#$&*
PHY 201
Your 'question form' report has been received. Scroll down through the document to see any comments I might have inserted, and my final comment at the end.
** Question Form_labelMessages **
Work and KE
** **
Please help me make sure I am understanding this concept.
When the force exerted on the object is in the direction of the object's motion, it tends to do positive work on the object and thus to speed the object up.
@&
Your statement is good provided you replace 'force' with 'net force':
When the net force exerted on the object is in the direction of the object's motion, it tends to do positive work on the object and thus to speed the object up.
*@
In this case the force exerted by the object is opposite to the direction of motion and we say that the object does negative work.
So when the object does negative work, work is actually done on it and it speeds up.
@&
When the net force exerted on the object is in the direction of motion, the net force exerted by the object, being equal and opposite to it, is in the direction opposite motion.
So when positive work is done on the object, negative work is done by the object. These two conditions are equivalent, and the result is that the object speeds up.
*@
@&
Saying the same thing with slightly different wording and different perspective:
When the object does negative work it is because the net force exerted by the object is in the direction opposite the motion. In this case the net force exerted on the object, being equal and opposite to the net force exerted by the object, is in the direction of motion, so the object speeds up.
*@
On the other hand if the force exerted on the object is directed opposite to the object's direction of motion it will tend to slow the object down.
In the case of the force exerted by the object is in the direction of motion and the object does positive work, expending kinetic energy in the process (the energy to do the work has to come from somewhere; in this case it comes from kinetic energy).
@&
This is correct, provided you say 'net force':
If the net force exerted on the object is directed opposite to the object's direction of motion it will tend to slow the object down.
In this case the net force exerted by the object is in the direction of motion and the object does positive work, slowing it down.
*@
@&
Positive work done by the net force on the object speeds it up. Negative work done by the net force on the object slows it down.
If the net force exerted by the object does positive work, the object slows down. If the net force exerted by the object does negative work, the object speeds up.
*@
?????
Does this mean if the force applied to the object is pushing the object in the direction it is already going? By being parallel and in the same direction, the force is causing the object to do more work?
@&
By being parallel and in the same direction, the force is doing positive work on the object, which will tend to speed it up. If the object was doing positive work, it would be 'spending' its energy and slowing down.
*@
I would think it would cause the object to do less work because the force is pushing the object. For example, as a kid my father acted like the force pushing me on my bicycle (the object) to help me speed up. In that case I was doing less work, therefore, increasing my total energy.
@&
@&
You didn't need to do much work, since the force you exerted on your father was the end result of his metabolism burning chemical energy. Your metabolism didn't need to do much other than help keep your balanced. You might well have helped, but as you understand the work you did would not have been sufficient to give you the same increase in speed.
Your father exerted a force on you, and by action-reaction you exerted a force on him. Presumably the force he exerted on you was in the direction of your motion, so he did positive work on you, tending to speed you up. The reaction force you necessarily exerted on him was in the direction opposite your motion (and his), and did negative work on him. Had it not been for the forces exerted by his muscles, he would have slowed as a result.
*@
*@
Or am I not comprehending this correctly. Is the force like riding a bicycle directly into hurricane force winds. The rider and bicycle is the object and the wind is the force that is pushing against me, thus requiring me to increase my work (positive) which means an increase in KE???
@&
When you walk you push your foot backwards, resulting in a frictional force between your foot and the sidewalk. You exert a backward frictional force on the sidewalk, which exerts a forward frictional force on you. Similarly, on a bicycle, your action causes the tire to push backwards on the road surface, which therefore pushes forward on the tire, propelling you forward.
The bicycle is a mechanism for very efficiently converting the work you do on it to work done on you and the bicycle itself.
Rolling friction and any headwind exert forces in the direction opposite your motion.
So the result of your work is a frictional force in the direction of motion, exerted by road friction on the tire. This force therefore does positive work on the system (you and the bicycle) while rolling friction and headwind exert forces in the direction opposite motion and do negative work on the system.
This gets a little confusing because you are part of the system, and you are doing positive work. However the work you are doing is internal to the system, and any force you exert on the bicycle results in an opposite force exerted on you. Between the work you do on the bicycle and the work the bicycle does on you, it's a wash; the you-and-bicycle system does zero net work as a result the forces you exert. However the process results in the road exerting a frictional force that does positive work on the you-and-bicycle system (and in your metabolism using up chemical energy), and the system therefore does negative work on the road.
So you're doing work, the you-and-bicycle system isn't doing any work within itself, but the system is doing negative work on the road, which in turn does positive work on the system.
Rolling friction and headwind do negative work on the system, which therefore does positive work against those forces, tending to slow the system.
If during an interval road friction does more positive work on the system than the magnitude of the negative work done by rolling friction and wind, the KE of the system will increase.
*@