PH 211 Winter 2014

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: fs > T. fs = T. fs < T. fs = smg. 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: • fs > T. • fs = T. • fs < T. • fs = smg. • fs = 0. 0 of 207 :01

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? Yes No 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? • Yes • No • Not enough information to say. 0 of 207 1

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 • slows down. • moves at uniform speed. • speeds up. • none of the above • 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 • slows down. • moves at uniform speed. • speeds up. • none of the above • There’s not enough information to answer this 135 of 207 1

Friction depends on normal force Static friction: fs≤μsFnormal, opposes cause Kinetic friction: fk=μkFnormal, 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: • The same • Greater on the left • Greater on the right • 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? • Car • Truck • Both experience the same acceleration. • 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? • Car • Truck • Both experience the same acceleration. • Can’t tell without knowing the final velocity of the combined mass. 126 of 207 1

Newton 3 Act on different objects! 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 Fn fk m2 m2g

And now …. this…..It is moving! T1 = T2 T2 T1 s Δz=Δs→a1=a2 z

T1 m1 m1g

T2 Fn m2 fk m2g

< 0

Demo http://www.physics.umn.edu/outreach/pforce/circus/ Hammer and nail

PH 211 Winter 2014