Please register to participate in our discussions with 2 million other members - it's free and quick! Some forums can only be seen by registered members. After you create your account, you'll be able to customize options and access all our 15,000 new posts/day with fewer ads.
It is late so bear with me: maybe I am misreading something, but I disagree that a bowling ball and a golf ball will take the same time interval to traverse a given distance down an incline or have the same final speed for that matter. The moment of inertia of the much larger radius object will come into play. If you write out a conservation of energy equation including rotational kinetic energy, you will see this to be the case.
Even if the radii are the same for two balls, say a racquet ball (hollow)and a billiard ball (solid), the distribution of mass (moment of inertia) still comes into play.
There is a difference in this experiment compared to a freely falling object in that torques come into play.
Make sense?
That makes sense. I actually have a lab where the kids roll objects with the same mass (spheres, cylinders and hollow cylinders) down an inclined plane to show just this effect. Free fall will be the same. However, I'm thinking that if the ramp angle is high enough and the run short enough, and you're not using photogates and millisecond timing, you might not see this difference.
I did a lot with inclined planes at the charter school because I had no equipment. We were able to get reasonable results even though rotational torque came into play when rolling an object vs. dropping it. (I had the students calculate the acceleration due to gravity using inclined planes. Not perfect but better than nothing when you have no equipment).
I often do a paper chromatography demo with high school students visiting my lab. We take strips of filter paper and fold them into cylinders and place droplets of food dye 3 cm from the bottom. Then we put the cylinder into 1 cm of salt water to elute. The different dyes (red, yellow, blue, etc.) elute at constant rates and the nature of the dye mixtures can be determined. This lab can be used for teaching about both spectroscopy (colors) and chromatography (separations).
Re different radii Yes they do but you are still wrong what you are not taking into account is that it is student work with a LOT of sloppy timing. And a short rolling/run time/distance. (I can't tell you how many times the students forget to time the rolling of the balls because they are talking to someone). But that is the whole point since after wards you can ask them questions (ususally as homework) re "why do you think" ....(which has a longer circumference, a bowling ball or a golf ball. If a bowling ball has a longer circumference .... etc.) They then get to "examine" their questions through an experiement of their own design (I usually have them do this at the end of the trimester for make up points). The best example are two students in my physics class that examined that very question using (instead of a 1 meter roll/run) a ten meter roll/run (luckly at that time the school had just built an access ramp with a small drain in the middle -which removed their having to build the ramp)
so yes you are right the balls should not roll down at the same time. (however the calculations lead to learning about motion, velocity and acceleration) what they do realize is an "ahah" moment which is slightly biased (incorrect) however it still lends itself to learning about falling bodies. It specially leads into investigations re the scientific method and validity of results etc.
ssorry it is time for me to drive off into the sunrise. If you want to discuss this further please remember that I won't be able to respond for 2 weeks.
Thanks and I'm sorry if I obfuscated the issue.
No offense, but I am a little disturbed by this post. Sorry, but I take my subject quite seriously and I am just hoping that you know that you can have two object with the same mass and same radius, roll down the same incline for a given distance and they may not arrive at the bottom at the same time, nor with the same speed and not due to some trivial issue with timing errors like the fact that human reaction time is on the order of tenths of a second and that will interfere with pushing the button on the stopwatch. (Though one can argue that a delay of both starting and stopping the stopwatch will cause an overall error less than "tenths" of a second but anyway, this is why you would need to run several, i.e. say six, trials and run simple stats on the data.)
I am sorry for you if you have to deal with students not even running the labs due to talking and having to teach from that point... but anywho... if you are teaching this material, I surely hope that moment of inertia is a concept that is familiar to you. If the DISTRIBUTION of mass is different for the two objects that I hypothesized above, the moment of inertia is different and... <insert argument about energy conservation here>... they will have different kinematic outcomes.
I do like your follow up with asking questions about what might cause the differences and allowing students to explore more experiments to test out their ideas. That is the nature of guided inquiry!
I'm on a break from the conf. so please excuse the brevety. You're quite right that there are many reasons why the balls should not roll down the ramp at the same speed or fall together but again I must remind you that I teach Physical Scthat is an 8th or 9th grade subject. When the students start in my class don't have knowledge of concepts like "distribution of mass", inertia, moment of inertia etc. this is a begining activity (done at the begining of the trimester) the whole idea is to get them used working in groups, measuring distances, timing, graphing, calculating speed (hence velocity -yes I do know that they are different but since the balls all roll in one direction for this activity they are the same), statistical analysis and acceleration.
To start with the balls are not timed from rest but when they cross the 0 mark on the boards. the timers are stopped when they cross the designated point (.25 m, .5 m, .75 m or 1 m). Second re the distribution of mass -yes you are right bowling balls, croquet balls and golf balls are mass produced and hence not perfect (I beleive that Jack Nicaulus (sp?) once gave an interview where he stated a preference for a specific ball and his not using untried balls due to the tendency of balls to wobble -my reference is my physics prof (1970's) who was a golfer and lecturing on the effects of the distribution of mass. those who are into golf can probably give a more accurate reference,) but they are good enough for the activity.
As you point out even the best, most fastidious student will make mistakes in hand timing due to human error and as you point the students (in my lab repeat each measurement 10x -average them then calculate the class average so there is some attempt to reduce error.) need to reduce error through repetition and averaging etc.
but again remember that these are 8th or 9th grade students doing activities for the first time. I teach, we discuss all the possibilities/sources of error and the students then can see when a fellow student group has a number like 0.75 when every one else has a range of 0.065 to 0.087 they can calculate what could be the error and decide as a class as to whether they should include the data. in the class report.
A source of deliverate error is in my doing the run these timings over a one meter course in a 5 ft board. That one meter is simply not enough time to reliably give the necessary seperation/distance. then the students go "Oh yeah we proved that all things fall at the same rate..." then is when the real questions are asked that they have to report on. As part of the original measuring they measured the diameters of the different balls. So if a croquet ball is x smaller than a bowling ball and y larger than a golf ball why do they .... etc.
This is a lot longer than I had planned but I will be available on Sunday. I do appreciate the comments and discussion but - Just out of curiosity wouldn't it be easier if you saw the lesson plan./ handout I give to the students then we can disscuss the "errors of my way?" I can sent it next in approximately one week if you would like.
I'm on a break from the conf. so please excuse the brevety. You're quite right that there are many reasons why the balls should not roll down the ramp at the same speed or fall together but again I must remind you that I teach Physical Scthat is an 8th or 9th grade subject. When the students start in my class don't have knowledge of concepts like "distribution of mass", inertia, moment of inertia etc. this is a begining activity (done at the begining of the trimester) the whole idea is to get them used working in groups, measuring distances, timing, graphing, calculating speed (hence velocity -yes I do know that they are different but since the balls all roll in one direction for this activity they are the same), statistical analysis and acceleration.
To start with the balls are not timed from rest but when they cross the 0 mark on the boards. the timers are stopped when they cross the designated point (.25 m, .5 m, .75 m or 1 m). Second re the distribution of mass -yes you are right bowling balls, croquet balls and golf balls are mass produced and hence not perfect (I beleive that Jack Nicaulus (sp?) once gave an interview where he stated a preference for a specific ball and his not using untried balls due to the tendency of balls to wobble -my reference is my physics prof (1970's) who was a golfer and lecturing on the effects of the distribution of mass. those who are into golf can probably give a more accurate reference,) but they are good enough for the activity.
As you point out even the best, most fastidious student will make mistakes in hand timing due to human error and as you point the students (in my lab repeat each measurement 10x -average them then calculate the class average so there is some attempt to reduce error.) need to reduce error through repetition and averaging etc.
but again remember that these are 8th or 9th grade students doing activities for the first time. I teach, we discuss all the possibilities/sources of error and the students then can see when a fellow student group has a number like 0.75 when every one else has a range of 0.065 to 0.087 they can calculate what could be the error and decide as a class as to whether they should include the data. in the class report.
A source of deliverate error is in my doing the run these timings over a one meter course in a 5 ft board. That one meter is simply not enough time to reliably give the necessary seperation/distance. then the students go "Oh yeah we proved that all things fall at the same rate..." then is when the real questions are asked that they have to report on. As part of the original measuring they measured the diameters of the different balls. So if a croquet ball is x smaller than a bowling ball and y larger than a golf ball why do they .... etc.
This is a lot longer than I had planned but I will be available on Sunday. I do appreciate the comments and discussion but - Just out of curiosity wouldn't it be easier if you saw the lesson plan./ handout I give to the students then we can disscuss the "errors of my way?" I can sent it next in approximately one week if you would like.
I would love to see a copy of the lab you do. We do physics second semester so we'll have plenty of time to discuss the error of your ways....
If you have a weight room at your school, do this. Take your students to the venue, and get the strongest player on the football team to meet you at that location. Get him to load a straight bar with heavy weights when it's on the floor. Ask him to lift it on to a weight rack which has steel extensions. The entire weight set will begin to roll forward. This is an example of conservation of energy. When the weight set is on the floor, it's potential energy. When the kid introduces outside energy, his own, and lifts the weights up and onto the rack, the rolling of the weights becomes kinetic energy, ala, the basis of all physics, a change of energy is what physics is all about, and you can see it in your everyday lives, change in energy doesn't mean that energy is lost, it's simply changed from potential to kinetic. Pay the football player appropriately...
Please forgive my tardiness in reply. I had to leave the conference because of a family emergency - my daughter tried to commit suicide. that has occupied me since. I am now back and she is with her mother for two weeks so that I will be able to email you the procedure next week -monday. Pleazse send me a message to me personally at my email address -ecaridad@yahoo.com I will attach the procedure.
Ivory, have you seen ESPN's "Sports Science" videos? I actually just flipped past one about the olympics (gymnastics and archery so far) and how they use energy conversion and forces and such. It's not an activity, but I think if I had watched those in high school it would have really piqued my interest in the subject matter. The individual segments are really short, so you could just select 5 minute pieces to introduce or engage students in a certain subject.
Please register to post and access all features of our very popular forum. It is free and quick. Over $68,000 in prizes has already been given out to active posters on our forum. Additional giveaways are planned.
Detailed information about all U.S. cities, counties, and zip codes on our site: City-data.com.
Please register to participate in our discussions with 2 million other members - it's free and quick! Some forums can only be seen by registered members. After you create your account, you'll be able to customize options and access all our 15,000 new posts/day with fewer ads.
