Questions

1. In tryouts of the national bobsled team, each competing team pushes a sled along a level, smooth surface for 5 meters. One team brings a sled that is much lighter than the others. Assuming that this team pushes with the same force as the others, compare the kinetic energy of the light sled to that of the others after 5 meters . Compare the momentum of the light sled to that of the others after 5 meters.

2. Suppose the rules were changed in previous question so that the teams pushed for a fixed time of 5 seconds rather that a fixed distance of 5 meters. Compare the momentum of the light sled to that of the others after 5 seconds. Compare the kinetic energy of the sled to that of the others after 5 seconds.

Questions1. You have been asked to analyze a collision at a traffic

intersection. Will you be better off to begin your analysis using conservation of momentum or conservation of kinetic energy? Why?

2. If a system has zero kinetic energy, does it necessarily have zero momentum? Give an example to illustrate your answer.

3. An object has a velocity toward the south. If a force is directed toward the north, will the kinetic energy of the object initially increase, decrease, or stay the same? Explain.

Conceptual Example: Speeds on Two Water Slides

Two water slides at a pool are shaped differently, but have the same length and start at the same height h. Two riders, Paul and Kathleen, start from rest at the same time on different slides.

(a) Which rider, Paul or Kathleen, is traveling faster at the bottom?

(b) Which rider makes it to the bottom first?

1. Does a car consume more fuel when its air conditioner is turned on? When its lights are on? When its radio is on while it is sitting on the parking lot?

2. Rows of wind-powered generators are used in various windy locations to generate electric power. Does the power generated affect the speed of the wind? That is, would locations behind the “windmills” have more wind if the windmills weren’t there?

Machines

A machine is a device for multiplying forces or simply changing the direction of forces.

The Lever

Work Input = Work Output

Input Force x Input Distance = Output Force x Output Distance

A lever can be a force multiplier.

But no machine can multiply work!!!!

Another Simple Machine:Inclined Plane

If we gain nothing for work, why bother?

Pulley: the lever in “disguise”

Inversed Example: Bicycle

Efficiency

Efficiency = (useful energy output)/(total energy input)

Question

Consider an imaginary miracle car that has a 100% efficient engine and burns fuel that has an energy content of 40 mega joules per liter. If the air drag and overall frictional forces on the car traveling at highway speed are 500 N, what is the upper limit in distance per liter the car could go at this speed?

Comparison of Kinetic Energy and Momentum

Question

Suppose a hunter is confronted with a charging bear. Which be more effective in knocking the bear down – a rubber bullet or a lead bullet of the same momentum?

Summary of Terms

Work. The product of the force and the distance through which the body acted on by the force moves.

Power. The time rate of work.

Energy. The property of a system that enables it to do work.

Potential Energy. The energy that a body possesses because of its position.

Kinetic Energy. Energy of motion.

Work-Energy Theorem. The work done on an object equals the change in kinetic energy of the work.

Conservation of Energy. Energy cannot be created or destroyed; it may be transformed from one form into another, but the total amount of energy never changes.

Machine. A device that increases ( or decreases) a force or simply changes the direction of a force.

Conservation of Energy for a Machine. The work output of any machine cannot exceed the work input.

1. Mountain highways often have emergency ramps for truckers whose brakes fail. Why are these covered with soft dirt or sand rather that paved?

2. Why can we not associate a potential energy with the frictional force as we did with the gravitational force?

3. A physics textbook is launched up a rough inline with a kinetic energy of 200 J. When the book comes momentarily to rest near the top of incline, it has gained 180J of gravitational potential energy. How much kinetic energy will it have when it returns to the launch point?

1. When you get your power bill, you are charged for the number of kilowatt-hours that you have used. Is kilowatt-hour a unit of power or a unit of energy?

2. Athletes will sometimes run along the beach to increase the effect of their workouts. Why is running on soft sand so tiring?

3. Which of the following is not a unit of energy: joule, Newton-meter, kilowatt-hour, watt?