1. Ch. 9 slides.ppt
WorkEnergy.ppt

2. Work Questions What forces act on the object?
For each of the forces that act on the object, is the work positive, negative or zero?
Is Wnet positive, negative, or zero? Does this agree with ∆K?
Does K increase, decrease, or stay the same
What is the sign of ∆K?
WorkEnergy.ppt

3. Pulling block
A 10.2 kg box is pulled by a rope 1 meter along a rough horizontal surface (µk = 0.4). The rope is pulled with 100 Newtons so that it makes a 30° angle with the horizontal. The box starts at rest.
What is the final speed of the box?
WorkEnergy.ppt

4. Gravity
A 100 g ball is dropped from rest at a height of 1m. What is the work done by gravity and what is the ball’s kinetic energy right before it hits the ground?
The same ball is now rolled down a ramp with that makes an angle of 30°with respect to the horizontal. The height of the ramp is 1m. Answer the same question as above and compare the work done by gravity and the ball’s final kinetic energy.
WorkEnergy.ppt

5. Car Power
Human-caused global warming is in large part caused by CO2 gas. One way to lower our CO2 emissions is driving cars less, using more efficient cars, and changing driving habits.
A Toyota Prius has mP = 1325 kg, while a Hummer H2 is 2.2 times that. They both can go 0 to 60 mph (26.8 m/s) in ~10 s. What is the work required to get the Prius to 60 mph from a stop and how much average power is used during this acceleration? How does the increased mass of the H2 affect these results?
Air resistance is very important to car efficiency. The Prius has KP = Ns2/m2, while the Hummer is 4.3 times that. How much work is required to drive a Prius 1 mi = 1.6 km at 60 mph? The Prius has over 50 mpg while an H2 gets about 10 mpg. Does drag or acceleration account for that difference?
If you double your speed, how does this affect the required work/mile and the power?
A Prius gets a higher mpg for city driving, but an H2 gets higher mpg for highway driving. Why are they different?
WorkEnergy.ppt

6. What is the rock’s maximum speed? What is
Sling shot
A 100 g rock is placed into a 10-cm elastic sling shot that has a spring constant of 5000 N/m. The sling shot is stretched 10 cm and then released so that the rock is launched at an angle of 50°.
What is the rock’s maximum speed?
What is
WorkEnergy.ppt

7. Block
A block of mass 1000-kg is compressed 10 m against a spring with spring constant k = 1000 N/m. The block follows the curve shown below. The track is frictionless, but the incline at the end of the track has coefficients of friction µs = 0.5 and µk = 0.25 (fk=1877 N). The incline makes a 40° with the horizontal and H = 10 m. C A B
How far up the incline does the block slide?
What is the speed of the block at point B?
WorkEnergy.ppt