Let's Do Work
What two things that have to happen for work to be done?
Work = Force x Distance: For work to occur, an object must be displaced in the same direction as the force acting on the object.
What variables effect an object's kinetic energy? Which variable has the greatest effect?
Mass and velocity effect KE. Velocity has the greatest effect because KE increases with the square of the velocity.
What is the formula for elastic potential energy? What are the variables in the equation and what do they mean?
PEe = 1/2*k*x2; k is the elastic constant, x is the distance the spring or rubber band is stretched or compressed.
What is the formula for power? What do each of the variables mean?
P = W/t; P is power, W is work, & t is time.
What are the states of matter from the lowest to highest entropy?
solids, liquids, gases
What is the Work-Energy Theorem?
Work = Change in KE
A 4 kg red mass travels at 3 m/s. A 2 kg blue mass travels at 5 m/s. Which has the greater kinetic energy of the two?
The blue mass has the greater kinetic energy.
A rubber band with a spring constant of 26.4 N/m has a relaxed length of 3.75 m. When a mass is attached to the end of the spring, it stretches the spring to 4.9 m. What is the elastic potential energy stored in the rubber band?
17.5 J
How much power is exerted when a 150 lb student walks 15 feet up the stairs in 25 seconds?
90 ft-lbs/sec
What's the one word definition of entropy?
disorder
Explain the difference between power and work when a 100 kg boy and a 50 kg girl run up a 15 m high flight of stairs. The boy reaches the top in 5 seconds and the girl reaches the top in 2.5 seconds. How much work does the girl do compared to the boy? How much power does the girl exert compared to the boy?
The boy does twice as much work as the girl. The boy and the girl exert the same amount of power.
What is thermal equilibrium? Will net energy be exchanged between these systems?
Thermal equilibrium occurs when two objects in physical contact are at the same temperature. There is no net energy exchanged between these systems.
How does the temperature of a substance relate to its kinetic energy?
Higher temperature substances have higher kinetic energies, lower temperature substances have lower kinetic energies.
What is the difference between power and work?
Work is force times distance. Power is the work (F*d) divided by the time (t) the work is done.
When a system's entropy is increased ____ energy is available to do work.
Less
Describe the transformation of energy for an object that is thrown upward and then falls back to earth.
The object is thrown upward with KE. This KE is converted to PEg as it travels upward. At the top of its upward arc, it has only PEg. It then falls back to earth and this PEg is converted back to KE. When it reaches the ground all of the energy in the object has been converted to KE.
Describe a substance with a high average KE. Describe a substance with a low average KE.
High temperature substances have a high average KE. Cold temperature substances have a low average KE.
What is the difference between internal energy and average kinetic energy?
Internal energy is the total energy of all the particles in a substance. Average kinetic energy is the KE of a typical individual particle in a substance.
A student mixed 200 mL of water at 50 degrees C with 200 mL of water at 90 degrees C. What is the final temperature of the water?
70 degrees C
When an egg is broken and scrambled, the entropy of the egg (increases/decreases) and the total entropy of the universe always (increases/decreases).
increases, increases
A 4 kg basketball is thrown upward with an initial velocity of 5 m/s. What is the potential energy of the basketball at the top of its' path?
50 J
A 55 kg skateboarder on a 4 kg skateboard is at the top of a ramp with a vertical height of 7 m. What is the skater's maximum kinetic energy at the bottom of the ramp?
4047 J
A 2 kg mass drops a distance of 20 m to the ground. If 50% of the initial potential energy goes into increasing the internal energy of the mass, determine the magnitude of that increase?
196 J
What is specific heat? Do substances with low specific heat cool down slowly or quickly?
Specific heat is the amount of heat energy required to heat up a unit mass (1 kg) of a substance 1 degree C in temperature. Low specific heat substances cool down quickly.
Two benches, a metal and a wooden one, are in the sun. Which one feels hotter? What property of matter will make this material feel hotter to the touch?
The metal bench, specific heat capacity