- #1
elvinc
- 10
- 0
Hi,
The simple definition of work done by a constant force on moving a constant mass is
(size of) force on object multiplied by (size of) distance object moves in direction of force while force is applied
In the ideal case of a puck on frictionless ice, an applied constant force will cause the puck to accelerate as long as the force is applied. The work done by the force on the puck transfers energy to the puck in the form of kinetic energy.
Ok – I understand that.
Take two scenarios of 1) pushing the puck on a rough surface and 2) lifting an object against gravity
In both case two balanced forces are acting on the object and the object (ideally) does not accelerate
Situation 1) the two forces acting on the puck are the pushing force and the friction. So which force is doing work on the block? Why, when we calculate the work done on the block do we use one force (say the pushing force) and ignore the other (friction)? Could we not argue that as the resultant force is zero then no work is done on the block? That won't work (pun intended) because we can't explain where the heat been the rubbed surfaces comes from.
Situation 2) is pretty much the same. We lift the object vertically against gravitational attraction of the earth. If the block does not accelerate the weight and lifting force are balanced and opposite. If the object is placed on a shelf at a height above the floor then it has gravitational potential energy so work must have been done on the block. Again how do we calculate the work done? Do we just multiply the lifting force by distance moved and ignore the weight? Yes, probably, but why?
Thanks
Clive
The simple definition of work done by a constant force on moving a constant mass is
(size of) force on object multiplied by (size of) distance object moves in direction of force while force is applied
In the ideal case of a puck on frictionless ice, an applied constant force will cause the puck to accelerate as long as the force is applied. The work done by the force on the puck transfers energy to the puck in the form of kinetic energy.
Ok – I understand that.
Take two scenarios of 1) pushing the puck on a rough surface and 2) lifting an object against gravity
In both case two balanced forces are acting on the object and the object (ideally) does not accelerate
Situation 1) the two forces acting on the puck are the pushing force and the friction. So which force is doing work on the block? Why, when we calculate the work done on the block do we use one force (say the pushing force) and ignore the other (friction)? Could we not argue that as the resultant force is zero then no work is done on the block? That won't work (pun intended) because we can't explain where the heat been the rubbed surfaces comes from.
Situation 2) is pretty much the same. We lift the object vertically against gravitational attraction of the earth. If the block does not accelerate the weight and lifting force are balanced and opposite. If the object is placed on a shelf at a height above the floor then it has gravitational potential energy so work must have been done on the block. Again how do we calculate the work done? Do we just multiply the lifting force by distance moved and ignore the weight? Yes, probably, but why?
Thanks
Clive