Chapter 10 Notes
10:1 Work and Energy
The physical definition of work is different that the everyday definition.
In order to move something, you have to apply a non-zero net force over a given time period.
Notice that the Work formula is not dependent upon TIME. For example..
If you move something at constant velocity through the vertical dimension, then the force required is the weight (mg) of the object, and
W = mgd
You try: Study the example and do practice problems 1 and 2 on page 199.
If an object does not move, there is no distance involved, and work is not done on that object.
When you make something move by doing work on it, that object gains energy.
The unit for work is the joule, so one joule equals one newton times one meter.
Homework: Set #1: Chapter 10 questions 1-5, problems 19-22 page 241
Work and Direction of Work
What happens if you apply a force on an object, but the object doesn't move in the direction you applied the force! Consider the diagram of Mr. Rodriguez's one horsepower pickup truck below:
If the truck moves horizontally a distance of 10 meters, then work is done on the car. But the work is in a different dimension than the applied force. So, how do we calculated the work done? Well, the secret here is that there is a force that is applied in the same dimension as the distance traveled: it is simply the horizontal component of the applied force, and you know how to find that: VECTORS!
You try: First find the horizontal component Fx of the 5000 Newton force applied by the horse:
Fx = 5000 cos 30 and you should arrive at 4.3 x 104 Joules of work done by the horse on the truck!
The effect of doing work on an object or system, no matter how it happened, is to give that object or system energy. If you made the object move, you gave it kinetic energy.
The rate of doing work is called power. Remember the two people we mentioned earlier, when one finished the same amount of work FASTER? This is an example of two different levels of POWER.
The unit of Power is the watt . One watt of power is equivalent to one Joule of work done every second, or one Joule/second.
Study the example problem on page 228-229 and work the practice problems 6-8 on page 229.
overheard in a local hardware store:
Customer: Do you have 2 4-watt bulbs?
Proprietor: Two what?
Customer: No, 4!
Proprietor: 4 WHAT?
This is our most common unit of electrical energy around the household. Note that the kilowatt-hour (KWH) is a unit derived from Kilowatts X hours, or power X time.
Since power = work/time and work is the equivalent of energy, then energy = power X time. So the KWH is energy, not power even though we often talk about paying the "power bill". We purchase energy from the local gas/energy company in units called KWH. You probably pay around $0.12-$0.15 per KWH for household energy.
Question: if you operate a 100-watt light bulb continuously for 3 hours, how much energy is used in KWH, and how much would it cost?
answer: energy = power times time.
100 watts = 0.10 kilowatt (kw)
0.10 kw X 3 hours = 0.30 KWH.
At $0.15/KWH, the above amount of energy would cost $0.045 (4.5 cents)
How much energy in KWH would be used by the same light bulb if it burned for only 30 minutes?
100 watts = 0.10 kw
30 minutes = 0.5 hours
Multiply power by time: 0.10 kw X 0.5 hours = 0.05 KWH.
How does a KWH relate to a joule?
A joule is a watt-second.
There are 1000 watts per KWH so a KWH is the same as 1000 watthours. There are 3600 seconds in one hour, so 1000 watt-hours is the same as 1000 X 3600 wattt-seconds or 3.6 x 106 watt-seconds, or 3.6 x 106 joules. A lot!
1.0 KWH = 3.6 x 106 joule
Lab Activity: "Your Power".. page 232. Addition: find out how a Horsepower compares to a watt, and find your power in Horsepower as well!
Homework: Set #2, Ch 10 problems27,29,30,31,32,34,36,37,41