http://www.youtube.com/watch?v=ivBs0DAL7WoTitle: Inertia and Newton’s First Law of Motion
9th Grade conceptual Physics
Principle(s) Investigated: This set of demonstrations are designed to help the students (9th grade) to understand the concept of inertia, based on which Newton’s First Law of Motion was founded. Students will observe several effects of inertia and describe how inertia works in different situations to maintain the original motion or motionless state. They will also form hypothesis on how inertia is related to the mass of an object based on their observations.
8.2c. Students know when the forces
on an object are balanced, the motion of the object does not change.
Boxes of raisins, one raisin box filled with iron nails, one transparent round pie pan, one marble, one metal weight (about 1 lb.), pieces of strings, one iron support with a ring, a ruler, one transparency, an erasable marker.
A. One boxes of raisins is stacking up on top of another box. The teacher will suddenly pull the bottom one to the right: the students are asked to predict on what the box on the top will do, and then record their observations as the teacher demonstrates.
B. One boxes of raisins is stacking up on top of another box. The teacher will gently push both to the right at a constant rate. The bottom box will hit on a bumper on the bench suddenly: the students are asked to predict on what the box on the top will do, and then record their observations as the teacher demonstrates.
C. A transparent round pie pan is cut to remove about a quarter of it. It is then laid onto the overhead projector so that the whole pan is clearly visible on the screen. A marble is rolled along the inner edge of the pan: the students are asked to predict on where the marble will go as it approaches the missing section, and then record their observations as the teacher demonstrates (http://www.youtube.com/watch?v=Qum9YS-U-OU).
D. Teacher introduces the concept of INERTIA (a tendency of an object to resist a change in motion) and helps the students to analyze the role forces have been playing in each situation, with the goal of deriving into Newton’s 1st Law of Motion.
E. The students are then asked to predict/observe the following demonstrations, and to analyze/explain their observations based on their understanding of inertia: A metal weight is hanging from a ring through a piece of string. Another piece of identical string is hooked to the weight from the bottom. The teacher will pull the string slowly (http://www.youtube.com/watch?v=bKjsHdBUauo) or rather quickly (http://www.youtube.com/watch?v=0B7bKncG_PE).
F. Teacher handouts two raisin boxes to students, and asks them do they feel the difference between the two boxes: the heavier one is filled with iron nails while the lighter one contains normal raisins. The two boxes are laid down on the overhead projector side-by-side along a pre-marked straight line on top of a transparency. Before the teacher pulls out the transparency with a swift motion, ask the students to predict which box will move more. Observe and answer the following questions: which one has greater inertia, the lighter one or the heavier one? What can we say about inertia and mass of an object?
Inertia is a tendency of an object to resist a change in motion. Unless a net force (unbalanced/uncanceled force) is exerted on an object, it will tend to remain at its original motion or motionless state, just like the raisin box on the top and the marble will do in demonstrations A, B, and C. In demonstration D, when the bottom string is pulled slowly, the force of the pull AND the weight of the metal are added to the top string, therefore the top string will break first. When the bottom string is jerked, the inertia of the weight prevents the jerking force to be passed on to the top string, therefore the bottom string break first. In demonstration F, the heavier/more massive raisin box moves less, indicating a larger inertia.
Applications to Everyday Life:
1. Headrest is very important in preventing head/neck injury when a car is struck by another vehicle from behind, otherwise our heads tend to jerky backwards because of inertia. For similar reason, fastening seatbelt is vital during head-on collisions to prevent our body from flying out from the front window due to inertia.
2. A loose head of a hammer may be secured by grasping the handle of the hammer and repeatedly strike the butt on a solid surface, because the inertia of the head keeps it moving downward even after the handle has stopped moving.
Part of this demonstration is modified from an activity suggested in “Hands-on Physics Activities with Real-life Applications” by J. Cunningham and N. Herr