ENERGY BASICS
What is the definition of energy?
The ability or capacity to do work.
What type of potential energy is stored in stretched or compressed objects?
Elastic potential energy.
What two factors determine an object’s kinetic energy?
Mass and speed.
State the law of conservation of energy.
Energy cannot be created or destroyed.
A machine outputs 90 J of useful work with 120 J of input. What is the efficiency?
75%.
What is Joules (J)
the unit for energy
If two objects have the same height but different masses, which has more gravitational potential energy?
The one with the greater mass.
If the speed of an object doubles, what happens to its kinetic energy?
It increases
Identify the system type: A sealed soda bottle that warms up in the sun.
closed system
A gym machine requires 240 J of input and produces 180 J of useful output.
What is the efficiency?
75%
What is the relationship between work and energy?
whenever work is done energy is transformed or transferred from one systme to another
What two factors that determine gravitational potential energy
Weight and Height
Two objects have the same speed. One has 3× the mass of the other. How does their kinetic energy compare?
The heavier object has 3× the kinetic energy
What does “useful work output” mean?
The work a machine is intended to do.
A 0.45 kg soccer ball is kicked at 18 m/s.
Find its kinetic energy.
72.9 J
what is the relationship between speed and kinetic energy?
the kinetic energy increases when speed increases
Why is potential energy called “energy of position”?
Because it depends on where an object is located in a system (its position)
Two objects have the same kinetic energy. One is twice the mass of the other. What must be true about their speeds?
The smaller mass must move faster to have the same KE
a cooler with a lid is used to keep object cool. what type of system is this?
Isolated system
A hiker places her 18 kg backpack on a rock ledge 4 meters above the trail.
How much gravitational potential energy does the backpack have?
705.6 J
Describe the difference between “energy being transformed” and “energy being transferred.”
Transformation = energy changes form.
Transfer = energy moves from one object/system to another.
An object is lifted to twice its original height. How does this change its gravitational potential energy?
PE doubles because height doubled
A student claims that a heavy object must always have more kinetic energy than a lighter object. Why is this incorrect?
Because kinetic energy also depends on speed; a lighter object moving faster can have more KE.
Compare open, closed, and isolated systems in terms of energy and matter exchange
Open: exchanges both energy and matter
Closed: exchanges energy, not matter
Isolated: exchanges neither energy nor matter
A worker pushes a cart using 120 N of force for a distance of 5 meters.
The machine produces 480 J of useful work output.
80%