Chapter 7: Kinetic Energy and Work. Energy and Work Kinetic energy Work done by a constant force...
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Transcript of Chapter 7: Kinetic Energy and Work. Energy and Work Kinetic energy Work done by a constant force...
Chapter 7: Kinetic Energy and Work
Energy and WorkKinetic energy
Work done by a constant force
Work–kinetic energy theorem
Work Done by a Gravitational Force
Work done by gravitational force
Change in kinetic energy
Tomato thrown upward
Lifting/lowering an object
Work Done by a Spring ForceHooke’s law
Work done by a spring force
Work Done by a General Variable Force
Work: variable force
Calculus
Divide area under curve
Add increments of W (numerically)
Analytical form?
Integration!!!
Sample Problem 7-8
Chapter 8: Potential Energy and Conservation of Energy
Introduction
Potential Energy and Conservation of EnergyConservative ForcesGravitational and Elastic Potential EnergyConservation of (Mechanical) EnergyPotential Energy CurveExternal Forces
Work and Potential EnergyPotential Energy
General Form
Gravitational Potential Energy
Elastic Potential Energy
(Non-)Conservative Forces
The system consists of two or more objects.A force acts between a particle–like object in the
system and the rest of the system.When the system configuration changes, the force
does work W1 on the particle–like object, transferring energy between the kinetic energy K of the object and some other form of energy of the system.
When the configuration change is reversed, the force reverses the energy transfer, doing work W2 in the process.
W1 = –W2 conservative force
Path Independence of Conservative Forces
The net work done by a conservative force on a particle moving around every closed path is zero.
The work done by a conservative force on a particle moving between two points does not depend on the path taken by the particle.
Sample Problem 8-1: A 2.0 kg block of cheese that slides along a frictionless track from a to point b. The cheese travels through a total distance of 2.0 m, and a net vertical distance of 0.8 m. How much work is done on the cheese by the gravitational force?
Conservation of Mechanical Energy
Mechanical Energy
Conservation of Mechanical Energy
In an isolated system where only conservative forces cause energy changes, the kinetic and potential energy can change, but their sum, the mechanical energy Emec of the system, cannot change.
Potential Energy Curve
Turning Points Equilibrium Points
– Neutral Equilibrium
– Unstable Equilibrium
– Stable Equilibrium
A plot of U(x), the potential energy function of a system containing a particle confined to move along the x axis. There is no friction, so mechanical energy is conserved.
1D Motion
Conservation of Energy
The total energy of a system can change only by amounts of energy that are transferred to or from the system.
The total energy E of an isolated system cannot change.
Thermal Energy/Friction
Sample Problem 8-8