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## Class 11 Physics (India)

### Course: Class 11 Physics (India)>Unit 9

Lesson 1: Introduction to forces and free body diagrams

# Types of forces and free body diagrams

Sal defines and compares tension, weight, friction and normal forces using free body diagrams.

## Want to join the conversation?

• Why is the 5 N force pushing down on the 10 N object a normal force?
(20 votes)
• Gravity naturally pushes down on the 5N object intern pushing down on the 10N shelf which if you follow it back was created by gravity which is a natural force to begin with.
(8 votes)
• If a girl who weighs 40 kg was to climb up a 20-foot tree at a constant acceleration, what would be the work done by the girl?

The formula for work is W=Force x Displacement
Newton's second law says F=ma, so if a = 0, wouldn't the force be zero and the work be zero. This doesn't make sense though because there has to be some work done.
(10 votes)
• you are saying constant acceleration not 0 zero acceleration , constant means it remains the same NOT 0 , so there has to be some acceleration in order for the girl to move , the question is wrong ^_^
(5 votes)
• At in the free body diagram at the top left of the screen, why is the normal force 5N upward. I understand why it is upward, but why is it 5N. Should it not be 10N because the shelf is 10N?
(8 votes)
• At i don't understand why the normal force is pushing downwards, i thought normal force reacts in the opposite direction of gravity.
(3 votes)
• No, not always. The normal force is (you guessed it) normal to the surface the object is in contact with. If you hold it against the ceiling, the normal force is in the same direction as gravity. If you hold it against the wall, the normal force is perpendicular to force of gravity. There is no law which states that the normal force must be in the opposite direction as gravity. Hope this helps!
(8 votes)
• In the last example with the shelf and the block of the shelf, why is the normal force acting downward. I would think that gravity is acting downward on the shelf but that force is counteracted by the tension force in the ropes holding the shelf(this is without the block on the shelf). When the block is on the shelf though, gravity is acting downward on the block but the force of gravity is counteracted by the normal force from the shelf keeping it from accelerating downwards.
(4 votes)
• The normal force is acting downwards because there is nothing under the shelf to counteract its force by providing an upward acting force (except the wires, but they provide tension). Plus, we have also got ourselves a block that ADDS to the weight of the shelf. This ADDED weight is what we are considering as the normal force.

Hope this helps
(5 votes)
• Is friction related to inertia?
(2 votes)
• Yes, it is, in fact, it can be due to friction an object has inertia. Inertia is defined as an object will be in motion as long as it is affected by an external force(usually friction, but not always). An example of friction being related to inertia: A book is sliding across a table and it stops after a certain amount of time. The book stops because of the frictional force that is affecting the book while it is sliding across the table. While that answers your question I also want to give an example of when friction is not necessarily related to inertia. For example, you are driving a car and suddenly you see a deer in front of you and you slam your brakes, meanwhile, your book is in the back seat and it flies and hits the back of your seat. The external force here is your seat that stops the book to keep flying. Friction may play a small role, but it did not play a major role relating to the inertia of the object. Sorry for a long response, but if you have any questions please do reply back. Just a clarification, I am taking my first year of Physics so if I make a mistake or need to add anything please suggest it. Thank you.
(5 votes)
• what happens to newton if you let go of an untied balloon?
(3 votes)
• when you let go of an untied balloon, the force of the balloons weight pulls the the balloon down and the force of the air pushing up from within the balloon pushes up on the balloon.
(2 votes)
• If I am holding a book in my hand would there be a normal force or are normal forces only for surfaces such as tables, floors, etc..?
(2 votes)
• There would still be anormal force applied.
(2 votes)
• Sal tells us that when the box falls onto the table or the Earth, the force of gravity pulling the box to the Earth and the force exerted by the table on the box cancel out, keeping the box from phasing through the table. However, this makes little sense because when forces cancel out then there should be no change in an object's velocity, so what actually stops the box from moving/phasing through everything?
(2 votes)
• On impact, the normal force exerts an additional impulse force on the box (and the table). The impulse force is responsible for the change in momentum of the falling object.

Because of Newton’s 3rd law, the force would act on both the object falling (the box) and the table. Sometimes the table is not able to overcome the stress caused by this additional force, so the table breaks and the object can continue it’s motion downward(probably damaged itself as well), with the amount of its original momentum lost being dependent on the amount of time spent impacting the table.

That is why dropping a box onto a table from a tall height can cause more damage to both the table and the box, than lightly resting the box on the table instead. The impulsive forces in these scenarios are different.
(1 vote)
• At the 5 minute mark, why is it friction that is the cause of the object not moving, couldn't it be mass?
(1 vote)
• Mass triggers friction (if on Earth or other body), and its the friction that causes the object to move:

At the 5 minute mark, the friction from all the previous moments already reduced the velocity of the object. It is at the 5th minute when it stopped moving.

Let's see it this way (let's say it looses 1 m/s each minute):

When launched:5 m/s
1st minute: 4 m/s
2nd minute: 3 m/s
3rd minute: 2 m/s
4th minute: 1 m/s
5 minute: 0 m/s

So at the 4.9th minute, its velocity is probably close to 0 but not exactly 0 (0.1 m/s). At the 5th moment, its 0. It stops.

In this case, if the example of Earth, its the mass of the object which causes the gravitional force between it and the Earth, triggering the friction to slow it down.
(3 votes)

## Video transcript

- [Instructor] In this video, we're gonna discuss different types of forces, but we're gonna do it in the context of free body diagrams. So let's say 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 a free body diagram where I am only going to draw the block. Remember, in free body diagrams, you only care about the forces acting on one of the 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 free body diagram. So you're gonna have a downward force, and it's magnitude is gonna be F sub g. We could also call that or w. And even though this block had 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 was the only force acting on the block, the block would accelerate downwards. But we're assuming that it's stationary. So there must be another force that is netting out against the force of gravity. Now, 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 its 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 free body 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 keeping it from accelerating 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 free body diagram for this block? Well, I'll do the same thing again. I will 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, 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 free body 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, trying to drag something across the ground and it doesn't move, and that's because there's friction between the object and 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 pass each other. Or sometimes it can even be due to molecular interactions where they're kind of sticky, where the objects are attracted to each other and you gotta pull passed 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 has a weight of 10 newtons. Sitting on that shelf I have an object that has a weight of five newtons. And let's say I have two wires and everything is symmetric, but this weight is right on the middle, and these wires are at 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 free body diagram for this five newton block that sits on the shelf? Well, that one is 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, five newtons, and that's gonna be counteracted by a normal force of the same magnitude but going upwards. So make sure I have enough space. So that's gonna be counteracted with the normal force which is going to be equal to five newtons upwards. And to be clear, this five 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. And 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 a 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 10 newtons. 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 a 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 of the five newton object is pushing down on our shelf. So what you have is another force that is pushing down. And 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 is going to have a magnitude of five newtons. Assuming that it's completely stationary, there must be some counteracting forces here. Where is that gonna come from? Well, that's gonna 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 one, and you're gonna have the tension from wire or rope two, T sub two. And because this thing is stationary, T sub one plus T sub two should be equal to 10 newtons plus five newtons. 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 free body diagrams.