Today’s Goals

• Introduce idea of Movie Review (due

March 13)

• Practicing With More Complicated Free

Body Diagrams to Solve Problems

• Learning How to Solve Problems on an

Incline

Test 2: Feb 26, 7pm, B51

“Movie” Review Part 1 (Your plan)

due in class March 13 – please print itGrammar and

Typos (2 pts)

Few if any grammatical errors and/or typos. In the real world, everything

you turn in should look professional. Practice now.

Description of Scene

( 5 points)

Based upon your description, it is very clear what is going on in the

scene (however, a youtube link is also helpful).

Appropriateness

(3 points)

You picked a scene related to class material and not obviously possible

or impossible (e.g. people flying), argument must be math based!

Formulas Provided

( 5 points)

Approach (5 pts.)

All of the formulas and only the appropriate formulas needed to calculate

the scene have been provided.

You have picked a reasonable approach with no flaws.

Identify variable(s)

to solve for

(5 points)

At least one variable is identified as what you will solve for to determine

if the scene can occur in real life. (Generally that variable is not time,

though you may need to solve for it to get something else.)

How you will find

out if variable(s)

reasonable

( 5 points)

You discuss how you will determine if your selected variable(s) are

actually achievable. This determination is either based on things that can

be found in papers, or based on more calculations.

Estimations

(5 points)

Any variables that need to be estimated are identified as well as how

estimations will be made.

To keep in mind

• Outlawed Scenes: Speed bus scene, circular bullet on

Wanted, Skyscraper movie poster, October Sky

(because it’s not fictional)

• Physics that is used a lot (sometimes several of

these): projectile motion, forces, friction, energy

(Ch.5) , impulse (section 6.1), pressure (section 9.2),

strain (section 9.3) if discussing bones breaking

• To include all of the necessary information, a good

Movie 1 proposal is typically almost one full page

• I can’t read your mind. The more you explain, the

better I can help you if you have something wrong.

Better to mess up on Movie 1 than Movie 2, which is

worth more points.

Tension

A 1200 kg elevator car

accelerates upward at 1

m/s2. Find the tension in

the cable.

The weight and tension the

cable can support limit the

acceleration that is safe.

Simpler, similar version:

Free Body Diagrams in Medical Treatment

Healing Depends on Supporting and sometimes gentle pulling

A setup similar to the one shown in the figure below is

often used in hospitals to support and apply a traction

(pulling) force to an injured leg.

(a)Determine the force of tension in the rope supporting the

leg (the upward force).

(b) What is the traction

force exerted on the leg?

Assume the traction force

is horizontal.

Medical Application: Support for Recovery

The trick with force

problems like this is to

figure out what to take a

free body diagram of.

The problem seems to suggest the leg, but you don’t know much about the leg.

Another thing FBDs are good for:

Inclined PlanesPhysics is infamous for sliding blocks down inclined planes

How boring! Why do we study it?

Many things we do involve inclines.Blocks are an easy way to simplify many complex objects.

In physics, we approximate

things as blocks or round

objects.

A real example: My Icy Driveway

When my driveway is a sheet of ice (ignore

friction), how fast do I need to be driving to get to

the top of my driveway?

Inclined

Driveway

Garage

3 m

(~10 feet)

Road~15°

Making 2 dimensions only 1!

x

y

A B

Why would I change x and y?

In Case A, need vx, vy, ∆x, ∆y?

Have to break up the vector components

In Case B: ∆y=0, vy=0

Acceleration changes though: ax=±9.8 sinθ

Better, unless if going around an inclined curve (Ch. 7)

Why ag,x=±9.8 Sinθ?

θ

y

x

y component of

acceleration

x component of

acceleration

Does the acceleration in the y

direction mean that it will

change velocity in y

direction? (Tricky question)

θθθθSAME ANGLE!

g

http://zonalandeducation.com/mstm/physics/mech

anics/forces/inclinedPlane/inclinedPlane.html

(Based on similar triangles argument)

Breaking up vector components:

Draw a line

parallel to y axis

from start of the

vector

Draw a line

parallel to x axis

from end of vector

Free fall is faster,

only part of

gravity pulls

downhill

My Icy Driveway

When my driveway is a sheet of ice (ignore friction

on slope), (a) how fast do I need to be driving to get to

the top of my driveway? (b) Is this feasible on an icy

day? (c) Is it feasible if my car was not in a garage?

Inclined

DrivewayGarage

3 m

Road~15°

Acceleration of starting for a typical car is only 0.5g.

So, how could I get out of my driveway?

When released, the cart accelerates up the ramp.

Which of the following is a correct free-body diagram for the cart?

A. B. C. D.

m1a m1a

w1 w1 w1 w1

T T T T

n n n n

A cart (weight w1) is attached

by a lightweight cable to a

bucket (weight w2) as shown.

The ramp is frictionless.

Q44

As long as the rope is free to move (e.g.

on a pulley), the tension in the rope is

the same at all locations on the rope.

Consider without plank

support

Net Force is Zero When an Object is in Equilibrium

If we want this 4 kg block (or leg) not to move when

the black board is removed, what weights should we

add to the ropes?

Draw the free body diagram when the black board is

removed.

25°65°

Atwood’s machineInvented in 1784 to verify constant

acceleration equations. Same principle

used in elevators and funicular railways.

Two masses hang from a pulley as shown in left

figure, with masses 2.0 kg and 4.0 kg. Find the

acceleration of each mass. Neglect the mass of the

pulley itself and the string.

2.0 kg

4.0 kg

Assumptions: massless

(very light) string and pulleym1=

m2=

Forces on Cars

A car accelerates down a

straight highway. Which of

the free-body diagrams

shown best represents the

forces on the car?

Friction prevents the wheels

from just spinning in place.

This is why a car on ice

sometimes can’t move.

(a) (b)

(c) (d)

(e) None of the Above

Q46

Consider the 3 situations below,

labeled A, B and C. Ignore friction.

After each system is released from rest, how do the tensions

in the strings compare?

A. A = B = C

B. A < B < C

C. A < C < B

D. B < A < C

E. B < C < A Q47

More challenging question

If you struggle to think

conceptually about it, it is

easier to determine if you

draw FBD & sum forces

Clicker Answers

Chapter/Section: Clicker #=Answer

44=A, 45=C, 46=D, 47=E