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mcouris
Joined: 16 Aug 2009 Posts: 4 Location: Palatine, IL

Posted: Sun Aug 16, 2009 5:43 pm Post subject: Simple question about work and force 


Question: A 5year old and her father each lift identical chairs from the floor to a table top, so which of the two did more work?
My twocents: Since Work = Force * Distance, and Force = Mass * Acceleration, doesn't the work increase if you have a higher acceleration of the same mass? Therefore, since the father is stronger, he can move the chair up to the table top faster, with more acceleration, so his force is greater than the force a child generates slowly lifting the chair up to the table top, so therefore the father did more work. Yet, my book says that both of them did the same amount of work. Can't the father, in an ideal situation, accelerate that chair against gravity, faster than the child can? Thanks, Matt. 

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hepcj Site Admin
Joined: 23 Jun 2007 Posts: 125

Posted: Mon Aug 17, 2009 7:37 pm Post subject: 


They both did the same amount of work. In an idealised case, the force required to lift a chair is just greater and in the opposite direction to the force pulling it toward the Earth under the action of gravity. If the chair has a mass m then this force is F=m*g.
As the table is the same height for both the child and the adult. Call it d. To move the chair on to the table requires that the work W= F*d = m*g*d.
Therefore the work W is the same for both parent and child.
In reality, the child might have more difficulty moving the chair and the adult might do the work more quickly but the work done is still the same for both.
The rate at which work is done is the power so the adult may use more power to do the same amount of work. 

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A physics teacher
Joined: 05 Sep 2009 Posts: 6 Location: Mauritius

Posted: Mon Sep 07, 2009 2:20 pm Post subject: 


The difference would be in the power if the father would lift the chair and carry it in a shorter time.
Hence since power = work done / time taken the father would have the greatest power if he lift the chair in a shorter period of time than the child. _________________ I am a physics teacher that teaches up to year 12. I would be happy to answer your questions and help you with any problem you may have.
I also blog at http://aphysicsteacher.blogspot.com 

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