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## Physics library

### Course: Physics library>Unit 3

Lesson 6: Tension

# Introduction to tension

Learn how tension, the force within or applied by a string or wire, counteracts gravity to keep objects stationary. Explore how tension varies in different scenarios, like in a system with multiple strings at different angles.  Created by Sal Khan.

## Want to join the conversation?

• If the block is not moving, and the forces are balanced, why doesn't the tension of both the ropes equal 100N? How can there be a tension of 200N in one of the ropes, if only 100N is pulling down on it?
• Imagine holding a something heavy (a physics textbook) in your hand with your arm straight down, by your side. Then raise your arm, still completely straight, up so it is at at an angle with you body. You will notice that it is much more difficult to hold the book in that position than if you let your arm hang straight down. It is the same principle.
• At he says he's adding 2 more strings and then considers the tension in only those 2 strings but what about the first string?
• The tension in the original string must equal that of the two new strings because they are connected at the ends, the forces can then be projected onto the x and y axis.
T0x=T1x+T2x
T0y=T1y+T2y
where T0 is the original light blue string
• When we've attached the two new wires, does the original blue wire not provide any force to support the block anymore? I mean is it now just extending the downward force of the block to the two new wires and otherwise just hanging around? And if this is the case could you please explain why? I can set up and do a problem like this but I just don't understand why that original wire no longer supports the weight anymore in any way. Thanks.
• The original blue wire will still provide the same amount of force to the block, and it still has the same amount of tension on it, but is still transferring that force to the block.

To put it another way, if you were to cut the blue wire, the block would fall because you removed the force that the blue wire supplies to the block.
• Is the T1x = T2 an application of Newton's 3rd law? some thing liddat?
• No, this is an application of Newtons first law. Since there is no acceleration in the horizontal plane, we assume that those forces are equal in magnitude and opposite in direction
• Why do we need to make the x and the y component of T1 wire?
• The fundamental reason behind this is that two mutually perpendicular vectors do not affect one another.Now, any vector can be resolved into two mutually perpendicular vectors, right? In this case T₁ᵪ and T₁ᵧ are those components.Now since T₁ᵪ is perpendicular to the 100N force acting downwards, it will not balance it, it will not add to it; it will not affect it in anyway.But the T₁ᵧ vector is parallel to the 100N force.Now, this T₁ᵧ MUST BE balancing the 100N downward force as the point, as Sal mentions, is not accelerating in any direction.So the 100N downward force is actually balanced by this T₁ᵧ=100N upwards force.
This is why we first MUST resolve the T₁ vector into two components, one perpendicular to the 100N force and one parallel to it.Hope I was clear enough :)
• At he says 'sine of 30 degrees is one half' what does he mean by that? That kinda made me confused :(
• What he is referring to is the trigonometric function sine. If you are confused, I suggest watching the videos on trig in one of the math playlists.
• If a string is stretched by two opposite & equal forces 10 N each.what will be tension in string
(1 vote)
• If the string is attached to the wall and you pull on it with 10 N of force, the string will have 10 N of tension, right? That's easy.

Now, ask yourself, how hard is the wall pulling on the string? It must also be pulling with a force of 10 N, otherwise the string would be accelerating toward you.

Now, if we replace the wall with another force that applies 10 N, does the string know the difference? No, right?
• So if T1 was directly above T2 instead of off to the side a bit, would you solve it the same way or would it just offset the force of tension in T2?
• Hey Whitney! You're right. In the case, that each wire is perpendicular to the walls, all of the force counteracting against the force of gravity would just be in the string going straight up from the mass.
That's just because of Newton's First Law, stating that the opposing force has to have the same magnitude AND has to point in the opposite direction (if their is no acceleration!)