Question #1) While we are making metric and imperial speed and weight connections lets make some kinetic energy connections in our brain. Lets image we have a car that weighs 1,587 kilograms that's about 3500 pounds and it's traveling at 11 meters per second that's about 25 MPH, this is you and three of your best friends driving in an average 4 door car at the speed you should be driving around your neighborhood. How many joules of kinetic energy does this combination of car, driver and passengers have? Warning, if you are doing this question as part of Mr. Cramer's class your answer will not be accepted without detailed work attached.
I know you're looking at all these numbers and you're thinking maybe auto mechanics is not for me. Don't worry I've never had to calculate how many joules of kinetic energy a car had to fix the brakes. I am hoping that you will start to understand the relationship of speed, weight and energy. We can punch these big numbers into a calculator as long as we know the order and we can do some rounding to help us make connections we didn't have before. This is what I'm talking about your weight in kilograms is about 45% of your number in pounds or if you want to round a little more about half. Your brain probably associates speed to miles per hour well,1meter per second is about 2.2 MPH.
If a millimeter is the smallest whole unit of measurement we are familiar with for length or linear measurement the joule or (J) is the smallest whole unit of measurement of energy or work. The formula to calculate how many joules of kinetic energy something has is KE=0.5 x m x v^2. Don't let this formula scare you it's really pretty easy and we're going to break it down and work with it. KE is Kinetic energy or joules of kinetic energy so if we do the problem on the other side of the = we will know how many joules something has. The other side says 0.5 x weight or mass x speed or velocity squared. Simply put if you want to know how many joules you have its half of your weight multiplied by your speed squared. If you are like me and have been living in the United States you probably aren't familiar with the units of weight and speed used in this formula so you will have to convert. Scientist use kilograms for weight/mass and meters per second for speed/velocity. Below are some conversions to help you make the connection between mechanic and scientist world.
U.S Customary Metric
Weight 1 pound = 0.4536 kilograms
Speed 1MPH = 0.447031354 meters per second
It's great that we understand when cars are moving they have kinetic energy and to stop a car you will have to remove it's kinetic energy but until we can understand kinetic energy in units it's like saying a person is tall but not having the slightest idea what an inch, foot, millimeter, meter or mile looks like. So for our first assignment lets put a value on the amount of energy a car has rolling down the road. Traditional brake systems just waste this energy and while hybrid cars are able to harness a portion of this kinetic energy there is still room for major improvement. Who knows, maybe if these assignments are easy for you, you'll be improving cars after you have spent some time in the repair field.
As soon as you get anything with weight moving it has kinetic energy, if you move it faster it has more kinetic energy, if you add more weigh at the same speed it has even more kinetic energy. Conversely (fancy talk for just the opposite) If this object were to slow down or lose weight it would lose kinetic energy. If you don't understand this idea just answer the following question- Would you rather be hit by a snow ball going 40mph or a Cadillac going 40mph? There is a right answer to this question and it's the reason cutting holes in your floor isn't going to do any good unless you're looking for a place to throw your double cheeseburger wrappers.
"What? that doesn't sound like a good idea. Well smarty pants, how do you slow down from a full speed run? That's what I thought, now your feet aren't good enough. What makes your car so special?"
"If you are measuring your size 12 shoes right now and adding 2 let me stop you before you put holes in your floor. If you get anything with mass (fancy talk for weight) moving it has kinetic energy (fancy talk for the energy of movement). Here is something you probably already know but didn't spend to much time thinking about.
"It's pretty simple, you built a sweet car that's super fast and now you want to add some brakes, just in case you want to slow it down to show off. Measure your feet add 2" for clearance and cut a hole in the floor, done!"