PHYS 1405 – Conceptual Physics I
Work, Power, and Energy
Leader: _________________________ Recorder: __________________________
Skeptic: _________________________ Encourager: ________________________
Materials
Yourself Hooky block
Stop Watch Spring Scale (5 N capacity)
Tape measure, metric 500 g mass
Introduction
In this activity we will introduce some of the basic notions of work and energy.
Safety
We will make several trips up and down stairs in this activity. If that is physically difficult or medically dangerous for you, then limit your activity accordingly.
Procedure
Attach a spring scale to the hooky block and pull the block across the floor at a constant velocity.
Q1) What is the net force acting on the block? Explain.
Q2) Does the spring exert a force on the block?
Q3) If the spring exerts a force on the block, how can the net force be 0?
Q4) Read the spring scale and record the force (you will only be able to make a rough estimate) it exerts while you pull the block at a constant velocity.
Q5) What is the force of friction on the block? Explain your answer.
Place the 500 g mass on top of the hooky block.
Q6) Do you think the spring will exert the same or a larger force to balance the friction force?
Q7) With the 500 g mass on the block pull the block with the spring and record the force.
Q8) Do you feel that you have to exert a larger effort to move the block with the 500 g mass on it?
Q9) Pull the block a greater distance along the floor. Do you have to exert a greater effort?
This combination of a force exerted over a distance is what we define in Physics as work. Work is define as the product of force and distance.
Q10) Work = _________ x __________
Q11) What are the SI units of force?
Q12) What are the SI units of distance?
Q13) What then are the SI units of work?
The combination of units you found in Q13) is called the joule after James Prescott Joule. The symbol for the joule is J.
Hold the hooky block in the air.
Q14) What forces are acting on the block?
Q15) If you let the block go, what force acts on it?
Q16) Does this force do work as the block falls?
Q17) Suppose the block weighs 1.0 N, and falls .50 m. How much work is done? (hint use the definition you wrote in Q10)
Suppose the force and the displacement are in different directions. A refinement of the definition of work is that only the piece of the distance that is in the same direction as the force will contribute to the work.
Q18) Suppose you walk up a stairwell. What force do you have to overcome to move up the stair well?
Q19) What direction does the force you answered in Q18) Point?
Q20) If you walk up a stair well, then what distance do you use to calculate the work that you do against gravity?
Check
your answer with your instructor at this point as you will need it for the next
part of the lab.
Obtain a metric tape measure and a stopwatch.
Have someone in the your group start at the bottom of the stairwell outside the classroom and time them as they walk slowly up the stairs.
Q21) Record the following data.
Time = ______ s
Use the tape measure to measure the relevant distance in m.
Distance = ________ m
You can obtain your weight in N by multiplying your weight in lbs by 4.5.
For example if you weigh 150 lb, the your weight in N is 150 x 4.5 = 675 N
Weight = _________ N
Calculate the work done against gravity.
Work = _____________ J
This time, time someone running up the stairs.
Q22) Record the following data.
Time = ______ s
Use the tape measure to measure the relevant distance in m.
Distance = ________ m
You can obtain your weight in N by multiplying your weight in lbs by 4.5.
For example if you weigh 150 lb, the your weight in N is 150 x 4.5 = 675 N
Weight = _________ N
Calculate the work done against gravity.
Work = _____________ J
Q23) Was work the same or different when you walked up the stairs compared to when you ran up the stairs?
Q24) If the work was the same what was different about the two situations?
We define the power as how much work is done per time, or Power = work/time.
Q25) What would be the SI units of power?
The combination of units you gave in Q25 is called a watt, named for James Watt who made significant improvements to steam engines in the 19th century. The symbol for watt is W.
Q26) Calculate the power for each case when you went up the stairs.
1st case P = ________ W
2nd case P = ________ W
Q27) Did you exert more power when you went up the stairs fast or slow?
Lift up the hooky block.
Q28) Did you do work, when you lifted it.?
Release the hooky block.
Q29) Did you recover the work that you did to lift the block?
Forces like gravity have a special property. Namely, we can store energy with them. When we store energy, we refer to that as potential energy. If the energy is stored by gravity, then we refer to the stored energy as the gravitational potential energy or GPE.
The GPE is equal to the work done in moving the object to its location, so
GPE = Weight x height or GPE = mgh.
Q30) If you lift a 0.300 kg object by .33 m, what is its GPE? (take g = 10 m/s/s).
Q31) The numbers in Q30) approximately describe lifting a 12 oz soda by 1 ft. So if you lift a 12 oz soda by 1 ft, how much work against gravity have you done?
Q32) What would be the GPE?
Drop the hooky block.
Q33) As the hooky block falls what happens to its motion?
Q34) What happens to the gravitational potential energy as the hooky block falls?
We introduce another type of energy, which we call kinetic energy. Kinetic energy is the energy an object has when it is moving. It is defined as KE = ½ mv2.
Q35) As the block falls, what happens to its kinetic energy?
Q36) Complete the following definitions
Work = _____________ x ______________
Power = ____________ / _______________
GPE = __________ x ____________ x ________
KE = ½ ____________ x _____________