PH 211 Winter 2014

Friday February 14

Lab calendar update

<< Tuesday 2/11 • • • << Thursday 2/13 Tuesday 2/18 Wednesday 2/19 Thursday 2/20 Lab 5 >> Lab 4 >> NO LAB MIDTERM 2 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=20e 597

Google moderator

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

http://www.google.com/moderator/#16/e=20e 889

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.

B.

C.

D.

E.

f

s >

T

.

f

s =

T

.

f

s <

T

.

f

s =  s

mg

.

f

s = 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.

f

s >

T

.

B.

f

s =

T

.

C.

D.

f

s <

T

.

f

s =  s

mg

.

E.

f

s = 0.

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.

80% 17% Yes 3% No No t en ou gh in fo rma tio n.

..

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 above 5. 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

76%

A. slows down.

B. moves at uniform speed.

C. speeds up.

D. none of the above E. There ’ s not enough information to answer this

3% 21% 0% 0% slo mo w s d ves ow at u n.

ni fo rm sp ee d.

sp ee ds no u p.

ne of th e ab ov e Th er e’ s n ot en ou gh in fo ...

Friction depends on normal force

Static friction: f s ≤μ s F normal , opposes cause Kinetic friction: f k =μ k F normal , opposes velocity

But how about this?

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 same 2. Greater on the left 3. Greater on the right 4. 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 same 2. Greater on the left 3. Greater on the right 4. 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. Car 2. Truck 3. 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. Car

54%

B. Truck

31%

C. Both experience the same acceleration.

6% 10%

D. Can ’ t tell without knowing the final velocity of the combined mass.

Ca r Bo th ex Tr uc per k ien ce th Ca e n’ sa t t m.

ell ..

w ith ou t k no w i..

Newton 3

𝑎,𝑏 = − 𝑏,𝑎 Act on different objects!

a b 𝑏,𝑎 𝑎,𝑏

𝑇 = 1 2 (𝐹 𝑎,𝑏 + 𝐹 𝑏,𝑎 ) 𝐹 𝑛𝑒𝑡,𝑎 = 𝐹 𝑏,𝑎 − 𝑇 𝐹 𝑛𝑒𝑡,𝑏 = 𝑇 − 𝐹 𝑎,𝑏 If F ab and F ba are not the same we get spontaneous motion!

Action-reaction pairs

Gravity: 𝐹 𝑎,𝑏 Tension: 𝐹 𝑎,𝑏 = 𝐹 𝑏,𝑎 = 𝐹 𝑏,𝑎 = 𝐺 𝑚 𝑎 𝑚 𝑏 𝑑 2 = 𝑇

And now …. this…..It is moving!

m 1 T 1 m 1 g

And now …. this…..Still moving

T 2 𝜃 m 2 m 2 g F n f k

And now …. this…..It is moving!

T 1 = T 2 T 2 T 1 s

Δz=Δs→a

1

=a

2 z

m 1 T 1 m 1 g

𝑇

1

− 𝑚

1

𝑔 = 𝑚

1

𝑎

1

T 2 −𝑇 2 + 𝑚 2 𝑔 sin 𝜃 + 𝑓 𝑘 = 𝑚 2 𝑎 2 F n 𝐹 𝑛 = 𝑚 2 𝑔 cos(𝜃) 𝜃 m 2 m 2 g f k

𝑇

1

− 𝑚

1

𝑔 = 𝑚

1

𝑎

1 −𝑇 2 + 𝑚 2 𝑔 sin 𝜃 + 𝑓 𝑘 = 𝑚 2 𝑎 2 𝑓 𝑘 = 𝜇 𝑘 𝑚 2 𝑔 cos(𝜃) 𝑎 1 = 𝑎 2

< 0

𝑇 1 = 𝑇 2

𝑇

1

− 𝑚

1

𝑔 = 𝑚

1

𝑎

1 −𝑇 1 + 𝑚 2 𝑔 sin 𝜃 + 𝜇 𝑘 𝑚 2 𝑔 cos(𝜃) = 𝑚 2 𝑎 1 −𝑚 1 𝑎 1 − 𝑚 1 𝑔 + 𝑚 2 𝑔 sin 𝜃 + 𝜇 𝑘 𝑚 2 𝑔 cos(𝜃) = 𝑚 2 𝑎 1 𝑚 1 + 𝑚 2 𝑚 2 𝑔 (sin 𝜃 + 𝜇 𝑘 𝑎 1 = cos(𝜃)) − 𝑚 1 𝑔

Demo

http://www.physics.umn.edu/outreach/pforce/c ircus/ Hammer and nail