# Week 10 (Oct. 14 - 18)

**Reading:**PSE Chap 7, Work and Energy

**Topics:**This week, we begin discussing the concepts of work, kinetic and potential energy, energy conservation, and power

**Lab:**

**Homework Problems:**Due on Wed. Oct. 25.

**David's sling:****Solution****.**King David, facing Goliath, swings a stone in the pouch of a sling above his head. The trajectory of the stone is in a circle whose plane is parallel to the ground. The sling is of length L = 1 meter; the stone is of mass m = 200 grams. The rotation rate is twice per second. Ignore gravity and drag (let's focus on the circular motion). (i) Draw a free body diagram for the stone at a particular instant. (ii) What is the speed of the stone? Is it accelerating? If so, in which direction? (iii) What is the tension in the string? What would happen if the string snapped?**Race track:****Solution****.**A 2000 kg race car rounds a curve at 150 mph. The radius of curvature of the track is 100 meters. What is the minimum coefficient of friction between the tires and the road so that the car does not slide?**Sled on a pond:****Solution****.**A sled of mass m is given a kick so it is sliding across a frozen pond at an initial speed of 2 m/s. The coefficient of kinetic friction between the sled and the ice is 0.10. Using energy considerations, find the distance the sled moves before coming to a stop. Hint: the friction force is doing work on the sled.**Climbing gymnast:****Solution****.**A 50 kg gymnast climbs a 10 meter high vertical rope in 5 seconds. Find the average power output of the gymnast in climbing the rope.

**Midterm:**We will have a Midterm on Monday, Oct. 17.

**I will be curving the grades for this class. Here is an estimate your midterm grade. Look at your current on-campus grade. If you are currently scoring**

Mid-term grade estimation:

Mid-term grade estimation:

above you are getting

###### ##############

93 A

85 A-

72 B+

68 B

else B-