DAY 45
NGSS Standard (this is what we're learning with this unit)
Analyze data to support the claim that Newton’s second law of motion describes the mathematical relationship amongthe net force on a macroscopic object, its mass, and its acceleration. [Clarification Statement: Examples of data could include tables or graphs of position or velocity as a function of time for objects subject to a net unbalanced force, such as a falling object, an object sliding down a ramp, or a moving object being pulled by a constant force.] [Assessment Boundary: Assessment is limited to one-dimensional motion and to macroscopic objects moving at non-relativistic speeds.]
#Goals: SWBAT...
1. Draw correct FBD's, with appropriate vector magnitude and direction.
2. Support classmates with helpful tutoring
3. Use FBD's to find Net Force
4. Describe the relationship between mass, net force, and acceleration
Warm-Up (4min)
read the scenario below, then answer the questions at the end
Two students are discussing their physics homework prior to class. They are discussing an object that is being acted upon by two individual forces (both in a vertical direction); the free-body diagram for the particular object is shown at the right. During the discussion, Anna Litical suggests to Noah Formula that the object under discussion could be moving. In fact, Anna suggests that if friction and air resistance could be ignored (because of their negligible size), the object could be moving in a horizontal direction. According to Anna, an object experiencing forces as described at the right could be experiencing a horizontal motion as described below in a dot diagram
Noah Formula objects, arguing that the object could not have any horizontal motion if there are only vertical forces acting upon it. Noah claims that the object must be at rest, perhaps on a table or floor. After all, says Noah, an object experiencing a balance of forces will be at rest. Who do you agree with? WHY?
CLASSWORK
1. 045A: Quiz #8: FBD's
2. 045B: Relationship between F, m, and a
Complete the chart on the left board. You should be able to find one pattern for each of the three sections
3. 045C: Complete the following practice problems: (15min)
0. Example: If an object is falling straight down, the Fair is 15N, the Fg is 20N, and the mass is 5kg, what is the acceleration?
1. Determine the accelerations that result when a 12N net force is applied to a 3kg object and then to a 6kg object.
2. A net force of 15N is exerted on an encyclopedia to cause it to accelerate at a rate of 5 m/s2. Determine the mass of the encyclopedia.
3. Suppose that a sled is accelerating at a rate of 2m/s2. If the net force is tripled and the mass is doubled, then what is the new acceleration of the sled?
4. Suppose that a sled is accelerating at a rate of 2m/s2. If the net force is tripled and the mass is halved, then what is the new acceleration of the sled?
Need help? Click below...scroll down until you see the problems, then click the "see answer link"
At Home Learning (HW)
Finding Acceleration via FBD's is what we'll cover Tuesday. We will solve problems involving friction.
Do all surfaces provide the same friction? What provides the most? The least?
045D: Take notes, and answer the edpuzzle questions for the following two videos
Introduction to Static & Kinetic Friction by Bobby (4:04)
Introduction to the Coefficient of Friction (4:59)
#Goals: SWBAT...
1. Draw correct FBD's, with appropriate vector magnitude and direction.
2. Support classmates with helpful tutoring
3. Use FBD's to find Net Force
4. Describe the relationship between mass, net force, and acceleration
Warm-Up (4min)
Copy or complete the following:
1. Complete the FBD on the board
2. Which requires more force to accelerate? A desk or a chair? Why?
3. If you apply the same force to a bike, and to a train locomotive, which will accelerate faster? Why?
#045: Quiz 6: FBD's
Answer the following (4min)
1. Ariana Grande drops a Pete Davidson bobblehead straight down from her 17th story apartment. Draw the FBD. Neglect air resistance
2. A truck is moving to the left and slowing down. Neglect air resistance. Draw the FBD showing the forces acting upon the truck.
CLASSWORK
1. #045A: Newton's 2nd Law
Read, then take Notes: (4min)
Write the equation for Newton's 2nd Law
Newton's second law states that the acceleration of an object is dependent upon two variables - the net force acting upon the object and the mass of the object.
The acceleration of an object depends (directly/inversely) upon the net force acting upon the object, and (directly/inversely) upon the mass of the object.
As the force acting upon an object is increased, the acceleration of the object is (increased/decreased).
As the mass of an object is increased, the acceleration of the object is (increased/decreased).
2. #045B: The Reality of Our First Free Body Diagrams (5min)
Are our FBD's accurate?
Watch, take notes, and answer the two Edpuzzle questions
https://edpuzzle.com/media/5808e44e892b65ec3e6b874d
3. #045C: Practice with the 2nd Law
A. A Net Force Causes Acceleration - Practice Problems
Navigate to: http://www.physicsclassroom.com/class/newtlaws/Lesson-2/Determining-the-Net-Force
Scroll halfway down the page to "A Net Force Causes Acceleration "
Answer in your head - compare your answers with your neighbor. Looking at the Descriptions of Motion, what is the net force for each scenario? (answers can be positive, negative, or zero)
Draw the FBD's in your notes. Try the two Check for Understanding problems on your own for practice. Get peer help if needed.
B. (SAVE FOR MONDAY) Complete the following practice problems:
1. Determine the accelerations that result when a 12-N net force is applied to a 3-kg object and then to a 6-kg object.
2. A net force of 15 N is exerted on an encyclopedia to cause it to accelerate at a rate of 5 m/s2. Determine the mass of the encyclopedia.
3. Suppose that a sled is accelerating at a rate of 2 m/s2. If the net force is tripled and the mass is doubled, then what is the new acceleration of the sled?
4. Suppose that a sled is accelerating at a rate of 2 m/s2. If the net force is tripled and the mass is halved, then what is the new acceleration of the sled?
Need help? Click below...scroll down until you see the problems, then click the "see answer link"
At Home Learning (HW)
1. Quiz Monday on FBD's. review the following... Day 43-45
2. Complete any unfinished practice problems from today for HW
3, #045C: Due Monday: Prepare for class by reviewing Newton's 1st and 2nd Laws along with Mr. P's class
Watch the video, and answer the EdPuzzle Questions: EDpuzzle
NGSS Standard (this is what we're learning with this unit)
Analyze data to support the claim that Newton’s second law of motion describes the mathematical relationship amongthe net force on a macroscopic object, its mass, and its acceleration. [Clarification Statement: Examples of data could include tables or graphs of position or velocity as a function of time for objects subject to a net unbalanced force, such as a falling object, an object sliding down a ramp, or a moving object being pulled by a constant force.] [Assessment Boundary: Assessment is limited to one-dimensional motion and to macroscopic objects moving at non-relativistic speeds.]
#Goals: SWBAT...
1. Use the concept "the stopping distance is proportional to the square of the velocity" to solve problems
2. Understand how internal vs. external forces affect the TME.
3. Use the Kinetic Energy (KE) equation to solve velocity problems.
4. Use the Work-Energy Equation (TMEi +Wext = TMEf) to solve medium difficulty energy problems with conservative and non-conservative forces
Not checking last nights HW assignment for credit
Warm-Up (4min):
Copy the diagram into your warm-up, then answer questions 1-3.
Use the following diagram to answer questions #1 - #3. Neglect the effect of resistance forces.
1. As the object moves from point A to point D across the surface, the sum of its gravitational potential and kinetic energies ____.
a. decreases, only
c. increases and then decreases
b. decreases and then increases
d. remains the same
2. The object will have a minimum gravitational potential energy at point ____.
3. The object's kinetic energy at point C is less than its kinetic energy at point ____.
CLASSWORK
1. #045A: HW Review
Link to the handout: HANDOUT
For each situation, indicate what types of forces are doing work upon the ball. Indicate whether the energy of the ball is conserved and explain why. Finally, fill in the blanks for the 2-kg ball.
The only force doing work is gravity (The Normal Force doesn't do work, as it's always perpendicular to the direction of motion). Since it is an internal or conservative force, the total mechanical energy is conserved. Thus, the 100 J of original mechanical energy is present at each position. So the KE for A is 50 J.
The PE at the same stairstep is 50 J (C) and thus the KE is also 50 J (D).
The PE at zero height is 0 J (F and I). And so the kinetic energy at the bottom of the hill is 100 J (G and J).
Using the equation KE = 0.5*m*v2, the velocity can be determined to be 7.07 m/s for B and E and 10 m/s for H and K.
2. #045B: PRACTICE PROBLEMS
4. Many drivers' education books provide tables that relate a car's braking distance to the speed of the car (see table below). Utilize what you have learned about the stopping distance-velocity relationship to complete the table.
5. Some driver's license exams have the following question.
A car moving 50 km/hr skids 15 meters with locked brakes. How far will the car skid with locked brakes if it is moving at 150 km/hr?
6. Two baseballs are fired into a pile of hay. If one has twice the speed of the other, how much farther does the faster baseball penetrate? (Assume that the force of the haystack on the baseballs is constant).
7. Use the law of conservation of energy (assume no friction) to fill in the blanks at the various marked positions for a 1000-kg roller coaster car.
8. If the angle of the initial drop in the roller coaster diagram above were 60 degrees (and all other factors were kept constant), would the speed at the bottom of the hill be any different? Explain.
9. Determine American ski jumper Lee Ben Fardest's (a mass of approximately 50 kg) speed at locations B, C, D and E.
10. An object which weighs 10 N is dropped from rest from a height of 4 meters above the ground. When it has free-fallen 1 meter its total mechanical energy with respect to the ground is ____.
11. During a certain time interval, a 20-N object free-falls 10 meters. The object gains _____ Joules of kinetic energy during this interval.
12. A rope is attached to a 50.0-kg crate to pull it up a frictionless incline at constant speed to a height of 3-meters. A diagram of the situation and a free-body diagram are shown below. Note that the force of gravity has two components (parallel and perpendicular component); the parallel component balances the applied force and the perpendicular component balances the normal force.
A. Of the forces acting upon the crate, which one(s) do work upon it?
B. Based upon the types of forces acting upon the system and their classification as internal or external forces, is energy conserved? Explain.
C. Calculate the amount of work done upon the crate.
STUCK? Here's the answers/solutions: LINK
At Home Learning (HW)
1. Complete the practice problems for #045B. I will check them for credit tomorrow.
2. Introduce yourself to our new concepts of momentum and impulse! :-) Take notes, and answer EdPuzzle questions:
(3:18) You Can't Run From Momentum! (a momentum introduction) - EDpuzzle
(3:57) Force of Impact Equation Derivation - EDpuzzle