is a language. It is used to describe the world around us. Can you tell me what this means? N i I i1 If you understand only how to do the math, you will need to know the numbers to see any meaning behind this equation. However, if you understand the meaning of the math, the equation itself tells you a great deal about how nature works. The equation says the following 1 The total torque acting on an object is the same as its moment of inertia multiplied by its angular acceleration. is a language. N i I i1 means that The total torque acting on an object is the same as its moment of inertia multiplied by its angular acceleration. You see i is any of N torques, I is moment of inertia and is angular acceleration. N means to sum all of what is behind it for every value of i from 1 to N. i1 2 But you still do not know the meaning of torque, moment of inertia and angular acceleration. 1
is a language. N i I i1 means that The total torque acting on an object is the same as its moment of inertia multiplied by its angular acceleration. But you still do not know the meaning of torque, moment of inertia and angular acceleration. Torque is a measure of how hard you are trying turn something. Moment of inertia tells us how hard it is to change how fast it turns. And angular acceleration measures how much it changes how fast it turns. 3 is a language. N i I i1 means that The total torque acting on an object is the same as its moment of inertia multiplied by its angular acceleration. Torque is a measure of how hard you are trying turn something. Moment of inertia tells us how hard it is to change how fast it turns. And angular acceleration measures how much it changes how fast it turns. This same equation can describe a grinding wheel, the hands of a clock, the motion of a wrench, and an infinite number of other situations. 4 If I plug in the numbers as an example, I only learn ONE of the situations!! 2
You need to remember algebra. Here are some of the basics If A B C A B B C B A C B If A B C A B / B C / B A C / B Distributive property A B C A B AC Commutative properties A B B A A B B A and similarly for square roots, squares, subtraction and division except that subtraction and division are NOT commutative! 5 Some simple examples If F F ma 1x 2x x F F ma 1x 2x x Distributive property m v v mv mv 1x 2x 1x 2x If ma z F TOTALz m F / a TOTALz z Commutative properties m1 m2 m2 m1 ma x a m x and similarly for square roots, squares, subtraction and division except that subtraction and division are NOT commutative! 6 m1 m2 m2 m1 m/ a a / m x x 3
Here are some other helpful concepts If and A C B C A B Ratios If and AC D BC E 7 even if YOU DON T KNOW C! Simultaneous Equations + a Ab B c 1 1 1 a Ab B c 2 2 2 da a A db b B dc c 1 2 1 2 1 2 usually we use this in such a way that one of the coefficients is zero A D B E even if YOU DON T KNOW C! Here are some more simple examples 8 If and F 1x 1 F F ma ma 2x 1 F 1x 2x even if YOU DON T KNOW m or a 1! Simultaneous Equations + 2F 6Q 10 12F 5Q 3 6 2 12 F6 6 5Q 6 10 3 usually we use this in such a way that one of the coefficients is zero Ratios If m1l T1 and m L K m m 2 2 T K 1 1 2 2 even if YOU DON T KNOW L! 0F35Q57 57 Q 35 4
Finding out which equation or set of equations to use while solving a problem in physics is the most difficult part of the process. It is also the most crucial part! Still, if you follow a few basic steps, the difficulty will be far less and you will need to spend much less time on PreAssignments, Homework and Exams. An example solved by a naïve student (Bailey D. Wonderdog s nemesis, the neighbors cat, for instance) will help us see what the rules are and how to apply them. 9 If you look in your textbook, you will find the equation IR At first this naively appears to be the simplest equation we can use for this problem. We might be tempted to guess that is the velocity, I is the impulse, and R is the radius. 10 Let s try this 5
IR When trying to plug in the numbers, we see our first challenge. There two different objects and each have different velocities. 11 Which one do we choose? To answer this, we must ask ourselves two things. 1. What physical quantity are we looking for? 2. What object is that physical variable related to? For this problem, the answers are 1. Impulse 2. The ball IR Thus, we would use the quantities associated with the ball in this problem. 12 Rule #1: We must know which object we are considering in a problem. 10 Plugging in the numbers, we see that I 0.5. R 20 If we plug this into the homework software, it will tell us we are incorrect. What went wrong. 6
IR Well, first of all, the velocity is in m/s and the radius is in cm. So, we have to convert one of the units to make them the same. You will learn how to do this in the lecture called Math for Physics. If we do it properly in this case, we find that 13 m 10 s 100 cm I 50 s R 20 cm 1 m Rule #2: Use the proper units. 1 IR Next we go back to the very first thing we learned in this lecture. The variables of physics are words in the language of math. If we look up the equation again and read carefully, we will find that it means 14 The voltage across a resistive element in a circuit is the same as the current through it multiplied by its resistance. The variables are not even close to what we wanted to use!!!! 7
Rule #3: Know the IR meaning of each variable. We now look up the word impulse in the index of our book or in the notes and find that the variable that represents it is J. We now find two equations that contain J on the website for the class. J Favg t and J mv 15 But which one should we use? Rule #4: Use what is known and unknown to sort out equations J mv that are not useful. is the only one of the two equations for which we have all of the information to solve. It reads The impulse on an object is the same as its mass multiplied by the change in its velocity. We know the mass and the change in the velocity of the ball. 16 The other equation needed force and time, neither of which is known. 8
J mv Now we just need to plug in the ball s mass (40 g) and its change in velocity. It had 10 m/s to begin with and ended with 10 m/s as well. Thus, J mv 10 g 0 m / s 0 g m / s 17 But the homework software stills says that we are incorrect! 18 J mv The arrows on top of the variables J and v tell us that they are vectors. When we subtract the initial from the final velocity, we must also take into account their direction. (One is north, the other south). Rule #5: ectors! Gotta use vectors!!! J mv 10 g 10 m / s 10 m / s 20 g m / s Which is the correct answer!!!!!!!! 9
In summary... Rule #1: We must know which object we are considering in a problem. Rule #2: Use the proper units. Rule #3: Know the meaning of each variable. Rule #4: Use what is known and unknown to sort out equations that are not useful. Rule #5: ectors! Gotta use vectors!!! Follow these rules when solving problems and you will find that physics is not so bad. 19 This is what DR. Mike means when he says you must use concepts to solve problems in physics. 10