6.1- Work Done By A Constant Force
- Work is a scalar quantity
- Can be either positive, negative, or zero
- If the displacement is zero, work is zero
- Without movement, work is zero
- Down = positive work
- Up = negative work
6.2- The Work-Energy Theorem & Kinetic Energy
- Work-energy theorem: work (w) done by the net external force equals the difference between the objects final kinetic energy and initial kinetic energy.
- If the net force does positive work, the kinetic energy increases.
- If the net force does negative work, the kinetic energy decreases.
- If the work is zero, the kinetic energy remains the same.
- In a circular orbit, the gravitational force is always perpendicular to the displacement of the satellite, and does no work.
- In an elliptical orbit, work can be done.
6.3- Work Done By The Force of Gravity
- Can be either positive or negative
- Only the difference in vertical differences should be considered when working with this force.
- Only measure vertical distance from the objects initial height to its final height.
- Gravitational potential energy (PE) is the energy that an object has by virtue of its position.
- Zero level heights can be taken anywhere, as long as both heights are measured from the same zero level.
- This deals only with conservative forces.
6.4- Conservative Forces, Non-Conservative Forces, and The Work-Energy Theorem
- When an object is moved from one place to another, the work done by the gravitational force does not depend on the choice of path.
- It only depends on the initial and final paths
- Conservative force: one that in moving an object between two points, does the same work, independent of the path taken between the points. Also, it does no net work on an object moving around a closed path.
- Examples: gravitational force, elastic spring force, and electric force.
- Non-conservative force: one that in moving an object between two points, the work depends on the path of motion between the points.
- Examples: static and kinetic frictional forces, air resistance, tension, normal force, and propulsion force of a rocket.
- PE is not defined for non-conservative forces.
- In normal situations, conservative forces (like gravity), and non-conservative forces (such as air resistance) act on an object at the same time.
- The work done by the net external force = the change in the objects kinetic energy.
6.5- The Conservation of Mechanical Energy
- The principle of conservation of mechanical energy: the total mechanical energy (TME) of an object remains constant as the object moves, provided that the net work done by external non-conservative forces is zero.
- TME remains constant all along the path between the initial and final points- the sum of KE and PE.
- KE and PE may be transformed into each other.
- KE is converted into PE, for example when a moving object coasts up a hill.
- PE is converted to KE when an object is allowed to fall.
- The principle of conservation of mechanical energy can be applied even when forces act perpendicular to the path of a moving object.
- Remember: The higher something is dropped from, the more energy!
6.6 Non-Conservative Forces and The Work-Energy Theorem
- Non-conservative forces can do either positive or negative work.
- Work is positive when it has a component in the direction of the displacement, and speeds up the object.
- Work is negative when it has a component opposite to the displacement and slows the object down.
6.7- Power
- Power is work done over time
- Power is a scalar quantity
- Average power is the rate at which the net force does work.
