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Mechanics Lecture 10, Slide 1 Announcements Quiz 4 on Friday Covers Unit 7 (Work and Energy) and Unit 8.

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Presentation on theme: "Mechanics Lecture 10, Slide 1 Announcements Quiz 4 on Friday Covers Unit 7 (Work and Energy) and Unit 8."— Presentation transcript:

1 Mechanics Lecture 10, Slide 1 Announcements Quiz 4 on Friday Covers Unit 7 (Work and Energy) and Unit 8 (Potential Energy)

2 Mechanics Lecture 10, Slide 2 Lecture 10: Center of Mass Todays Concept: Center of mass Finding it Using it

3 Mechanics Lecture 10, Slide 3 Center of Mass

4 Mechanics Lecture 10, Slide 4 Cube Example

5 Mechanics Lecture 10, Slide 5 Cube Example: Using Volume Mass Density ρ = mass density (mass/volume)

6 Mechanics Lecture 10, Slide 6 Cube Example: CM Calculation

7 Mechanics Lecture 10, Slide 7 x = L x = 0 Exercise Show mathematically that the center of mass of a uniform stick of length L and mass M is at x = L/2. dm

8 Mechanics Lecture 10, Slide 8 x = L x = 0 Exercise Show mathematically that the center of mass of a uniform stick of length L and mass M is at x = L/2. dm

9 Mechanics Lecture 10, Slide 9 Center of Mass for System of Objects

10 Mechanics Lecture 10, Slide 10 The disk shown in Case 1 clearly has its CM at the center. Suppose the disk is cut in half and the pieces arranged as shown in Case 2 In which case is the center of mass highest? ACT: Disk vs. 2 half-disks Case 1Case 2 x CM x A) Case 1 B) Case 2 C) same

11 Mechanics Lecture 10, Slide 11 The disk shown in Case 1 clearly has its CM at the center. Suppose the disk is cut in half and the pieces arranged as shown in Case 2 In which case is the center of mass highest? A) Case 1 B) Case 2 C) same ACT: Disk vs. 2 half-disks Case 1Case 2 x CM x x x x

12 Mechanics Lecture 10, Slide 12 If the net external force is zero, the CM wont accelerate Kinematic Quantities

13 Mechanics Lecture 10, Slide 13 ACT: System of particles A) 00 B) 0H/2 C) 0H/3 D) H/4H/4 E) H/40 x y x y H Three tiny equal-mass magnets are placed on a horizontal frictionless surface at the corners of an equilateral triangle. When the magnets are released, they attract and quickly slide to a single point. What are the coordinates of that point?

14 Mechanics Lecture 10, Slide 14 C) 0 H/3 X Y (symmetry) x y H

15 Mechanics Lecture 10, Slide 15 = -1 m Homework Problem = m

16 Mechanics Lecture 10, Slide 16 Homework Problem

17 Mechanics Lecture 10, Slide 17 m5m5 CM moves toward m 5 ABCDABCD cm

18 Mechanics Lecture 10, Slide 18 m5m5 CM moves toward m 5 cm ABCABC

19 Mechanics Lecture 10, Slide 19 CheckPoint: System of Two Masses Two objects, one having twice the mass of the other, are initially at rest. Two forces, one twice as big as the other, act on the objects in opposite directions as shown. A) A CM = F/M to the right B) A CM = F/(3M) to the right C) A CM = 0 D) A CM = F/(3M) to the left E) A CM = F/M to the left Which of the following statements about the acceleration of the center of mass of the system is true? F2F2F M 2M2M About 42% answered correctly.

20 Mechanics Lecture 10, Slide 20 CheckPoint Revote: System of Two Masses Which of the following statements about the acceleration of the center of mass of the system is true: C) The actual masses are accelerating away from each other at the same rate so the center of mass does not change. D) CM acceleration is defined as the sum of the total forces divided by the total mass. Total mass is 3m. The forces are not equal, therefore the greater force will determine the direction of acceleration. E) The acceleration of the center of mass is equal to the external force divided by the total mass. C) A CM = 0 D) A CM = F/(3M) to the left E) A CM = F/M to the left F2F2F M 2M2M

21 Mechanics Lecture 10, Slide 21 ACT: System of 2 Masses A) B) C) D) Two objects, one having twice the mass of the other, are initially at rest. A force F pushes the mass M to the right. A force pushes the mass 2M upward. What is the acceleration of the center of mass? M 2M2M F

22 Mechanics Lecture 10, Slide 22 CheckPoint: Two Men Pulling A) -3 m B) 0 m C) 3 m 3 m 0 m3 m Two guys who weigh the same are holding onto a massless pole while standing on horizontal frictionless ice. If the guy on the left starts to pull on the pole, where do they meet? Where is the center of mass of the system initially? Does the center of mass accelerate?

23 Mechanics Lecture 10, Slide 23 ACT: 2 guys pulling, II A) -3 m B) -1 m C) 0 m D) 1 m E) 3 m A large skinny guy with mass 2M and a smaller guy with mass M are holding onto a massless pole while standing on frictionless ice, as shown below. If the little guy pulls himself toward the big guy, where would they meet? 3 m 0 m3 m 2M2M M Where is the center of mass of the system initially? Does the center of mass accelerate?

24 Mechanics Lecture 10, Slide 24 A) -3 m B) -1 m C) 0 m D) 1 m E) 3 m A large skinny guy with mass 2M and a smaller guy with mass M are holding onto a massless pole while standing on frictionless ice, as shown below. If the little guy pulls himself toward the big guy, where would they meet? 3 m 0 m3 m 2M2M M m m 1 m ACT: 2 guys pulling, II


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