #4, pg 125 8-10 (extended to Friday)
Friday HW pg 125 11-14. #15 and 16 are extra credit