Hands-on Activity Power Drag Lab

Learning Objectives

After this activity, students should be able to:

• Describe the factors affecting the concept of work and power in physics.
• Determine the work done on an object mathematically.
• Determine the power used in moving an object mathematically.
• Explain how time and work are related in the concept of power.
• (Optional Activity Extension) Describe equilibrium of forces and be able to sum up forces and use Newton’s second law of motion.

Educational Standards

Each TeachEngineering lesson or activity is correlated to one or more K-12 science, technology, engineering or math (STEM) educational standards.

All 100,000+ K-12 STEM standards covered in TeachEngineering are collected, maintained and packaged by the Achievement Standards Network (ASN), a project of D2L (www.achievementstandards.org).

In the ASN, standards are hierarchically structured: first by source; e.g., by state; within source by type; e.g., science or mathematics; within type by subtype, then by grade, etc.

NGSS: Next Generation Science Standards - Science

Common Core State Standards - Math

Materials List

Each group needs:

• a wooden plank with a hook on the side
• a spring scale
• two small 1-kg weights
• a ruler or meter stick
• a timer (a cell phone may be used)
• a copy of the Power Drag Lab Sheet
• (optional) a calculator

Worksheets and Attachments

Pre-Req Knowledge

Students need to have basic algebra skills. They should know the concepts of mass and weight, be able to do measurements using basic lab tools, and know the concepts of basic kinematics (speed, velocity, distance, displacement, and acceleration) and forces.

Introduction/Motivation

Today we’re going to learn about work, power, and energy, and how they are all related, both conceptually and mathematically. We’re going to get started by looking at an example related to power.

(Show students Figure 1 and pose the following questions: Is the statement correct? Why or why not?)

(Let students give their opinions. Encourage them to draw on their previous knowledge about physics and on lived experiences (if students have used a snowblower and/or shoveled snow). After a short discussion, show the following video on work and power (3:16 minutes): https://www.youtube.com/watch?v=ZqCMR7PjZRU.

While they watch the video, instruct students to take notes and complete the Power Drag Introduction Sheet.)

Procedure

Background

Work, Force, and Distance

In physics, work is defined as the application of force (F) to move an object over a distance (d). Work is measured in Joules (j) and can be calculated using the following equation:

W = F x d

Students may have heard the word “work” in non-physics contexts, such as doing work on math homework problems. This is not work in the scientific sense because the student is not moving an object over a distance. Students may have misconceptions that work can occur when an object is being moved over a distance at a constant velocity; however, when an object is moving at a constant velocity, there is no net force acting on it, and therefore no work is being done.

Work and Power

Power is measured in Watts (W) and is defined by how fast work is done over a given period of time (t):

P = W / t

As an example, two people are moving two identical televisions to the top floor of a building. It takes Person A five minutes to carry the television up the stairs; Person B uses a pulley to lift the television to the top floor in just two minutes. Both people did the same amount of work because they were lifting identical objects (the same mass, the same force needed to lift it) and moving them over the same distance (to the top of the building). However, Person B generated more power because they moved the television in a shorter amount of time.

Before the Activity

• Make copies of the Power Drag Lab Sheet.
• Gather materials and place them in a central area in the room.

With the Students

1. Divide the class into groups of three and distribute copies of the Power Drag Lab Sheet.
2. Have the students assign roles: secretary (take notes), materials manager (get the materials and return them when done), and the leader.
3. Have students read the instructions on the Power Drag Lab Sheet before letting the materials manager gather the materials.
4. Have students follow the Power Drag Lab Sheet instructions, reproduced below:

   

1. Set the wooden plank on the table or on the floor and place a 1-kg steel block on top of the wooden plank.
2. Attach the spring scale to the wood using the hook provided on the wood.
3. Measure 0.5 m from the front of the wood and mark it.
4. Start pulling the wood with the 1-kg weight using the spring scale at constant speed.
5. Measure the force on the scale and record it.
6. Keep moving until the 0.5 mark is reached. Measure the time it takes to pull it the whole length.
7. Compute for the work done using W = F x d.
8. Compute for the power using P = W/t.
9. Repeat Steps 3-8, but this time pull the spring scale faster (constant speed).
10. Record all measurements and computations in the Power Drag Lab Sheet.
11. Add another 1-kg block on the wooden plank.
12. Repeat Steps 2-8.

5. As students work, remind them to record all measurements and computations in their Power Drag Lab Sheet. 

6. Check in with student groups. If groups finish at different times, ask them to use what they saw in the lab to think about the statement from the Introduction: “A snowblower can do more work in less time, so it has more power than the person shoveling.”

Vocabulary/Definitions

equilibrium: When all forces added up vectorially and the sum is zero.

force: A push or pull upon an object resulting from the object’s interaction with another object. Any action that tends to maintain or alter the motion of a body or to distort it.

mass: Amount of matter. Amount of inertia of an object.

net force: The sum of all the forces acting on an object.

power: Calculated by work/time.

speed/velocity: How fast an object moves. Speed = distance/time Velocity = displacement/time

work: Calculated by force x displacement; also the change in kinetic energy of an object.

Assessment

Pre-Activity Assessment

Video Introduction: Students watch a 3:16-minute video to introduce them to work and power: https://www.youtube.com/watch?v=ZqCMR7PjZRU. As students watch the video, have them complete the Power Drag Introduction Sheet. Review students’ answers using the Power Drag Introduction Sheet Answer Key.
 

Activity Embedded (Formative) Assessment

Lab Measurements: Students complete the Power Drag Lab Sheet while they work through the activity. Ensure that students are accurately recording data on their sheets and following the direction to complete all of the trials.

Post-Activity (Summative) Assessment

Summative Assessment: Students complete the Power Drag Post-Lab Assessment. Review students’ answers using the Power Drag Post-Lab Assessment Answer Key.

Troubleshooting Tips

Make sure that when the students use the spring scale, they use the side with Newtons (N) to measure force.

Activity Scaling

For more advanced students, such as those in an AP Physics 1 Class:

1. Have students examine the forces acting on the block by considering forces in equilibrium on both the horizonal and vertical axes. Ask students to draw a free body diagram. Encourage them to consider the pulling force (F_a), friction (F_f), weight (F_g), and the normal force (F_N). Students should determine that if the block is pulled at a constant speed, the sum of forces along the horizonal axis (the path of motion) should equal zero. This means that the magnitude of the friction force is equal to that of the pulling force. Along the vertical axis, the magnitude of weight is equal to the normal force.
2. Have students determine the coefficient of friction on the surface (e.g., table). Provide students with a way to measure the mass of the block with the weight (e.g., a spring scale that can measure grams, a kitchen scale, or a triple beam balance). They find the coefficient using the equation µ = F_f / F_N. Building on their exploration of forces in equilibrium, students can calculate the coefficient by substituting equivalent magnitudes for F_f and F_N, finding that µ = F_a / F_g.

References

Copyright

Contributors

Supporting Program

Acknowledgements

This digital library content was developed by the University of Houston's College of Engineering under National Science Foundation GK-12 grant number 1855147. However, these contents do not necessarily represent the policies of the NSF and you should not assume endorsement by the federal government.