Chapter Objectives and Summaries
for the Oxford University Press text by Walding, Rapkins and Rossiter:-
" NEW CENTURY Senior Physics - Knowledge, Processes and Reasoning "
CHAPTER 9 Work and Energy.
Knowledge of subject matter
- List various sources and types of energy.
- State examples of energy transformations.
- Define work, kinetic energy, potential energy and power and state their units (W = Fs.cosØ; P = W/t; KE = ½ mv2; EPE = 1/2kx2; GPE = mgh).
- Use the definition of work to calculate energy transfer.
- Solve simple problems involving energy changes.
- State the principle of conservation of energy.
- Describe between elastic and non–elastic collisions.
- Describe elastic potential energy. Solve simple problems involving elastic potential energy.
- State Hooke's law. Solve simple problems involving Hooke's law.
- Describe energy changes occurring in a swinging pendulum and objects moving in a vertical circle.
- Interpret given physical situations on terms of inter-conversation of energy.
- Distinguish between elastic and inelastic collisions.
- Analyse force-displacement graphs in term of work and energy.
- Calculate spring constant from experimentally obtained graphs.
- Identify and analyse modern day technology (high jump bags, bumper bars etc.) in terms of energy considerations.
- Write laboratory reports on energy conversion experiments.
- Devise and design relevant equipment to overcome deficiencies in common equipment.
- Identify examples of energy transfer
COMPLEX REASONING PROCESSES
- Analyse changes in kinetic and potential energy in vibrating springs.
- Analyse collisions to determine whether they are elastic or inelastic.
CHAPTER 9 SUMMARY
- Energy is not lost in any transfer, it just gets transferred from one place to another or from one form to another. This is called the Law of Conservation of Energy.
- Energy is the capacity to do work.
- Work is defined as the product of the force and the distance moved in the direction of the applied force. (W = F s). Work has the units newton metre or N m. The newton metre is called the Joule (J).
- For forces applied at an angle Ø: W = F s cos Ø
- Efficiency is a measure of the useful energy output compared to the energy input.
- Bodies that are moving have kinetic energy (EK, KE): EK = ½mv²
- Work done equals change in kinetic energy: W = D EK
- Bodies that can do work because of their position have potential energy (EP, PE or U): EP = mgh.
- The work done is a measure of the change in potential energy: W = D EP.
- Gravitational potential energy is defined as the energy associated with the state of separation between bodies that attract each other via the gravitational force.
- Elastic potential energy is the energy stored in a spring or other elastic body by virtue of its distortion, or change in shape.
- The relationship F = - kx is called Hooke's Law where k is the spring constant. The stiffer the spring the greater the spring constant.
- When Hooke's law is no longer obeyed, the object is said to have reached the elastic limit.
- Kinetic and potential energy are said to be forms of mechanical energy.
- Within an isolated system mechanical energy is conserved. KE + GPE = constant.
- There are two types of collisions: elastic and inelastic: In an elastic collision, kinetic energy is conserved.
- In an inelastic collision, the total kinetic energy is not conserved.
- Power is a measure of the rate of energy output - it has the units joule per second (J s-1). One J s-1 is called one watt (W)
- Random vibrations or motions of atoms and molecules within an object are examples of internal energy (Ei).
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