Presentation on theme: "Warm-up: 1. What is the kinetic energy of a charging rhino that has a mass of 1500 kg and a velocity of 5 m/s? 2. What is the potential energy of a 2 kg."— Presentation transcript:
Warm-up: 1. What is the kinetic energy of a charging rhino that has a mass of 1500 kg and a velocity of 5 m/s? 2. What is the potential energy of a 2 kg water balloon held off of an 80 m tall building? 3. Is it possible for one object to have both KE and PE at the same time? If so, give an example.
A quick recap… 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. Energy transferred into work: When would W = KE? When would W = PE?
Example #1: A 1 kg soccer ball reaches a maximum height of 30 m after a goalies drop kick. How much work could the ball do on a midfielders head? W = PE (the work it can do is equal to its energy) W = mgh = (1kg)(10m/s 2 )(30m) = (1kg)(10m/s 2 )(30m) = 300J = 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? W = KE (the brakes have to do work to stop the cars KE) F·d = ½ mv 2 F·d = ½ mv 2 (2500N) d = (1/2)(1000kg)(10m/s) kgm/s 2 d = 50,000 kgm 2 /s kgm/s 2 d = 50,000 kgm 2 /s 2 d = 20m d = 20m Example #2:
Practice problems… 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? W = KE (the work the car can do is equal to its KE) W = KE (the work the car can do is equal to its KE) 62,500J = ½mv 2 62,500kgm 2 /s 2 = ½ m(10 m/s) 2 m = 1250kg m = 1250kg 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 catchers glove back.25 m before completely stopping the ball. If a baseball has a mass of.2 kg, how fast is the pitchers fastball? W = KE (the work required to stop the ball is equal to its KE) W = KE (the work required to stop the ball is equal to its KE) F·d = ½mv 2 F·d = ½mv 2 v = 40 m/s v = 40 m/s
Internal Energy Definition: energy due to the random motion of molecules. Also called thermal energy, because: 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 (this is absolute zero - the lowest possible temperature)
Internal energy is a combination of KE and PE on a microscopic scale, which makes it impossible for us to measure individually…we need to use temperature as an indicator of internal energy. Cold is simply the absence of internal energy.Cold is simply the absence of internal energy. Cold cannot flow into something, you can only remove energy from it to lower its temperature. Example: A refrigerator doesnt put coldness into the food, it removes energy from the food.
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 say 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. An object has energy, an object can do work. An object has energy, an object can transfer heat. (Biological Energy)
Application Problems 1) 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? (1Cal. = 4,186 J) (about 84 times) 2) How many Calories did you burn in the Harvard Step Test? (your work/4186 J) 3) How many minutes would you have to keep stepping in order to burn off a 280 Calorie Snickers bar? (280 Cal./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) (Enter pounds, inches, and years into your equation): Males: 66 + (6.23*W) + (12.7*H) - (6.8*A) = Cal/day Females: (4.35*W) + (4.7*H) – (4.7*A) Multiplication Factors: 1.2 – little or no exercise – (light exercise, 1-3 days per week) 1.55 – (moderate exercise, 3-5 days per week) – (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 bodys 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).