Work done (physics)

2 and 3 are correct

work done is scaler not vector, so 1 is NOT correct
the formula of work done = F x S , so 2 is correct
3 is also correct, coz the unit of work done is a kind of energy, we use Joule to measure energy or u can use Nm-1 to measure energy

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1 no
work is a saclar, why?
Although work depends on force, work itself, like energy, is a scalar. If the force is in the same direction as the displacement, work done is positive. If the force is in the opposite direction as the displacement, work done is negative.


i) because the displacement and the force is in the same direction, the quantity(product of displacement and force) has no direction, so it is a scalar
ii)work done is a kind of energy, we often use work to define energy, can energy has direction?of course not!!












2 yes??? the statment is implicit
'''''''''''''''''''one thing we must make it clear: is the displacement in the direction of the force?'''''''''''''' if not, the work done should also include cosθ

The force must have a component parallel to the direction of motion. If not, we should resolve it into components parallel and perpendicular to the displacement.
**only the component of force that is parallel to the displacement contributes to work.













3, work done = F s
unit for work done = N m (s is displacement)

by F = ma, unit for m is kg, unit for a is m s^(-2)
unit for force = kg m s^(-2)


once again, unit for work done = kg m s^(-2) m = kg m^2 s^(-2)---------------------(1)



the "basic unit" unit for energy can be calculated in many ways, here i would use K.E
(you may use P.E.= mgh, or Energy = Power * time )


K.E = 1/2 m v^2
unit for KE = kg [ms^(-1)]^2 or unit for KE = Joule (commone sense)
= kg m^2 s^(-2)------------------------------------------------------(2)

hence, by combining the cases( (1) and (2) ),
unit for work done = kg m^2 s^(-2) = unit for KE = Joule























I give you one unit table, it will be helpful to you


Units

Physical quantitySymbolSI units

Momentumpkgm-1s-1
impulseIkgm-1s-1 or Ns
ForceFN or kgms-2
EnergyEJ or kgm2s-2
WorkWJ , kgm2s-2 or Nm
PowerPW , Js-1 or kgm2s-3

2007-08-03 23:11:25 補充：
2 yes??? the statment is implicit **********one thing we must make it clear: is the displacement in the direction of the force?****************if not, the work done should also include cosθ

2. Right. It's the definition.
1. Wrong. W=F*s.
In most case in physics, vector times vector would give a scalar.
As F and s are vector, W is scalar.
Work done can be +/-.
+ => the object is gaining energy by F.
- => the object is losing energy by F.
3. Right. Work = F*s = change in ENERGY by a force.