Work, Power and Energy Continued
Clarification… • Energy is possessed by an object. • Work is done when energy is transferred from one object to another. • In other words, work is a measurement of how much energy was gained or lost by an object. • Equations: W=KE W=PE W=F X D
Examples… • A 1 kg soccer ball reaches a maximum height of 30 m after a goalie’s drop kick. How much work could the ball do on a midfielder’s head? • W=PE PE=mgh PE=(1kg)(10m/s2)(30m) PE=300J • When a certain car locks up the brakes and slides, it has a frictional stopping force of 2500 N. If the car has a mass of 1000 kg, how far will the car slide if it slams on the brakes traveling at 10 m/s? • D=W/F W=KE KE= (0.5)(1000kg)(10m/s)2 KE=50,000J D=50,000J/2500N D=20m
Practice problems • 1) A car smashes into a stationary cow while traveling at 10 m/s and does 62,500 J of work on the cow. How much mass does the car have? m=KE/(.5)v2 m=62,500J/(.5)(10m/s)2 m=1250kg • 2) A baseball catcher applies 640 N of force to stop the incoming fastballs thrown by his pitcher. When the pitcher throws a fastball, it knocks the catcher’s glove back 25 cm before completely stopping the ball. If a baseball has a mass of .2 kg, how fast is the pitcher’s fastball? v=√KE/(.5)m W=KE W=F X D W=(640N)(.25m) W=160J v=√160J/(.5)(.2kg) v=40m/s
Internal Energy • Definition: energy due to the random motion of molecules. • Also called “thermal energy,” due to the relationship between the internal energy of an object and its temperature: • High temperature = large internal energy • Low temperature = small internal energy • Even ice at 0 C has a fairly large amount of internal energy. • Zero internal energy = -273 C (“absolute zero” is lowest possible temperature)
Internal energy is a combination of kinetic and potential energies on a microscopic scale, which makes it impossible for us to measure individually…we need to use temperature as an indicator of internal energy. • We need to get rid of the idea that something is “cold,” and replace that with “low internal E.” • Cold cannot flow into something, you can only remove energy from it to make it feel cold. • Example: A refrigerator doesn’t put coldness into the food, it removes energy.
Heat • Definition: a transfer of energy that results in a change in temperature. • Heat is another measurement of energy, so it has units of Joules. • Just like we said with Work, an object cannot contain Heat. Instead, we that heat is “transferred” whenever there is a temperature change. • Heat always flows from an object with higher internal energy to something with less internal energy. • “I have energy, I do work. I have energy, I transfer heat.”
Biological Energy • Our bodies transform and use energy • There are three types of molecules that provide energy to our bodies: fats, carbohydrates, and proteins. • The amount of energy that we receive from these molecules is measured in Calories. • 1 Calorie = 4186 J • Each of the three energy-carrying agents has its own Caloric value: • 1 gram fat = 9 Calories • 1 gram carbohydrate = 4 Cal (don’t count the fiber) • 1 gram protein = 4 Cal • Since Calories are in units of Joules, they can tell us how much work we can do with our food energy.
Application Problems • How many times would you have to lift a 10 pound weight (equivalent to about 50 N of force) through a distance of 1 m in order to burn 1 Calorie? (about 83 times) • 2) How many Calories did you burn in the Harvard Step Test? (Work/4186) • 3) How many minutes would you have to keep stepping in order to burn off a 280 Calorie Snickers bar? (280/above answer) • This would be pretty bad news if we had to burn all our Calories by doing mechanical work. What are some other ways that our bodies use energy? (Think about the fact that our body temperature is maintained at 98.6 degrees.)
Human Metabolism • Even when inactive, an average adult male has a basal metabolism rate of 90 W!!! That means he burns over 1850 Cal per day just by being alive!!! • Calculating Basal Metabolic Rate (BMR): • Males: 66 + (6.23*W) + (12.7*H) - (6.8*A) = Cal/day (enter pounds, inches, years) • Females: 655 + (4.35*W) + (4.7*H) – (4.7*A) • Multiplication Factors: • 1.2 – little or no exercise • 1.375 – (light exercise, 1-3 days per week) • 1.55 – (moderate exercise, 3-5 days per week) • 1.725 – (heavy exercise, 6-7 days per week)
The Biological Process • Whenever fats, carbs, or proteins enter our body, they are broken down into the chemical glucose. Everything else gets dumped to the large intestine for elimination. • Glucose is the “greatest common denominator” of biological energy. • Whenever we do not use all of the energy that we have consumed, our body builds the extra glucose into fat, because fat is the most efficient means of storing energy. • If the body’s extra energy was stored as carbs instead of fat, then “cellulite” would take up 3 to 4 times as much space as it does now!! (…but it would also burn off at a faster rate). • When our body needs energy to perform a task, the glucose is processed into a chemical called adenosine triphosphate (or ATP), which can be used by any part of our body to generate energy. • 1 molecule glucose 36 molecules ATP
Glucose yields from some food types: • Fruit sugar (fructose): 1 fructose = 1 glucose = 36 ATP • Refined sugar (sucrose): 1 sucrose = 2 glucose = 72 ATP • Milk sugar (lucrose): 1 lucrose = 1 glucose = 36 ATP • Fats: anywhere from 4-100 glucose = 144-3600 ATP!!! • The brain is constantly surveying the energy levels in your body, and when it realizes that energy is needed, it signals the pancreas to release insulin, which triggers the individual cells to release stored-up glucose, which is promptly processed into ATP, which is finally used by cells and tissues to do work.