Inertia

Bowen, Sheila J.                     Gwendolyn Brooks Junior High
                                     333-6390
                           

Objective(s):
 
 Students will be able to explain inertia.
 Students will be able to identify the factors that determine inertia.
 Students will be able to compare the acceleration of an object 
 with a net force to an object without a net force.
 Students will be able to state Newton's first law of motion.

Apparatus Needed:

 Card table, tablecloth, two china plates, heavy plastic plate, light 
 plastic plate, styrofoam or paper plate, plastic drinking glass, index 
 card, two pennies, five nickels, paper disc, small plastic disc, 
 three-wheeled cart or rollerskate, dot/tape time, two identical 
 shaped objects of different weights, 500 gram weight, distance/time 
 posterboard graph, average/speed posterboard graph, paste or tape.

Recommended Strategy:

 Students should have prior knowledge of speed and acceleration 
 measurements and graphing.
 Demonstrate inertia by snatching a tablecloth from under two china 
 plates.
 Student teams are to complete and write up the following activities.  
 Each write up should include a prediction and an explanation of what 
 happened and why.

 Activity 1 - Place a heavy plastic plate in the center of the table on 
 a tablecloth.  Then snatch the tablecloth from under the plate.  
 Repeat this activity using a light plastic plate and then a styrofoam 
 plate. 
 
 Activity 2 - Place an index card flat on top of a plastic drinking 
 glass.  Place a penny on the center of the card.  Hold the base of 
 the glass with one hand.  Then using the forefinger of the other 
 hand, flick the card forward so it moves out from under the 
 penny.  Repeat this activity using a plastic disc and then a paper 
 disc.

 Activity 3 - Stack 4 nickels one on top of the other.  Using your 
 forefinger, flick another nickel sharply against the bottom coin of 
 the stack.  Repeat this activity using the penny and then the plastic 
 disc.

 After the activities, conduct a classroom discussion of factors of 
 inertia observed in each activity.  Next have each student handle two 
 objects, identical in shape and appearance but different in weight, 
 to distinguish which has the greatest inertia.

 Set up demonstration using a three-wheel cart or rollerskate with an 
 accelerometer on top.  Attach the end of two tapes from the dot/tape 
 timer to the cart.  Start the timer and give the cart an initial 
 push.  Impress on students to observe the accelerometer.  Have two 
 students, one for each tape, mark and cut the tape into a series of 
 four dot strips.  Take the first strip of one of the students 
 and paste or tape it in the lower left corner vertically on the 
 distance/time posterboard graph.  Then take the second strip and 
 paste it with its lower left corner next to the upper right corner of 
 the first strip.  Continue pasting each successive strip until you 
 get to the top of the graph.  Draw a line connecting the top of the 
 strips.  With the second set of tape strips, lay them one at a time 
 next to each other on the horizontal axis of the average speed/time 
 posterboard graph.  Start with the first strip to the end of the 
 graph.  Draw a line across the top of the strips.  Next tie a cord to 
 the cart and place the other end through a pulley at table level and 
 then up to a pulley on a ringstand.  Attach the 500 gram weight to 
 the end of the cord.  Once again attach two tapes from the dot/tape 
 timer to the cart.  Repeat the demonstration releasing the weight.  
 Then tape the strips in the same way to each of the posterboard 
 graphs.
 The class should then compare and explain both graphs.  Then discuss 
 inertia, its factors and Newton's first law.