Ch 05: Intro to Forces (Dynamics)WorksheetSee all chapters
All Chapters
Ch 01: Intro to Physics; Units
Ch 02: 1D Motion / Kinematics
Ch 03: Vectors
Ch 04: 2D Motion (Projectile Motion)
Ch 05: Intro to Forces (Dynamics)
Ch 06: Friction, Inclines, Systems
Ch 07: Centripetal Forces & Gravitation
Ch 08: Work & Energy
Ch 09: Conservation of Energy
Ch 10: Momentum & Impulse
Ch 11: Rotational Kinematics
Ch 12: Rotational Inertia & Energy
Ch 13: Torque & Rotational Dynamics
Ch 14: Rotational Equilibrium
Ch 15: Angular Momentum
Ch 16: Periodic Motion
Ch 17: Waves & Sound
Ch 18: Fluid Mechanics
Ch 19: Heat and Temperature
Ch 20: Kinetic Theory of Ideal Gasses
Ch 21: The First Law of Thermodynamics
Ch 22: The Second Law of Thermodynamics
Ch 23: Electric Force & Field; Gauss' Law
Ch 24: Electric Potential
Ch 25: Capacitors & Dielectrics
Ch 26: Resistors & DC Circuits
Ch 27: Magnetic Fields and Forces
Ch 28: Sources of Magnetic Field
Ch 29: Induction and Inductance
Ch 30: Alternating Current
Ch 31: Electromagnetic Waves
Ch 32: Geometric Optics
Ch 33: Wave Optics
Ch 35: Special Relativity
Ch 36: Particle-Wave Duality
Ch 37: Atomic Structure
Ch 38: Nuclear Physics
Ch 39: Quantum Mechanics

Concept #1: Intro to Forces & Newton's Laws


Hey guys we're gonna start talking about forces so this is also the beginning of a topic in physics called dynamics. We've done a lot of motion problems and that was called kinematics that area of physics. This area with forces is called dynamics. So much like other sections other topics I'm gonna explain this in an order that's a little bit different from your book possibly a little bit different from the professor. I'm going to follow an order that I think makes the most logical sense in terms of help you learn. So let's get started. So you can think of forces basically just a push or a pull, the unit to measure force how strong of forces is Newton's or a Newton which is abreviated N, Newton is the most bad ass physicist and scientist ever. That's all the history will give you.Lot of people thinks its If you think it's Eienstien but it's actually him, will represent forces as a pull. Now this is just a preference that I have. You can certainly draw as a push but I much prefer if it's a pull. And the reason for that is it's easier to visualize this would make more sense later but you can just trust me for now so if I tell you that you're pushing a box you might want to try like this. But we actually want to draw forces as puls instead, so I'm gonna move this even if I told to push or move this tent over here. OK. If it gets a little bit more complicated but still pretty easy. Let's say there a force of five Newton thirty degrees below the x axis. So you can move it over here. It's the same five Newtons and it's still 30 degrees below the x axis just the same. So we would basically draw it over there it's easier to visualize. So. What I want to do is I want to go over all different types of forces to know in a logical sequence. The first one I'm gonna do is we can talk about generic forces. Basically this is nerve of the other forces right. And we're going to refer to them as simply applied forces. So if it's not gravity or normal or whatever we're gonna call these applied forces, the most usual one is you is a person applying force on an object we call that an applied force. For example you pulling on a box or a little kid pushing a car or whatever a toy car. So if you pulling a rope and then the other one is if you pull on a rope then that force instead of called being called an applied force can also be called a tension. Okay. So here's a box here's a rope. Let's say you're pulling over here. When you're a little dress here. Boom. You push or you pull with a force applied force whatever that forces gets transmitted into the rope and the tension on the rope would be the same. So if you push if you pull on a rope or push, I cant push a rope. But if you pull the rope with a force of 10 then the tension in the rope is 10. OK. And what the rope does it basically transfers your force from here to here. And as long as the rope is massless these two forces will be identical. OK. So I can say the force applied to be the same as tension, In physics most ropes will be massless and most of the times they won't break. Right that's the only thing that special tension is that the rope might actually break. But in most of the times it won't. And we can just assume that it's not going to happen. All right. So Newton's laws, Newton come up with these laws in 16 mid 16 hundreds and you should know them. The first one is inertia. I go one these very quickly because I'm sure there's tons in your book and a professor probably spoke about this for a while. Inertia is just a tendency that objects have to maintain their velocity. So it's a tendency to maintain velocity. You can think of it as maintaining velocity or tendency to resist change same thing as maintain in velocity. So the idea is that you're going to maintain your velocity unless you're acted upon by a net force. So objects tend to keep doing what they're doing. Unless there's a force acting on it if you're not moving and there's not an air force on you you won't move. If you are moving with five meters per second and there's no forces acting on you you gonna move five meters per second forever. Now that's kind of weird in every day life because if you throw something it's eventually going to stop, things don't move forever but that's because the world is full of friction and if it wasn't for friction things would move on forever and that would be all kinds of weird. The reason I say that force is because an object could be acted upon by let's say five to the left and a force of five Newtons to the right. And then they would just cancel which is the same thing as not having any forces at all. OK so here's the idea. Motion requires no force. This is a very big misconception that for something to continue to move you have to continue to push it. You don't you just throw something it's going to move on forever as long as there's no forces on it. Acceleration however does require force. So you don't require force to continue to move. You need a force to change your velocity to change how you're moving. The second one. Newton's second law is the most important out of the three in terms of these problem solving. And it's just the math equation. Some of Forces equals in which is arguably the most important equation in all of physics. Maybe all of science. So at least in some basic level. So I can rewrite this in terms of A. And it would look like this. A equals the sum of all forces over the mass. Right. So this states that the sum of all forces on an object is equal to the mass of that object times the acceleration of the object has. I can write it this way so the acceleration an object has is the forces are divided by its mass. And I want to show what happens if I have. The same force on two different masses. Let's say there is a force of 10 that's applied to an object of mass 1. Obviously again an acceleration of 10 meters per second squared. But let's say I have a force the same force of 10. But now the mass is 2. So we get five meters per second squared. So notice that if the mass goes up. Acceleration goes down. Acceleration is change in velocity over change in time. So assume the time stays the same, mass increasing. If you have more mass you're going to accelerate less. Where are you going to accelerate slower or your change in velocity be slower.

Remember inertia is a property that objects have it's a tendency that objects have to maintain their velocity. Objects don't want to change velocity. And the more energy you have the less you change the velocity or the more you maintain your velocity. Inertia is maintaining your velocity. The more mass the more you maintain your velocity so the more mass the more inertial. So mass can be thought of in physics as the quantity of inertia. That's one definition. How much resistance to change in velocity Do you have. OK. I only mention this here if two objects are pushed by the same force the heavier one will accelerate less or at a slower pace. OK. So mass is the quantity of inertia that is the amount of resistance to delta-V. So if you were to push a small car and a large truck even at the absence of friction let's say it's a complete frictionless ice ring or something. It would be the car would accelerate faster the small object would accelerate faster even without friction because it has less resistance to. Change in velocity. OK. The third one I'm sure you've heard about this one. This is Action-Reaction. Right so your boyfriend cheats on you and then you go and you hook up with his best friend or something to try to get back at him. So every action results in reaction of equal magnitude and these are forces. So every force results in another force a reaction force of equal magnitude but opposite direction. OK and this looks like this. The force of A on B. equals the negative of the force of B on A. All this negative does is tell you that they are in opposite directions. If I were to punch someone in the face their face is also hitting my head and the force is the same. Their face probably more fragile than my hand so that's why hurts more for them. Right. All forces exist in Action-Reaction pairs. So for example we're going to talk about this later. But already you know if you're around the earth the earth pulls you down. That force is called weight. And it's you can write this as mg well guess what if if the Earth pulls on you with 500 you're pulling in the earth with 500 as well. So the idea is that you're basically getting closer towards each other. The problem with that is that obviously the earth is huge and there's a lot of other people pulling in the earth. So it's not really going to move because of you at least not a whole lot. Right. Negligible miles. So all forces with no exception exist in action reaction pairs. Now let's start solving some force problems which is this is the core we're gonna do for a whole bunch of videos from now on. So steps to solve force problems. There's three and we're going to do this over and over again. The first thing is we're going to draw what's called a free body diagram. Free body diagram. Which is very often abbreviated is F. B. D. And almost every professor likes using this. Every book talks about it. And then we're going to write some of our forces because I mean this is the second law. And then we're going to solve this. OK. Now free body diagram. We'll talk about this more as we go to the videos. It's the only time in physics and one of the few times in physics one where you have to be very picky about the way you draw diagrams. Everything we've done so far. We're sketches for the most part but there are specific rules in how to draw free body diagrams. And often this will be part of your great when you're solving a question on your test. All right. So let's do some examples here. Find the Block's mass and this first one here. So I'm asking for mass I know 30 Newtons. This is a force the only force here and the acceleration is six. So which equation should I use, I should use was a (F=ma) obviously but before we do that let's draw free body diagram so free body diagram this is not a free body diagram because a free body diagram has to be a dot. That has forces connected to it ok but only forces can be connected to it to the dark. That's the world. So I can write F=30. If I'm pushing to the right you would expect that the acceleration goes to the right. However I can draw this like this because acceleration is armed forces and in a free body diagram I can only have force arrows, force vectors touching the dots pretty sensitive like. OK. So this is your complete free body diagram. A dot with forces coming out of it. Now I'm going to write the sum of all forces on this object equals the mass of this object. So this is the sum of all forces on the objects is the mass of the objects and the acceleration of the object. You wouldn't usually write all this I'm just making that point for now and I can cover that a few more times as necessary. But anyway the only force here is 30 it's going to the right I gonna say that going to the right is positive. So it's positive 30 the mass is what I'm looking for. And the acceleration is six.

So mass is 30 over six. Mass is five. And that's in kilograms. The SI unit for mass. OK very straightforward. (F=ma) plug in the numbers you're done. Let's look at this one here. Let's check the south find the acceleration so I don't know what is the acceleration of this box. Lets draw a free body diagram. I have a I'm gonna call this F. This one to the left I'm gonna call it F1 equals thirty have to equals 10. They might be wondering what one is to the left and to the right. Should one be positive for the other one to be negative. It will be. So this is my complete free body diagram right there. Some of our forces equals (ma) And what I'm going to do here is I have two forces I'm gonna make two little brackets. The masses for and the acceleration I'm looking for. Now these forces are going the opposite direction so they have to have opposite sides right. So I'm gonna say they're going to the right is positive this is going to be our standard. So these guys basically becomes 10 and this is a negative 30, so plus 10 negative 30. Now I don't have to put a negative here. Right. It is negative but I didn't have to put a negative there because there's already an arrow pointing to the left so I know that it's it's going to be negative. I don't have to explicitly put it there. Also 10 minus 20 and minus 30 is negative 20 equals 4a. And if I move the 4 over again native 20 over 4 this is negative 5 meters per second square. I got a negative. Does that make sense. It should because my strongest force is Pulling to left. So it's gonna think of it as a little tug of war. It's going to win. So my acceleration will fall the strongest force and let's do the last one here. I'm asking what is the magnitude of this force F free body diagram. Notice that it looks very similar to this. I just can't have a little box it's going to be dot. It's kind of silly. So I'm looking for this F here. There's a 20 here that's my free body diagram. OK. So the sum of all forces equals (ma) the forces are F Negative. And in 20 pauses. The mass is five and the acceleration is 2. OK. And then if we move everything around I get that negative half equals 10 minus 20, this is a 10 minus 20 negative F is negative 10. So F is positive 10. Now. Very important. When we highlight the answers here. I got a positive 10. Does that mean that that force is actually to the right. No. When you plug in forces you should plug them as under some of our forces with their signs indicating their direction. But your final answer will be when you do this your final answer will be positive. And the reason for that is that this is just the magnitude of the Force. So when you do (F=ma) you look for a force you are gonna plug in a force saying it with respective sign and whatever answer you get is the magnitude of the force we're going to have a chance to talk more about this in future videos and make this a little more clear. So anyway that's it for this intro video in forces. Hope it clears things up a little bit and let's continue on to the other ones.