Problem Solving Day
and
Movie Project Introduction
Two things we’re doing today.
- Doing some problem solving. Going to do a broad range of problems.
- Movie project introduction too.
Movie Project Introduction
• See syllabus and my website.
• 5% of total grade
Website has full description of project. Briefly:
Pick a movie scene in which one or more of the physical principles we have discussed in class is happening.
Movie Project Introduction
• Draft 1 due: March 20.
• Final paper due: April 20.
Website has full description of project.
Best: Fictional movies where something real
is actually happening.
Wingardium
LeviOsa
Fmagic
Fg
[Fun to think about but BAD EXAMPLE for this project!]
Now I’m going to go through a movie project example. With a scene that half of the world rolled their eyes at the first time they saw it.
As if this scene wasn’t bad enough as is, now I’m going to make you apply physics to it.
Let’s start by SETTING UP THE PROBLEM. What question do I want to ask about this scene?
Legolas slides down this incline.
Does he get to the bottom in a feasible
amount of time?
SET UP THE PROBLEM FIRST. What question am I trying to answer?
Then: What physical principles do I need to use? THIS IS SOMETHING YOU’LL HAVE TO DO ON THE TEST TOO! Understand which equations and physics principles .
Since I want you actively thinking about it I’ve done this as a clicker question.
Legolas slides down this incline.
Does he get to the bottom in a feasible
amount of time?
What physical principles should I use to analyze this question?
A.
B.
C.
D.
Answer: A.
Kinematics and Newton’s laws.
Conservation of energy and friction.
Conservation of energy (only).
Kinematics (only).
Q59
What kind of problem is it?
• Location or velocity vs. time important?
—> Kinematics
• Are you considering velocities for some change in
height?
—> Energy conservation
• Are there forces influencing the scenario?
—> Consider frictions, tensions if necessary
• Are you dealing with a collision or explosion?
—> Momentum/impulse
—> Conservation of momentum
You might need to combine a few concepts!
For this assignment and for the upcoming test, one of the first difficulties will be simply identifying what concept to use on a problem. THERE’S NO HARD RULE OF
THUMB, SO YOU SHOULD STAY ON YOUR FEET AND WORK ON THINKING CRITICALLY. Sometimes there are more than one way to solve things, so just try!
You might ask yourself: what physics is acting in this scene? Can I assume a frictionless surface? Should I not?
Legolas (m=80 kg) slides down a 12m ramp that is inclined
30o from the horizontal. His initial velocity down the ramp
is 2 m/s. How long should it take him to get to the bottom
of the ramp, assuming it is frictionless?
(note: in the film, it takes him ~5.5 s)
I’ve made estimations from the video as you will be expected to do. What information is relevant? How did I estimate things in this video?
Now, this is meant to be a problem solving day, and some of you asked me to do some problems on inclines, so here you go.
The answer is 1.85s. COULD THIS STILL BE FEASIBLE? OBVIOUSLY FRICTIONLESS IS NOT A GREAT ASSUMPTION. Thus I’ve phrased my question wrong.
Legolas (m=80 kg) slides down a 12m ramp that is inclined
30o from the horizontal in 5.5 seconds. His initial velocity
down the ramp is 2 m/s. What is the force of kinetic
friction between his shield and the ramp?
So obviously we haven’t accounted for all the physics. Let’s try this phrased as another problem, since from the film we know how long it took him to slide down.
Something else has to be acting on his shield to slow it down… Friction, of course!
Legolas (m=80 kg) slides down a 12m ramp that is inclined
30o from the horizontal in 5.5 seconds. His initial velocity
down the ramp is 2 m/s. What is the force of kinetic
friction between his shield and the ramp?
You will need to cite external references to understand and discuss the feasibility of your movie outcome. See rubric on movie project description on my website.
Other online examples
(You can follow these links when I post
these slides on my website after class)
“The Physics of Action Movies”
https://www.youtube.com/watch?v=1kUGyZNKsws
“Good and Bad Examples of Physics in Movies”
https://www.youtube.com/watch?v=ud592YKhePI
What kind of problem is this?
A wooden block is pulled 5m across a frictionless surface
using a rope. The tension in the rope is 30 N; and the net
work done on the block is 105 J. What angle does the rope
make with the horizontal?
1. Draw it!
2. Write knowns and unknowns.
3. Look at equation sheet.
vector components, definitely something about work, seem to be no velocities involved here, no friction, not thinking about movement vs time so no kinematics, so
what’s happening?
Have them do this and tell them I’m going to ask a clicker about this.
What kind of problem is this?
A wooden block is pulled 5m across a frictionless surface
using a rope. The tension in the rope is 30 N; and the net
work done on the block is 105 J. What angle does the rope
make with the horizontal?
A.
B.
C.
D.
E.
Work-energy theorem.
Conservation of energy.
Impulse.
Work done by gravity.
Definition of work.
Q60
What will you use?
DO THE PROBLEM ON LIGHT BOARD.
Gun recoil
The first problem with this is that he had his hands out—the gun should have recoiled his hands into his chest.
Gun recoil
Where should the recoil have acted?
The first problem with this is that he had his hands out—the gun should have recoiled his hands into his chest.
It’s going to snow next week…
Enjoy the weather!