Define "energy".
The ability to do work.
Work and energy are calculated in what unit?
The Joule
Define "work" in physics terms.
The action of a force acting through a distance to transfer energy to an object.
(a) What are the two main types of energy? AND
(b) What is meant by the term "mechanical energy"?
a) kinetic and potential energy
b) the sum total of the kinetic and potential energies of an object as a whole (do to motion or position)
What does it mean to say that energy is "conserved"?
It cannot be created or destroyed, but can only change forms and be exchanged between objects.
The total amount of energy in the universe is constant.
What objects have kinetic energy?
Objects in motion
The amount of kinetic energy depends on what?
mass and velocity
What is the mathematical formula for kinetic energy?
KE = 1/2(mass)(speed squared)
Which object has the greater amount of kinetic energy?
OBJECT A: mass= 1 kg speed= 10 m/s
OBJECT B: mass= 0.5 kg speed= 20 m/s
OBJECT B (doubling the speed has a greater affect on KE than halving the mass)
object A KE = 50 J
object B KE = 100 J
A swinging pendulum (m= 2 kg) has at it maximum height a gravitational energy of 9 J relative to its rest position. How fast will the pendulum be moving as it passes through its rest position? (assume no friction or air resistance)
v= 3 m/s
KE + PE = 9 J at the bottom of the swing, PE= 0 ==> KE +0 = 9 J ==> KE = 9 J
1/2(mass)(speed squared) = 9
1/2 (2)(speed squared) = 9 ==> speed squared = 9 ==> 3 squared = 9
What do all forms of potential energy have in common?
They store energy in the position/condition of an object or the particles making up that object.
This type of potential energy results from lifting an object higher above a reference height.
gravitational potential energy
This type of energy may result from stretching or compressing a material.
elastic potential energy
This type of energy results from the positions of different types of atoms within a substance.
(Hint: food and gasoline store energy in this form)
chemical potential energy
The amount of gravitational potential energy stored by an object depends on what 3 factors?
Mass, gravitational acceleration, height
A device that converts mechanical energy into electrical energy is a ____.
GENERATOR
A device that converts electrical energy into mechanical energy.
MOTOR
Describe the principle of "electromagnetic induction" by which a generator produces electrical energy from mechanical energy.
A moving (changing) magnetic field induces an electrical current to flow through a wire. By moving a magnet relative to a wire, electrical current and energy are produced.
What electromagnetic principle explains how a motor can change electrical energy into motion.
The moving charges create a magnetic field that interacts with a magnet, producing a magnetic force that moves the wires.
Why is a continual input of mechanical energy needed to produce a continual output of electrical energy from a generator (even in a frictionless system)?
The current induced by the motion produces a magnetic field that opposes the motion. Work must continually be done against this magnetic resistance to keep the generator running.
The amount of work done when a crate is pushed 10 meters across the floor using a 50 Newton force.
(HINT: 1 Joule = 1 Newton x 1 meter)
500 Joules
Which object has the greater gravitational potential energy?
Object A : mass = 10 kg ; height = 20 m
Object B: mass = 30 kg ; height = 10 meters
Object B has more GPE (mass x g x height)
How much work is required to accelerate a 10 kg cart to a speed of 10 m/s over a flat, frictionless surface?
(Hint: it is equal to the object's gain in kinetic energy)
W = change in KE = 1/2mv² = (5 kg)(10 m/s)² = 5 x 100 = 500 Joules
How much more distance is needed to stop a car moving at 90 mph compared to the distance needed to stop a car moving at 30 mph? (hint: distance is proportional to the amount of kientic energy each car carries).
W is propertional to KE, which is proportional to the square of the speed.
90 mph is 3x faster than 30 mph and carries 3x3 (9) times for energy, so 9x greater stopping distance.
An object (m= 1 kg) is lifted to a height of 1 meter above the floor. It is then dropped. In the absence of friction or air resistance, how much kinetic energy does the object have just before hitting the floor?
9.8 Joules This is equal to the amount of gravitational energy the object had just before being dropped (mgh = 1 x 9.8 x 1 = 9.8 J)
Since mechanical energy (KE + PE) is conserved, any loss in potenetial energy must result in an equal gain in kinetic energy