C3 – Interactions transfer momentum Finish C3 slides Finish C2 and C3 Lecture Worksheet C4 - Particles and Systems General Physics

Solutions to HW Download ProbViewer 1.4 Password for daily HW problems www.physics.pomona.edu/sixideas/sicpr.html Password for daily HW problems phys130cn General Physics

Quiz Problem 1 – C4X.1 Consider a system consisting of two billiard balls on a pool table. If the balls collide, is the interaction between the ball an internal or external interaction? A. Internal B. External General Physics

Two Minute Problem – C4T.1 mod Which of the following things does NOT qualify as being an “extended object”? A. An electron B. An atom C. A rock D. A human being General Physics

Force Rate at which momentum flows General Physics Stopped here Did the C2-C3 lecture worksheet. Most students completed 2 or 3 of the problems. Needed help with first one. General Physics

Impulse Questions In terms of impulse and momentum, why do airbags in cars reduce the chances of injury in accidents? When jumping from a significant height, why is it advantageous to land with you knees bent? General Physics

Mass and Weight In physics, mass and weight are completely distinct concepts Where m = mass (does not = weight), a scalar = is the gravitational field vector = weight, a force and vector General Physics

Momentum flow and motion - 1 Multitap The initial arrow show the ball’s initial momentum. The taps represent the impulse being delivered to the ball due to the gravitational interaction. You add the tap impulse to the initial impulse to get the momentum after the tap. General Physics

Momentum flow and motion - 2 Reservoir Momentum is only changing in the z direction because there is only a gravitational interaction acting on the ball. The x reservoir and y reservoir don’t change. The x has a positive value for momentum because the ball has a horizontal initial velocity. No interactions act in the x-direction. General Physics

Conservation of Momentum A system’s total momentum is the vector sum of the momenta of its constituent particles The total momentum of an isolated system is conserved; that is, it does not change with time General Physics

Center of Mass is the system’s center of mass are the positions of the system’s N particles are the masses is the system’s mass General Physics

Center of Mass, Example Both masses are on the x-axis The center of mass is on the x-axis The center of mass is closer to the particle with the larger mass General Physics

Center of mass - column vector notation General Physics

Think-Pair-Share – C4B.2 Consider a system consisting of two point particles A and B. The particles are 25 cm apart, and the distance between particle A and the system’s center of mass is 5 cm. What is the mass of particle B compared to the mass of A? General Physics

Think-Pair-Share – C4B.3 In a certain reference frame, a 0.20-kg point particle lies 22 cm due north of the origin, another 0.15-kg particle lies 38 cm due west of the origin, while a 0.40-kg particle lies at the origin. Locate the center of mass of this system in this reference frame. General Physics

Center of mass activity - 1 You will need Meter stick Fulcrum Mass hangers masses General Physics

Center of mass activity - 2 Use column vector notation for two examples Same mass Different masses Find location of and compare to experimental result General Physics

System’s Momentum A system’s center of mass responds to its external interactions exactly as a point particle would respond to those interactions General Physics

Think-Pair-Share A uniform piece of sheet steel is shaped as shown. Compute the x and y coordinates of the center of mass of the piece. Use column vectors – good Matlab problem Rcm = (1/M)*((3M/6)[15 5] + (M/6)[5 15] + (2M/6)[10 25]) Rcm = [11.6 13.3] General Physics

Motion of the Center of Mass A projectile is fired into the air and suddenly explodes With no explosion, the projectile would follow the dotted line After the explosion, the center of mass of the fragments still follows the dotted line, the same parabolic path the projectile would have followed with no explosion General Physics

Center of Mass Dick Fosbury introduced a new high jump technique - the Fosbury Flop - at the 1968 Olympics. Raised world record 30 cm! General Physics

Inertial Reference Frames How do we distinguish between an inertial and noninertial reference frame? Test Newton’s first law An object in motion stays in motion and an object at rest stays at rest A reference frame attached to a truck that starts and stops is not an inertial reference frame Earth and space station okay if we ignore gravitational interaction General Physics

Interacting with the earth Consider the interaction of you and the earth. If you jump up, you receive an upward impulse from the earth and the earth receives a downward impulse from you. Using a reference frame attached to the center of mass of the system of the earth and you, the earth does move, but the distance is too small to detect. General Physics