Download Power, Simple Machines, braking

Physics Regular 1516 Williams
Rollercoaster Physics:
Power, Simple Machines,
braking
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Name:___________________________No:___________
Work and Skidding Cars
Helpful equations:
Kinetic Energy: KE = ½ mv2
Work: W = F d
1. If work is defined as a push or pull that makes an object move faster or lifts it up to give it
more potential energy, then would you say that brakes do positive or negative work?
2. When you slam on the brakes on your car, where does most of the energy of motion go?
3. If you double your speed, what happens to your kinetic energy?
4. If you double your speed, what happens to the amount of work done when you brake?
5. Assume that “stronger brakes” deliver larger stopping forces. If you get stronger brakes,
what happens to the distance you skid for any particular starting speed?
6. How are kinetic energy and skidding distances related?
7. Suppose you skid four meters when you are traveling at 20mph. How far will you skid in the
same car when you are moving at 40 mph?
8. How far would you skid when you are moving 80 mph?
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Name:________________________________No:________
POWER UP LAB!
Objective: Can you exert 1 horsepower? Some students can…The purpose of this activity is to determine your power when going up
stairs and comparing this power to other every day items.
Variable list: your weight, your height, time to run up stairs, shoe size, height of each stair, mother's maiden name, number of stairs,
length of stairs, acceleration of gravity, weight of the earth,
Purpose Questions: (SI units for everything)
1. What variables do you need to know in order to determine your Power for running up a flight of stairs?
2. What variables do you need to consider in order to determine the Work you do to run up a flight of stairs?
3. What is the Force you need to exert to move up a flight of stairs equal to?
Materials:
stopwatch
meter stick
your muscles
your brain
calculator / pencil
Method: Pick someone in your group will to run up the stairs. Time them going up the stairs for the following cases: Casual walk.
Walking quickly (two stairs at a time), Running up the stairs (quickly and carefully!)
Data: (show sample calculations for Trial #1)
Trial #
Time
Force (lbs)
Force
(s)
(N)
Casual
Distance (m)
Work (J)
Fast walk
Running
Useful facts:
2.205 pounds (lbs) = 9.81 Newtons (N)
100 cm = 1 meter (m)
746 Watts = 1 horse power (hp)
Sample Calculations (4 Steps!):
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Power (W)
Power (hp)
Analysis: (use data from student who ran up the stairs)
1. How fast do you have to go up these stairs to equal the power of a 100 W light bulb?
2.
A package of plain chocolate M&M’s™ that has 240 Calories. There are 4,184 J in a Calorie. How many stairs do you need to
climb to "burn off" those Calories?
3.
According to Washington University website, 1 gallon of gas releases 125 MJ of energy. Assuming that cars are about 25%
efficient with this energy, how many horsepower does your car demonstrate during a 1/2 hour drive at 60 mph? (assume 30
miles/gallon)…c'mon, this is just mostly conversions; I'm hoping you are good at conversions by now!
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Simple machines mini-lab
Goal: Demonstrate input and output forces and distances and efficiency for a lever and an inclined plane
SHOW ALL WORK!
Lever
The sketch above shows before the small mass lifts the large mass on left and after lifting on right. Show that
1. Energy is conserved, so work by small mass = work on large mass
2.
Find the AMA and the IMA
Inclined Plane
Find the weight of your cart. If there is no scale available, assume the mass of your cart is 500 g. Show below: din, dout, Fin, Fout (note:
no direct measurement of Fin)
Assuming 100% efficiency, compute Fin.
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Power/work review and simple machine basics
1. If you have a mass of 60 kg, how much do you weigh? (the force required to lift you)
2. If you lift up 60 kg 15 cm, how much work have you done?
3. If it takes you 0.6 seconds to do the above work, how powerful are you?
4. Imagine you have a mass of 70 kg and there are 20 steps, each 15 cm tall. How much work do
you do to lift yourself up these 20 steps? (hint: find the TOTAL distance!)
5. If it takes you 8.0 seconds to do the above work, how powerful are you?
6. Suppose a wheelchair ramp is 20 m long (hypotenuse) and lifts a person + chair with a total
mass of 80 kg up 1.5 m. (kinda hard problem, but it’s guided, so follow along…..)
a. What weight is being lifted? (this is force you get from the machine: Fout)
b. How much work is done against gravity?
c. How hard must you push? (this is force you put into the machine: Fin)
d. How powerful are you if you push the person up the ramp in 15 seconds?
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Unit 12 Physics Themed – Vocabulary and Equations – Work, Energy & Power in Rollercoasters
Ei = Ef (conservation of energy)
GPE = mgh
KE = ½mv2
ME = KE + GPE Wt = mg
W = Fd
P = W/t
AMA = Fo/Fi
IMA = di/do
Eff. = Wo/Wi
Eff. = Po/Pi
f = µN
Fnet = ma
ac = v2/r
Fc = mac (circ.) g’s = ac/9.8
x = circumference = 2πr
v = √(2g∆h) = √(19.6∆h)
1609 m = 1 mi
60 mph = 27 m/s
Symbols:
PE, KE, GPE, m, g, h, v
Vocabulary:
Joule
Newton
Kinetic energy (KE)
Potential energy (PE), Work
t
Δx
Δv
Equation
a
vyi vyf ∆y
v = ─── a = ───
vyf = vyi + at
Δt
Δt
    
Δx = v0 Δt + ½ at2
(vyi + vyf)
vf2 = vi2 + 2a Δx vf = v0 + a Δt
∆y = ───── t
   

Δx = vx Δt
2
vyi = v sinƟ vx = v cosƟ
∆y = vyit + ½at2
    
p = mv pi = pf
2
2
v
yf = vyi + 2a∆y
(m1v1 + m2v2)I = (m1v1 + m2v2)f
   

I = Δp = mΔv = FΔt
1 hp = 746 W 1 lb = 0.4536 kg 1 mi = 1609 m 1 W-s = 1 J 1 ft = 0.3048 m
Gravitational potential energy Deformation energy,
Sound energy
(GPE or PE)
Light energy, Thermal energy,
Mechanical energy,
Total energy, Mechanical energy (ME)
Elastic potential energy
Efficiency, Friction, Frictionless
Chemical energy, Nuclear
Conservation of energy
energy, Electrical energy
Pendulum, Perpetual motion machine
Internal energy,
Unit Objectives - Williams
1.
2.
3.
5.
6.
7.
8.
9.
10.
11.
12.
I understand all the vocabulary & math of this unit and all demos, videos, equations, and class assignments.
I remember objectives & vocabulary from previous units.
I understand that positive work requires a parallel force components and increases the energy of the object
I know that work can be negative and what this implies
I can compute work, power and various energy forms in absolute and relative terms based on speed, mass, height
and force direction values or changes in these values
I know conservation of energy including ME & friction converts useful energy (ME) to internal energy
I know common forms of energy and can identify them
I am able to compute power and can explain how it differs from work
I have memorized the current cost of energy locally per kilowatt hour
Given information on work, power, time etc., I can compute energy cost using the factor label method
I understand the relationship between work, force, energy and distance for brakes
13.
14.
I can recognize the relationships between W, P, t, d, K, etc graphically
I understand elastic systems including how force and stored energy vary with elongation/compression
4.
I can compute work where forces are at various angles relative to the displacement of the object
15. I understand what conservation energy means including common examples of real world exchanges of energy
16. I can to track the exchange energy for masses moving up and down hills while ME remains constant
17. Compute/graph energy totals & categories using GPE, KE, ME including using correct units and including how
changes in mass, speed and height change the results
18. Compute weight/mass
19. Identify and understand other forms of energy in unambiguous situations
20. Follow energy flow in pendulums and rollercoasters including consequential changes in speed and position when
heights are changed as well as novel situations; understand why first hill must be tallest
21. Conceptually, I can account for friction/thermal energy including the unavoidable energy flow in that direction
DuPage ROE Objectives
401. I can identify if masses have kinetic and/or potential energy at a given instant.
402. I can identify potential energy as a function of position.
403. I can identify kinetic energy as a function of velocity.
404. I can calculate gravitational potential energy and kinetic energy.
405. I can identify an isolated system and analyze it.
406. I can identify that energy is transferred between different forms.
407. I can solve problems using conservation of mechanical energy.
408. I can apply the mathematical definition of work as the product of Force and displacement.
409. I can identify situations of positive work, negative work, zero work.
410. I can identify work as a change in energy.
412. I can analyze the rate of energy change of a system in terms of power.
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Work, Power, Simple Machines Calendar: 2015-16 (Williams)
Bold and underlined means put in journal notes.
Mod
1
2
Date
3
4
Fr:03/11/16
Mo:03/14/16
Tu:03/15/16
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We:03/16/16
6
7
8
9
Th:03/17/16
Fr:03/18/16
Mo:03/21/16
Tu:03/22/16
10PTC
We:03/23/16
11-ED
Th:03/24/16
End Q3
Plans

Th:03/10/16 
(12-01) notes: Braking & simple machines
Work & skidding cars together (packet)
Trebuchet work time
 (12-02) notes simple machine calculations, tradeoffs
Trebuchet work time
 (12-03) notes Coaster strategies (power, work, etc.)
 Power up lab
 Simple machines mini-lab, then HW time
 Possible group/pairs quiz
Trebuchet work time, H10-23
 Clickers (8), HW time (H12-06 due in class, quiz?)
 Work, power, simple machines Quest (~50 pts)
Evening Parent Teacher Conferences, 6P - 9P
 Launch day! (Trebuchet)
Early Dismissal (11:30, provided no emergency days?)
 Physics video
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Homework
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H12-01
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H10-21 (yes, TEN-21)
H12-02
H10-22
H12-03
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H12-04
H12-05
Study for quest
Launch day tomorrow!