Ch 6 Work and Energy. Work in 1 dimension. Work is when a force is applied and an object is displaced If no displacement occurs or no force is applied then no work is done. The definition of work when the force is parallel to the displacement .
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Ch 6 Work and Energy
In the SI system, the units of work are joules:
As long as this person does not lift or lower the bag of groceries, he is doing no work on it. The force he exerts has no component in the direction of motion.
Work done by forces that oppose the direction of motion, such as friction, will be negative.
The work done may be positive, zero, or negative, depending on the angle between the force and the displacement
If there is more than one force acting on an object, we can find the work done by each force, and also the work done by the net force
Wtotal=W1 + W2 + W3 … = Σ W
6-3 & 6-4 KE, PE and Work Energy Principle
Mechanical Energy is the sum of the kinetic and potential energy of an object
Energy was traditionally defined as the ability to do work.
Not all forces can do work, but ME can
ME= KE + PE
Potential energy can also be stored in certain materials when they are compressed; the figure below shows potential energy yielding kinetic energy.
The force required to compress or stretch a spring is:
where k is the spring constant, and needs to be measured for each spring.
Negative because it is a restoring force, acting in the opposite direction of displacement
6-5 Conservative and Nonconservative Forces
Conservative forces are ones that do not depend on, or are independent, of the path taken. An example is gravity
If the object were to return to the starting point no net work would be done
Potential energy can only be defined for conservative forces.
6-6 Mechanical Energy and Its Conservation
Total mechanical energy ME= KE + PE therefore
This is the Law of Conservation of Energy
Power is the rate at which work is done
In the SI system, the units of power are watts:
The difference between walking and running up these stairs is power – the change in gravitational potential energy is the same.