Newton Confustion

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Newton Confustion

Postby Clive » Thu Sep 15, 2005 1:03 am

Can anybody tell me if 1 Newton meter ( 1 Joule) is the same as holding 1Kg for 1 second.

like if you put 10 kg on scale for 10 second it 10 x01 = 100 Joules or 100 Nm.


is this right

Clive :shock:
Clive
 

Re: Newton Confustion

Postby Guest » Thu Sep 15, 2005 1:21 am

Clive wrote:Can anybody tell me if 1 Newton meter ( 1 Joule) is the same as holding 1Kg for 1 second.

is this right

Clive :shock:


No, it is wrong on two levels

1) A joule is work. Your muscles may be wasting energy as heat, but no work is being done on the object if you just hold it. Work is force x distance (and in vector notation, a dot product). If there is not movement in the direction of the force, no work has been performed.

2) F = ma. A 1 N force accelerates a 1 kg mass at 1 m/s^2. The acceleration of free fall on the earth is about 9.8 m/sec^2. Thus, gravity exerts a 9.8 N force on the 1 kg mass, and you have to exert an equal but opposite force of 9.8 N to keep it from falling. Still, if there is no movement, there is no work, according to the physics definition. If the force of gravity were different, say on the moon, the force to hold it would differ, but it is still a 1 kg mass.

There is another concept in physics called impulse which is the product of force and time. It is equal to the momentum change (difference in mass times velocity). It is important in rocket performance. But it doesn't apply here, as the force you are applying exactly offsets gravity, and there is no net impulse or net work. Still, your body strains and uses more energy to hold the weight; it is wasted as heat, and may make you sweat.
Guest
 

Re: Newton Confustion

Postby Guest » Sun Nov 13, 2005 9:52 pm

Anonymous wrote:
Clive wrote:Can anybody tell me if 1 Newton meter ( 1 Joule) is the same as holding 1Kg for 1 second.

is this right

Clive :shock:


No, it is wrong on two levels

1) A joule is work. Your muscles may be wasting energy as heat, but no work is being done on the object if you just hold it. Work is force x distance (and in vector notation, a dot product). If there is not movement in the direction of the force, no work has been performed.

2) F = ma. A 1 N force accelerates a 1 kg mass at 1 m/s^2. The acceleration of free fall on the earth is about 9.8 m/sec^2. Thus, gravity exerts a 9.8 N force on the 1 kg mass, and you have to exert an equal but opposite force of 9.8 N to keep it from falling. Still, if there is no movement, there is no work, according to the physics definition. If the force of gravity were different, say on the moon, the force to hold it would differ, but it is still a 1 kg mass.

There is another concept in physics called impulse which is the product of force and time. It is equal to the momentum change (difference in mass times velocity). It is important in rocket performance. But it doesn't apply here, as the force you are applying exactly offsets gravity, and there is no net impulse or net work. Still, your body strains and uses more energy to hold the weight; it is wasted as heat, and may make you sweat.
Guest
 


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