Chapter 6 - work and energy Flashcards
When is work accomplished?
work is accomplished when a force moves an object through a certain distance
What is work defined as?
The product of the magnitude of the displacement times the component of the force parallel to the displacement.
Does the Earth do work on the Moon? The Moon revolves around the Earth in a nearly circular orbit, kept there by the gravitational force exerted by the Earth. Does gravity do (a) positive work, (b) negative work, or (c) no work on the Moon?
The gravitational force FB exerted by the Earth on the Moon acts toward the Earth and provides its centripetal acceleration, inward along the radius of the Moon’s orbit. The Moon’s displacement at any moment is
tangent to the circle, in the direction of its velocity, perpendicular to the radius and
perpendicular to the force of gravity. Hence the angle u between the force FB G
and the instantaneous displacement of the Moon is 90°, and the work done by gravity is therefore zero (cos 90° = 0). This is why the Moon, as well as artificial satellites, can stay in orbit without expenditure of fuel: no work needs to be done against the force of gravity.
What is the work energy principle?
The net work done on an object is equal to the change in the object’s kinetic energy.
What happens if positive network W is done on an object? (use work-energy principle)
It tells us that if (positive) net work W is done on an object, the object’s kinetic energy increases by an amount W.
What is potential energy?
the energy associated with forces that depend on the position or configuration of an object (or objects) relative to the surroundings.
Define the change in potential energy.
the change in potential energy associated with a particular force is equal to the negative of the work done by that force when the object is moved from one point to a second point
Alternatively, we can define the change in potential energy as the work required of an external force to move the object without acceleration between the two points,
What is the principle of conservation of mechanical energy?
If only conservative forces do work, the total mechanical energy of a system neither increases nor decreases in any process. It stays constant—it is conserved.
