Chapter 5: Work, Energy and Power Flashcards
- Define work done.
- What is the equation for work done?
- How is work done calculated if the force is applied at an angle to the direction of motion?
- Work done is defined as the product of the force and the distance over which the force is applied. In mathematical terms
- work done = force x distance moved in direction of force
W = Fx
where W is the work done in Nm or J (joule). - The component of force F in the direction of motion is Fcosϴ. This takes place instead of F.
- What is energy?
- What type of quantity is energy?
- What is the SI unit for energy?
- Energy is the capcaity for doing work.
- Energy is a scalar quantity, with magnitude but not direction.
- The SI unit for energy is the joule (J), the same unit as for work done.
- Define kinetic energy.
- Define gravitational potential energy.
- Define chemical energy.
- Define elastic potential energy.
- Define electrical potential energy.
- Define nuclear energy.
- Define EM/radiant energy.
- Define sound energy.
- Define thermal energy.
- Kinetic energy is the energy due to the movement of an object with mass.
- Gravitational potential energy is the energy of an object due to its position in a gravitational field.
- Chemical energy is the energy contained within the chemical bonds between atoms, it can be released when the atoms are rearranged.
- Elastic potential energy is the energy stored in an object as a result of reversible change in its shape.
- Electrical potential energy is the energy of electrical charges due to their position in an electric field.
- Nuclear energy is the energy within the nuclei of atoms, it can be released when the particles within the nucleus are rearranged.
- Radiant energy is the energy associated with all electromagnetic waves, stored within the oscillating electric and magnetic fields.
- Sound energy is the energy of mechanical waves due to the movement of atoms.
- Thermal energy is the sum of random potential and kinetic energies of atoms in a system.
What is the principle of conservation of energy?
The principle of conservation of energy states that the total energy of a closed system remains constant: energy can never be created or destroyed, but it can be transferred from one form to another.
- Define kinetic energy.
- How can kinetic energy be calculated?
- Comment on the proportionality between kinetic energy, mass and speed.
- Kinetic energy is the energy associated with an object as a result of its motion.
- The kinetic energy of an object in linear motion can be calculated for its mass m and speed v using the equation
KE = 0.5mv2 - For objects travelling at the same speed, the KE is directly proportional to the mass. For a given object, the KE is directly proportional to the square of its speed.
Give the derivation for kinetic energy.
- What is gravitational potential energy?
- How can you calculate the change in GPE of an object in a uniform gravitational field?
- Gravitational potential energy is the capacity for doing work as a result of an object’s position in a gravitational field.
- You can calculate the GPE Ep of an object from its mass m and the change in height h using the equation
* Ep* = mgh.
How can the origin of the equation for GPE be explained?
The origin of the equation for GPE can be explainedusing the idea of work done by a force. Take an object of mass m being lifted vertically upwards through a height h at constant speed (so its KE does not change). Hence the force applied must be equal to its weight mg. The work done W by this force is transferred into GPE. Therefore
- Ep* = W = force x distance moved in direction of force
- Ep* = (mg) x h = mgh
There are many situation where KE and GPE are exchanged: as an object falls, its GPE decreases and its KE increases.
Use the principle of conservation of energy to derive an equation for the exchange of KE and GPE.
From the principle of conservation of energy, where there are no resitive forces involved, the GPE lost is equal to the KE gained.
- Define power.
- Using the defining equation, give an alternative definition of power.
- What are the units of power?
- Power is the rate of work done. As an equation, this is written as P=W/t
where P is the power and W is the work done in a time t. - Power can also be defined as the rate of energy transfer.
- Power is measured in joules per second (J s-1) or in watts (W).
4.
Derive the equation P = Fv.
There are situations where a constant force has to be exerted on an object to maintain constant speed.
For example, a car travelling at a constant speed with a net force of 0 (as rate of work done by forward force from car engine is equal to rate of work done against the car by frictional forces).
Take a constant force F which moves the car a distance x in a time t.
When can the equation P=Fv be used?
The equation can be used in any situation where a constant force is necessary to maintain a constant speed [i.e. a swimmer travelling through water at constant speed or a skydiver falling through the air at terminal velocity].
How is the efficiency of a closed system calculated?
efficiency = [total output energy / total input energy] x 100
