Kinetic v. Potential (9th grade Physical Science lab ) Students will: Measure the height, distance traveled, and time interval for a ball rolling down a ramp. Calculate the ball’s speed and the potential and kinetic energy of the object. Analyze the results to find the relationship between potential energy and kinetic energy. Materials Needed: • • • • • • • • • electronic scale (measure in kg) board for ramp (at least 1 meter) or V taped meter sticks box ball (golf, racquet, large marble...smooth is better) masking tape meterstick (for measuring) stack of books, at least 45 cm high catch box (a plastic tub or cardboard box) stopwatch Teacher notes: groups of 2 or 3 students (like groups); 2.5 to 3 days (data collection day 1; calculations and discussion day(s) 2 & 3 ) 1. This can be used as a review for calculating speed (8th grade standard) in addition to introducing the relationship between KE and PE. 2. The students do not need to memorize the formulas, teach them how to use the formulas. This is not about doing the math, it is about understanding how they can better understand the relationship through the math! 3. IF YOU ARE MAKING COPIES FOR SUDENTS- GIVE PAGEs 1 & 2 ONLY ON DAY ONE AND PAGES 3 & 4 ONLY WHEN 1 AND 2 ARE COMPLETE AND YOU HAVE CHECKED THE QUESTIONS OR COMPLETED A GROUP DISCUSSION! The students do not need a copy of the procedure to keep, one for each group class set is workable! (page 2) The data tables and analysis pages can be provided as copies or the students can create the data tables and/or do the calculations in their notebook or on their own paper 4. Students can continue on to part 2 on their own if they show understanding in group questions (formative assessment), if not you can have a full class discussion before continuing. There should not be a discussion about the KE and PE at this point, question 1 is about the time changing and question 2 is about having the data needed to calculate the speed (v) of the ball. 5. Parts 2 & 3- the math. You can start as a group and let them finish or do it all together as a class. Always make them show their work- the formula and plug in the values. This will show you if there are any errors in the set up. You can walk them through this if needed- don't wait for slower groups or other groups will get bored and then chaos! Adapted from: Work and Energy HMH PART 1. DATA COLLECTION 1. Prepare space for lab. Gather materials needed and put everything else away. 2. Measure the mass of the ball. (Hint: Make sure to set the balance to zero before you start.) • Record the mass in your table in all three “Ramp Height” columns. 3. Place a strip of masking tape across the board close to one end. • Measure the distance from the tape to the opposite end of the board. • The distance must be measured in meters (m), or centimeters (cm). • Record this distance in the row labeled “Length of ramp.” 4. Use a catch box at the end to catch the ball (can be made by cutting out one side of a box) 5. Make a stack of books approximately 15 cm high. • Build a ramp like the one shown in the drawing by setting the taped end of the board on top of the books. • Place the other end of the board in the catch box. • Measure the vertical height of the ramp at the tape. • Record this height in your data table in the row labeled “Ramp Height 1.” Making Time Measurements 6. First trial, Ramp Height 1: • Place the ball on the ramp at the tape. • Let the ball go, so that it rolls down the ramp. • Use a stopwatch to measure the seconds the ball takes to travel to the bottom of the ramp. • Record the time in your table. 7. Second trial and third trial, Ramp Height 1: • Repeat step 6 two more times, and record the results in your table. • After the third trial, calculate the average travel time and record it in your table. (Hint: To calculate the average time add the times for the three trials together and then divide the total by 3.) 8. Repeat steps 5-7 for two more ramp heights: • Trials for Ramp Height 2: Using a stack of books that is approximately 30 cm high, repeat steps 5–7. • Trials for Ramp Height 3: Using a stack of books that is approximately 45 cm high, repeat steps 5-7. Record the data collected from Part 1 in Table 1. TABLE 1: Data collected Height 1 Height 2 Height 3 Mass of ball (kg) Length of ramp (m) Height of ramp (m) Time ball traveled, first trial (s) Time ball traveled, second trial (s) Time ball traveled, third trial (s) AVERAGE TIME (s) GROUP QUESTIONS: Discuss and answer together with your group before starting part 2. 1. Explain what happens to the collected data as the height of the ramp is increased? 2. From the data collected, describe how you could find the average velocity of the ball as it traveled down the ramp. PART 2: CALCULATIONS- BECAUSE THE MATH PROVIDES THE EVIDENCE! 1. Calculate the average speed of the ball for each change in ramp height. • Use the equation below to find the average speed. Show your work. • Record your answers in Table 2. average speed = length of ramp average time ball traveled Average speed for Ramp Height 1: Average speed for Ramp Height 2: Average speed for Ramp Height 3: Fun fact: the final velocity is twice the average velocity in the special case that the object starts from rest and accelerates at constant velocity (there physics equations that can prove/support this fact, so for right now just trust me on this!) 2. Multiply the average speed by 2 to obtain the final speed of the ball and record the final speed in Table 2. Record the analyzed data from Part 1 in Table 2. TABLE 2: Analysis of Data Height 1 Average speed (m/s) Final speed (m/s) Initial potential energy of ball (J) Final kinetic energy of ball (J) Energy difference (J) Height 2 Height 3 PART 2 GROUP QUESTIONS: Discuss and answer together with your group before starting part 3. 1. Describe what it means for an object to have potential energy (PE). 2. How is kinetic energy different from potential energy? (just consider the definitions of each) 3. In part 3, you will be calculating the potential energy of an object due to gravity. Try to describe in words or in a sketch/drawing where and why the ball has PE due to gravity (PEg). P3a. rt 3: Energy calculations Calculate and record the initial potential energy (PEg) of the ball for each change in ramp height. The acceleration of an object due to gravity (g) is approximately 9.8 m/s2. • Use the equation below to find the initial potential energy of the ball (due to gravity). • Record your answers in your data table 2. Reading the formula: The potential energy due to gravity is equal to mass times gravity times height. PEg =mgh PE =mass of ball (9.8 m/s2 ) height of ramp Show your work! IMPORTANT! mass must be in kg & height must be in m Initial potential energy for Ramp Height 1: Initial potential energy for Ramp Height 2: Initial potential energy for Ramp Height 3: Calculate the final kinetic energy (KE) of the ball for each change in ramp height. Use the equation below to find final kinetic energy (KE). Record your answers in Table 2. SHOW YOUR WORK! Final KE for ramp height 1: Final KE for ramp height 2: Final KE for ramp height 3: PART 3 INDEPENDENT QUESTIONS: 1. Compare the PE and the KE of the ball at each height. Describe what happened to the PE of the ball from the top to the bottom of the ramp. 2. Compare the PE and the KE of the ball at each height. Describe what happened to the KE of the ball from the top to the bottom of the ramp. 3. Hypothetically, if someone bet you $100 that you don’t know the PE and KE of the ball was in the middle of the ramp, could you prove them wrong? Take a chance and try to figure it out here- you can describe I words and/or draw a picture but you MUST give a value for KE and PE and tell me HOW you came up with that value (even if you get the wrong answer you will get points for trying!) PART 3 GROUP QUESTIONS: 1. Why did you need to calculate the average speed in Part 1? 2. A roller coaster has an initial PEg value of 1200 J and a final KE value of 850 J. What reasons can you think of that could explain the difference?
