If you're seeing this message, it means we're having trouble loading external resources on our website.

If you're behind a web filter, please make sure that the domains *.kastatic.org and *.kasandbox.org are unblocked.

Main content
Current time:0:00Total duration:8:03

Video transcript

in this video we're going to discuss different types of forces but we're gonna do it in the context of free body diagrams so let's say that I have a table here and I have a block that is sitting stationary on that table what are all of the forces that are going to act on this block well to do that to think about that I can draw Freebody diagram where I'm only going to draw the block remember when Freebody diagrams you only care about the forces acting on one of the objects in your system so if we're looking at only the block what's going on we're going to assume that the block is on earth we're assuming that it's stationary well if it's on earth the block has some weight you have the force of gravity acting on the block and so let me draw that in my Freebody diagram so you're going to have a downward force and it's magnitude it's going to be F sub G we could also call that or W and even though this block at contact with a table which maybe has contact with the earth weight or the force of gravity is a long-range force even if this block was in orbit even if it wasn't in orbit it would still have gravitational interactions with the earth the earth would still be pulling on it but going back to this free body diagram if this were the only force acting on the block the block would accelerate downwards but we're assuming it's stationary so there must be another force that is netting out against the force of gravity and what would that be well that would be the force of the table pushing on the block and this force of pushing in a direction that is perpendicular to the surface of an object that's known as normal force and it's magnitude you could denote as capital F sub n let's do another example but this time instead of having the block on a table let's say it is hanging from a string which is attached to the ceiling but once again everything is stationary draw a Freebody diagram for that well once again I am only concerned with the block it's still on earth we're assuming so you're going to have the force of gravity acting downwards on the block but what's it keeping it from downwards well you might say well you got the string that's holding it up that is pulling on it and that pulling force is known as tension so what you would have here is an upward force that Nets out against the force of gravity and sometimes its magnitude is denoted by capital T or it might be a F sub T now let's make things a little bit interesting let's try to kind of combine these things and we'll actually introduce a new force so let's say that we this is the ground right over here I have a block on the ground and I have a situation where I am pulling on this block using a rope with a force of magnitude let's just call this the force of tension I am pulling on that block but the block is not moving what would be the Freebody diagram for this block well I'll do the same thing again I'm gonna draw the block now in the vertical direction you have the same thing that you saw in that first scenario you're going to have the force of gravity or the weight of the block pulling downward on the block and that's going to be counteracted by the normal force of the ground on the block the ground is holding up the block is one way to think about it it's keeping it from accelerating downwards so the normal force is acting upwards but what about the horizontal direction I already said that I'm pulling to the right with a force of magnitude F sub T so let me do that on my Freebody diagram so this would be F sub T but I said it's stationary so there must be something that is counteracting that that is netting against that going in that direction what force would that be well that would be the force of friction we've all experienced trying to pull on something try to drag something across the ground and it doesn't move and that's because there's friction between the object in the ground and friction fundamentally it could be because the surfaces of the two objects are rough and you kind of have to grind them past each other or sometimes it can even be due to a molecular interactions where they're kind of stick where the objects are attracted to each other and you got to pull past that and so in this situation you have the force of friction counteracting this pulling force the force of tension the force of friction and the force of friction is really interesting because it always goes against the direction of sliding it always goes against motion now with all of these examples out of the way let's try to do a more complex scenario let's say that I have a shelf and it is it has a weight of 10 Newtons sitting on that shelf I have an object that has a weight of 5 Newtons and let's say I have two wires and everything is symmetric this weight is right on the middle and these wires are both of the ends of the shelf and this is wire one and this is wire two and they are attached to the ceiling and for the sake of simplicity we're gonna assume that the wires have no weight in actuality they would but for the sake of this argument let's assume that they are weightless what would be a Freebody diagram for this five Newton block that sits on the shelf well that one's actually pretty straightforward and it's analogous to this first scenario that we saw you have your block you have the force of gravity pulling down with a force of magnitude 5 Newtons and that's going to be counteracted by a normal force of the same magnitude but going upwards so let me make sure I have enough space so that's going to be counteracted with the normal force which is going to be equal to 5 Newton's upwards and to be clear this 5 Newtons this is equal to the weight the magnitude of the weight of the object so that was pretty straightforward the free body diagram for just the block it's really important to see that because notice in the free body diagram all you see is the block but now let's draw the free body diagram for the shelf so if I have the Shelf right over here pause this video and try to do that well we know its weight it's 10 Newtons so we can do that first so it has a weight of 10 Newtons so the force the magnitude of the force of gravity downwards is Newton's is that the only downward force well no you have this object that's sitting on it and gravity is pulling down on that object with the force of five Newtons and that causes that object to push on our shelf so that pushing force is actually a normal force it's due to the gravity on that five Newton object but the end result is a five Newton object is pushing down on our shelf so what you have is another force that is pushing down it is going to be a five Newton force and really we should view that as a normal force it's a contact force it's a pushing force of the five Newton object on the 10 Newton shelf so this says going to have a magnitude of 5 Newtons assuming that it's completely stationary there must be some counteracting force this year where is that going to come from well that's going to come from the pulling forces of these wires so you're going to have the tension from rope one we could call that T sub 1 and you're gonna have the tension from Y or a rope to t sub 2 and because this thing is stationary T sub 1 plus T sub 2 should be equal to 10 Newton's plus 5 Newton's so I'll leave you there we've done a nice survey of various forces you might see in a first-year physics class and we've been able to think about them in the context of Freebody diagrams