Chap 6 and 7 Flashcards
Energy and work def
A scalar quantity associated with the state of one or more objects.
Work is energy transferred to or from an object by means of a force acting on the object.
Work done if F is const and moving in a straight line
w = fdcos(theta)
Difference between work done by the force on the object and work done against the force
work said to be done by the force on the object if the force is in the direction of motion (acute angle)
work said to be done against the force of the force is opposite to the direction of motion.
Network
work done by all forces acting on an object (scalar)
A variable force (also known as vector fields)
We use the integral way to calculate this (line integral).
Conservative and non-conservative forces and examples
- Cons if the work done in moving a particle between two points does not depend on the path taken.
Also, if the work done by it in moving an object around any closed path is 0.
ex: gravity, spring force, electrostatic force. - non-cons if work done in moving a particle between two points does depend on the path taken.
ex: friction and drag.
Hooke’s law
The restoring force exerted by a spring is directly proportional to the amount of stretch and is directed opposite to the stretch. F = -kx
Work done by a spring force derived from an integration of the spring force
w = -0.5kx^2, negative bcs it’s always opposing motion.
N.B: This only happens when we chose our starting position to be x = 0, where the spring is neither compressed nor stretched. Otherwise, we have to use other integrals for example, when compressing from x = 0.2 to x = 0.4
consider a single force acting on a system. If the force does positive work, then the KE of the system will _____ and if the force does negative work, the KE of the system will ____
- It will increase
- It will decrease.
The work done on a system by a force represents ______
the amount of energy trasnferred (or removed from) the system by the force.
Doing work against a conservative force vs doing work against a non-conservative force.
- For conservative, energy can be recovered, but for a non-conservative force, energy cannot be restored.
Why is PE referred to as stored work?
bcs it can be recovered and converted into other forms of energy anytime.
Work done by conservative forces and PE
- work done against a cons force leads to gain in PE. Work done by a const force leads to loss in PE
Wc = - changeU
Note: This is very different from the work-energy theorem.
Work done by tension in circular motion
It’s zero bcz on very small parts it is perpendicular to the displacement.
Conservative or non-conservative? Allows you to choose any path you like.
Work done by conservative forces is the same no matter what path is taken, but the same is not true for non-conservative forces.
