Phys. 121: Tuesday, 14 Oct.● Reading: Begin ch. 12 for Thurs.; finish by next week.● Written HW 7: due by 2:00 pm.• Written HW 8: ch. 9, problems 2, 9, 12, 24, 51, and 63. Due in one week.● Mastering Phys.: Sixth assign. due Thursday. Extra credit problems now available.● Exam 1: I will add 1 pt. (curve) to score shown.

Written problems (9 and 10) may be re-worked for

half the credit back if you missed more than 0.1 on

them; go to an Office for Student Learning tutoring

session this week, then bring the corrected problems

back to me, to do so

A. A force that is applied at a random time.B. A force that is applied very suddenly.C. The area under the force curve in a force-versus-

time graph.D. The time interval that a force lasts.

Impulse is

Reading Question 9.2Clickers:

How is momentum exchanged in collisions?We're usually not as interested in the detailsof the forces, butonly in the totalmomentumtransferred. Thisis called theimpulse. Impulseis given by theintegral of forceover time.

Clickers: What are the dimensionsof impulse?

• a) The same as those of force• b) The same as those of energy• c) The same as those of acceleration• d) The same as those of distance• e) The same as those of momentum

The cart’s change of momentum px is

A. –20 kg m/s.

B. –10 kg m/s.

C. 0 kg m/s.

D. 10 kg m/s.

E. 30 kg m/s.

Clickers:

A 2.0 kg object moving to the right with speed 0.50 m/s experiences the force shown. What are the object’s speed and direction after the force ends?

A. 0.50 m/s left.

B. At rest.

C. 0.50 m/s right.

D. 1.0 m/s right.

E. 2.0 m/s right.

Clickers:

A mosquito and a truck have a head-on collision. Splat! Which has a larger change of momentum?

A. The mosquito.

B. The truck.

C. They have the same change of momentum.

D. Can’t say without knowing their initial velocities.

Clickers:

Collisions: Elastic vs. Inelastic• Energy is always conserved in collisions! But sometimes

it takes other forms (heat, sound) besides kinetic or potential energy.

• In an Elastic Collision, the kinetic energy of the objects is conserved also. (Example: drop a ball onto the floor

which bounces ALL the way back up.)

• In a Totally Inelastic Collision, the objects stick together after colliding. (Example: drop a lump of clay onto the

floor which sticks without bouncing at all.)

• Most collisions are neither perfectly elastic nor inelastic (most dropped balls will bounce back to only part of their

original height).

Most real collisions (of large objects) are neitherperfectly elastic nor perfectly inelastic, butsomewhere in between.

Strategy for collision problems:

• Always conserve total momentum of colliding objects! Special case: if one object is VERY much more massive than the other (a brick wall or the Earth versus a ping-pong ball, for example), then treat the massive object as fixed in place instead.

• If perfectly elastic, use conservation of kinetic energy to solve for the final velocities. If perfectly inelastic, assume that the objects stick together and only use momentum conservation to solve for final velocities (or masses or other unknowns).

For a totally inelastic collision betweentwo objects, the objects stick together!

Clickers:

The 1 kg box is sliding along a frictionless surface. It collides with and sticks to the 2 kg box. Afterward, the speed of the two boxes is

A. 0 m/s.

B. 1 m/s.

C. 2 m/s.

D. 3 m/s.

E. There’s not enough information to tell.

Ballistic Pendulum: How can we use energyconservation if a collision is inelastic? Answer:work the problem in two separate parts.

Clickers: one ball is at rest on a level floor. A second, moving ball collides elastically with

the first, and they then both move in the same direction. What can you conclude about the

masses of the two balls?• a) No conclusion is possible.

• b) The masses of the balls must be the same.

• c) The originally stationary ball must have more mass than the moving ball.

• d) The originally stationary ball must have less mass than the moving ball.

• e) Neither ball can have any mass.

Clickers: While moving inventory at the gym,you drop several balls toward the floor. Can anyof these ever bounce back to higher than theiroriginal height, if they were dropped from rest?

a) Yes; physics will allow it. b) No: that's magic and only works for Harry Potter. c) Ask again when it's not 49ers week, and maybe I will care then.