# Question about tension

1. Jan 1, 2012

### johnathon

If I attach an object to the end of a string and hold it still I get that the tension will be equal to the weight of the object. But what if I start to move the string up. How does that affect the tension?

2. Jan 1, 2012

### Staff: Mentor

In order to accelerate the object upward you must increase the tension in the string. (Analyze the forces acting on the object and apply Newton's 2nd law.)

3. Jan 1, 2012

### Staff: Mentor

Moving upward means accelerating upward right? If so then gravity is pulling down and you are pulling up and thus string tension is increased. If you accelerate upward fast enough you could break the string in which case tension returns to zero

4. Jan 1, 2012

### johnathon

So if I lift up the string with a force of 2N, the tension also increases by 2N (applying Newton's 3rd law)?

Last edited: Jan 1, 2012
5. Jan 1, 2012

### Staff: Mentor

Whatever force you exert on the string is the string tension.

If you increase the force you pull up on the string by 2N, then the tension must increase by 2N. (Treating the string as massless.)

6. Jan 1, 2012

### johnathon

So is the tension the equal and opposite reaction to my pulling?

7. Jan 1, 2012

Exactly.

8. Jan 1, 2012

### johnathon

Ah, brilliant. Thanks

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