Physics120108: First Week of Autumn

A dinosaur from the future came in to our class one day and ate the previous scribe. Then it ran away. Since the previous scribe is unable to scribe and I'm due for scribing anyways (yes, scribing is worth marks), I'm taking advantage of this blog drought as I do this post over the weekend. Here's an abbreviated overview of what we did Friday, September 26 to Monday, September 22nd.

FRIDAY, SEPTEMBER 26

Redo Coefficient of Sliding Friction Lab
Chapter 9 Study Guide (Impulse and Change in Momentum and The Conservation of Momentum) Worksheet (homework)
Chapter 7 Momentum Worksheet (homework)

THURSDAY, SEPTEMBER 25

Sang along to the M Times V song
Watched Momentum Video (same-same guy) (5 points and hand-in)

WEDNESDAY, SEPTEMBER 24

Dynamics Test

TUESDAY, SEPTEMBER 23

Corrected Forces Review and Forces Evaluation: Using Concepts

MONDAY, SEPTEMBER 22

Forces Review, Forces Enrichment, Forces Evaluation: Using Concepts, Frictional Forces, and Advanced Forces was assigned last Friday. These should be done by now.
5.2 Using Newton's Law: Read pp. 93-103. Questions #16, 18, 19, 22, 23 (pp. 123-126).
6.3 Application of Vectors: Read pp. 123-126. Questions #31, 35 (p. 131)

5.2 Using Newton’s Law Read pp. 98-103 Questions #16, 18, 19, 22, 23 (pp. 106-107)

16. You are driving a 2500.0-kg at a constant speed of 14.0 m/s along an icy, but straight and level road. While approaching a traffic light, it turns red. You slam on the brakes. Your wheels lock, the tires begin skidding, and the car slides to a halt in a distance of 25.0 m. What is the coefficient of sliding friction (μ) between your tires and the icy roadbed?

18. A 4500-kg helicopter accelerates upward at 2 m/s2. What lift force is exerted by the air on the propellers?

19. The maximum force a grocery bag can withstand and not rip is 250 N. If 20 kg of groceries are lifted from the floor to the table with an acceleration of 5 m/s2, will the bag hold?

22. A sled of mass 50 kg is pulled along snow-covered, flat ground. The static friction coefficient is 0.30, and the sliding friction coefficient is 0.10.
a. What does the sled weigh?
b. What force will be needed to start the sled moving?
c. What force is needed to keep the sled moving at a constant velocity?
d. Once moving, what total force must be applied to the sled to accelerate it 3.0 m/s2?

23. A force of 40 N accelerates a 5.0-kg block at 6.0 m/s2 along a horizontal surface.
a. How large is the frictional force?
b. What is the coefficient of friction?

6.3 Application of Vectors Read pp. 123-126 Questions #31, 35 (p. 131)

31. A street lamp weighs 150 N. It is supported equally by two wires that form an angle of 120° with each other.
a. What is the tension of each of these wires?
b. If the angle between the wires is reduced to 90.0°, what new force does each wire exert?

35. You slide a 325-N trunk up a 20.0° inclined plane with a constant velocity by exerting a force of 211 N parallel to the inclined plane.
a. What is the component of the trunk’s weight parallel to the plane?
b. What is the sum of your applied force, friction, and the parallel component of the trunk’s weight? Why?
c. What is the size and direction of the friction force?
d. What is the coefficient of friction?