### Summary

In this lesson, students will explore conservation of mechanical energy and the relationship that it has with work.

### Essential Question(s)

What effect does increasing the potential energy of an object have on the work it does on another object during a collision?

### Snapshot

**Engage**

Students are challenged to list as many things that they already know about potential energy, kinetic energy, and work.

**Explore**

Students make a hypothesis based on the essential question and devise a procedure to test the hypothesis.

**Explain**

Students state their hypothesis, explain their investigation and state their conclusion.

**Extend**

Students calculate the amount of force applied to the car and create a graph that shows the affect of increasing potential energy on the force applied to the car and the distance it traveled.

**Evaluate**

Students are given a scenario of a car involved in a rear collision. Students will figure how far the car will travel and if a nearby pedestrian is in danger.

### Materials

Toy cars (1 per group)

Rulers (1 per group)

Meter stick

Marbles (a whole bag)

Textbooks for stacking

Electronic scale(s)

Graph paper

Scientific calculator

Sticky notes

### Engage

Pair students up. Write the terms, "potential energy", "kinetic energy", and "work" on the board. Use the How I Know It strategy for students to write everything they know about these terms and how they know it. Challenge them to think back to previous science classes and experiments that they've done in the past.

The How I Know It strategy requires the students to think deeply about how they know the terms. If students can't really think how they know or remember these terms, lead a class discussion and start drawing those experiences and knowledge out as a group.

Allow pairs to share what they know and clarify any misconceptions. Review as needed so students are refreshed on the terms.

Before moving on to the Explore section of the lesson, have pairs join to make quads.

### Explore

Post, either on the chalk board or SMART board, the essential question: "What effect does increasing the potential energy of an object have on the work that is done on another object during a collision?"

Instruct groups to collaborate for a few minutes and write a one-sentence hypothesis. Allow groups to share their hypotheses.

Show students the following materials: five marbles, toy car, a ruler, a meter stick, marbles, books for stacking, and an electronic scale.

Challenge students to use any of the materials (they don't have to use everything) to design an investigation that will test their hypothesis and answer the essential question. Make sure the students know to gather quantitative data.

Before they begin, instruct groups to make a list of all the variables that can be measured when potential energy, kinetic energy, and work are involved. They will need a prior knowledge of the three equations. Also encourage them to make a list of variables they should keep constant.

### Explain

Use the 3-2-1 strategy and have each group orally give an account of three things they learned from their investigation, two questions they have, and one thing they found interesting.

It would also be good for students to explain any sources of error for the experiment and address parts of the procedure that could be refined.

Pose the question: "In this investigation, how does potential energy eventually become the work that is applied to the second car?"

Allow students to answer and explain their reasoning to the question.

### Extend

Instruct students to calculate the potential energy of each of their trials.

Say, assuming that all the potential energy is gone at the bottom of the ramp, what is the kinetic energy the car possesses? (The answer is equal to the initial potential energy.)

Say, assuming that all of the kinetic energy becomes work done on the second car, what force was placed on the second car?

Instruct the students to calculate the force of the car as the potential energy increases.

Now students are ready to create the graphs! Have them graph the increasing potential energy vs. the force applied on the car and the distance the car traveled. Student will need two y-axes (one on left and one on right, labeled force and distance) and the potential energy will be on the x-axis.

Discuss the trends they notice in the graph. Consider reviewing independent and dependent variables.

Ask students to separate from their groups so they can work individually for the Evaluate section of the lesson.

### Evaluate

Give the students the following scenario: You are sitting at a stoplight at the bottom of a hill. A car at the top of the hill has brake failure. It is freely rolling down the hill and will collide with the rear of your car. The car is 1245 kg (similar to yours), the height of the hill is 155 m and it collides with your car applying 3255 N of force. A woman is pushing a stroller 600 m in front of you. When you car lurches forward, will you strike the woman and the stroller?

Use the Exit Ticket strategy. Give students a sticky note and instruct them to write yes or no (indicating yes they will hit her or no they will not), followed by the distance their car will travel. Have them stick the note to your door on the way out. Make sure they put their name on the note.

### Resources

(2009).

*Marble Energy Lab: Teacher's Guide*. The Physics Classroom. https://www.physicsclassroom.com/getattachment/lab/energy/e4tg.pdf