Direction of Acceleration

Mary Spaulding, Clearbrook-Gonvick Public School, Clearbrook Minnesota, based on an original activity from Physics Principles and Problems (1995) page 75.

Summary

In this short lab, students will observe the acceleration of a object as it initiates acceleration, maintains a steady speed, and slows to a stop. In this lab, a Tonka truck is used. Students will observe that positive acceleration occurs when an object starts to move. When an object is moving at a steady speed, no acceleration occurs and when it slows to a stop, negative acceleration is being demonstrated.
The student will diagram the motion of an accelerometer.
A small bubble level (level) is attached to the truck. The truck is pulled at a steady speed. Students observe the location of the bubble when the truck is started
(positive acceleration), moving at a steady speed (zero acceleration), and when it is stopped (negative acceleration).


Learning Goals

This activity is designed to visually represent the direction of acceleration. Students will learn that constant speed in a straight line has no acceleration, that as a moving object speeds up, acceleration is graphically in a positive direction, and that as a moving object slows down, the acceleration is graphically in a negative direction.
Other skills developed include observation, predicting, diagramming, graphing and writing.

Context for Use

Used in grade 11-12 physics class.
Time needed: 20-30 minutes.
Students need to work in lab groups of 2-4.
Follows discussion of the definition of acceleration. Students should be familiar with the terms motion, velocity, and acceleration.

Description and Teaching Materials

Materials:
-Small bubble level
-Medium size toy truck
-Heavy string or light rope
-Tape

Directions:
With the tape, attach the level to the truck with the bubble centered. Tie the string to the truck so it will pull the truck in a straight line. Pull the truck forward at a steady speed. Students should observe and take note of the location of the bubble when (1) the truck begins to move, (2) the truck is moving at a steady speed and,
(3) when the truck slows down. Have the students sketch what happens to the bubble in each instance and have them indicate with arrows the direction of acceleration. Ask them to predict what would happen if truck was pulled from the opposite end, in the opposite direction.

Outcome: The bubble will move forward as the truck starts to move forward. This indicates that the truck
accelerates in a forward direction. When the bubble is centered, the truck is not accelerating. (Objects moving in a straight line at a steady speed have no acceleration.) As the truck coast to a stop the bubble moves toward the back of the truck. The acceleration is in the opposite direction of the motion. As soon as the truck starts to move in the backwards direction, the bubble will move toward the back of the truck. This indicates that the truck is accelerating in the same direction as it starts to move. Again, when the bubble is centered, the truck is moving at a steady speed and acceleration is therefore zero. As the truck starts to slow down, the bubble moves toward the front. The decelerating truck is now accelerating in the opposite direction.

Teaching Notes and Tips

Any object that can be pulled may be used, but objects with wheels make for a smoother visual of this concept. A 'homemade' accelerometer can be used in place of the bubble level. The concept of zero acceleration at a constant speed is easily visualized in this lab.

Assessment

After completing this lab, discussion of their observations will reinforce what they have observed. A written lab report including sketches and graphs of the 3 different accelerations with explanations will be submitted.

Standards

9-12.II.D.1. laws of motion

References and Resources