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### Course: Physics library>Unit 3

Lesson 6: Tension

# Tension in an accelerating system and pie in the face

The second part to the complicated problem. We figure out the tension in the wire connecting the two masses. Then we figure out how much we need to accelerate a pie for it to safely reach a man's face. Created by Sal Khan.

## Want to join the conversation?

• Was there a missing video? I did not see this problem in the previous videos (Introduction to tension part 1 & 2).
• me too so confusing....
• Why does he uses the 'coefficient of friction' instead of seperating it into 'coefficient of STATIC friction' and 'coefficient of KINETIC friction'?
• It is the coefficient of static friction because the pie is not slipping down the hand.
• Hi, Can you tell me where I can find the first part of this video? I've tried all the links mentioned in questions but none of them works!
• My intuition says that the mass of the pie is important to determine the acceleration necessary,
but Sal says that it cancels out. Wouldn't a pie with more mass need a greater acceleration?
(1 vote)
• Acceleration = velocity/time.
As you can see, there is no mass involved here.
It is quite natural to think that more mass, more the acceleration needed, but, apparently, that's not the way it works.
• Where is the previous video?
• The previous video is mild and medium tension.
• what is the formula for finding tension
• There's a no particular formula for tension. Tension is just a kind of force usually applied to a string. If you have an object of mass m hanging on a string with a downward acceleration due to gravity, g, on that object, the tension on the string, is the same as the force of gravity on the object, F = mg.

You may need to take the mass of the string into account for some problems where the string is particularly long or thick, but the mass of the string is almost always negligible for most physics problems.
• What is the video called that has the problem for the beginning?
• Hi just wondering where is the first part to this problem?
• Here's the video :

I don't know why its on YouTube, though and not on Khan Academy.

Hope this helps :)