PH 211 Winter 2014

Friday February 14

Lab calendar update

<< Tuesday 2/11 Lab 5 >><< Thursday 2/13 Lab 4 >>• Tuesday 2/18 NO LAB• Wednesday 2/19 MIDTERM 2• Thursday 2/20 Lab 5

Midterm pickup

Tuesday 14:00 – 15:30

• Compare your exam and solutions.• Come to office hours for questions.• If asking for re-grading, use form from web-

site, and follow procedure.

Google moderator

Which topic would you like to discuss in class on Monday?

http://www.google.com/moderator/#16/e=20e597

Google moderator

Which topic would you like to discuss in class on today?

http://www.google.com/moderator/#16/e=20e889

On question 6.44, the problem states that a bag is dropped on a conveyor belt moving at a constant velocity. I am confused about how to determine the acceleration and/or force imparted on the bag."

"Can you explain how to do problem 6.28 from MasteringPhysics? "A 1400 kg steel beam is supported by two ropes." rope 1 is 20° left of vertical, rope 2 is 30° right of vertical. What is the tension in ropes 1 and 2?"

A box on a rough surface is pulled by a horizontal rope with tension T. The box is not moving. In this situation:

A. fs > T.

B. fs = T.

C. fs < T.

D. fs = smg.

E. fs = 0.Slide 6-75

A box on a rough surface is pulled by a horizontal rope with tension T. The box is not moving. In this situation:

A. fs > T.

B. fs = T.

C. fs < T.

D. fs = smg.

E. fs = 0. fs

> T.

fs = T.

fs < T.

fs = 0.

46%

0%2%0%

53%

A box has a weight of 100 N. It sits on the floor. It is then pulled by a 30 N horizontal force.

Does the box move?

A. Yes

B. No

C. Not enough information to say.

Slide 6-78

A box has a weight of 100 N. It sits on the floor. It is then pulled by a 30 N horizontal force.

Does the box move?

A. Yes

B. No

C. Not enough information to say.

YesNo

Not enough in

form

ation...

3%

80%

17%

An object is held in place by friction on an inclined surface. The angle of inclination is increased until the object starts moving. If the surface is kept at this angle, the object

1. slows down.2. moves at uniform speed.3. speeds up.4. none of the above5. There’s not enough information to answer this

An object is held in place by friction on an inclined surface. The angle of inclination is increased until the object starts

moving. If the surface is kept at this angle, the object

A. slows down.B. moves at uniform speed.C. speeds up.D. none of the aboveE. There’s not enough

information to answer this

slows d

own.

moves a

t unifo

rm sp

eed.

speeds u

p.

none of the ab

ove

There’s not e

nough in

fo...

3% 0%0%

76%

21%

Friction depends on normal force

Static friction: fs≤μsFnormal, opposes cause

Kinetic friction: fk=μkFnormal, opposes velocity

But how about this?

𝑣 , �⃗�

In case a the weight of the hanging block is 10 N, in case b the tension force is 10 N. Compare the accelerations of the two

systems. (the big block is the same in both cases) The acceleration is:

1. The same2. Greater on the left3. Greater on the right4. I have no idea

In case a the weight of the hanging block is 10 N, in case b the tension force is 10 N. Compare the accelerations of the two

systems. (the big block is the same in both cases) The acceleration is:

1. The same2. Greater on the left3. Greater on the right4. I have no idea

A compact car and a large truck collide head on and stick together. Which vehicle undergoes the larger acceleration during the collision?

1. Car2. Truck3. Both experience the same acceleration.4. Can’t tell without knowing the final velocity

of the combined mass.

A compact car and a large truck collide head on and stick together. Which vehicle undergoes the larger

acceleration during the collision?

A. CarB. TruckC. Both experience the

same acceleration.D. Can’t tell without

knowing the final velocity of the combined mass.

CarTru

ck

Both experie

nce th

e sam...

Can’t tell w

ithout k

nowi..

54%

10%

31%

6%

Newton 3

�⃗� 𝑎 ,𝑏=− �⃗� 𝑏 ,𝑎

�⃗� 𝑎 ,𝑏�⃗� 𝑏 ,𝑎

a b

Act on different objects!

𝑇=12(𝐹 𝑎,𝑏+𝐹 𝑏 ,𝑎)

𝐹 𝑛𝑒𝑡 ,𝑎=𝐹 𝑏 ,𝑎−𝑇 𝐹 𝑛𝑒𝑡 ,𝑏=𝑇 −𝐹 𝑎 ,𝑏

If Fab and Fba are not the same we get spontaneous motion!

Action-reaction pairs

Gravity: Tension:

And now …. this…..It is moving!

T1

m1

m1g

And now …. this…..Still moving

T2

m2

m2g

fk

Fn

𝜃

And now …. this…..It is moving!

zs Δz=Δs→a1=a2

T1

T2

T1 = T2

T1

m1

m1g

𝑇 1−𝑚1𝑔=𝑚1𝑎1

T2

m2

m2gfk

Fn

𝜃

𝐹 𝑛=𝑚2𝑔 cos(𝜃)

−𝑇 2+𝑚2𝑔sin (𝜃 )+ 𝑓 𝑘=𝑚2𝑎2

𝑇 1−𝑚1𝑔=𝑚1𝑎1

−𝑇 2+𝑚2𝑔sin (𝜃 )+ 𝑓 𝑘=𝑚2𝑎2

𝑓 𝑘=𝜇𝑘𝑚2𝑔cos (𝜃)

< 0

𝑇 1=𝑇 2

𝑇 1−𝑚1𝑔=𝑚1𝑎1

Demo

http://www.physics.umn.edu/outreach/pforce/circus/

Hammer and nail