# Investigating gravity: Predicting time to hit the ground, and finding that dropped and horizontally shot objects hit the ground at the same time.

#### Summary

In this physics lab, students drop and shoot horizontally "Nerf" balls from a variety of known heights. Students use lab notebooks to record the amount of time it takes for the ball to hit the ground for each trial. Students plot height vs. time data in Excel, and they create a trend line for the dropped and shot balls. Students will use the trendline to predict as accurately as possible (with error bars) how long it will take a ball to hit the ground from a certain distance. Students will also discover that launched and dropped balls take the same amount of time to hit the ground when started from the same height.

## Learning Goals

This activity is designed for students to: 1. Use experimental data to create a graph that models a height dropped vs. time to hit ground system. 2. Use a line of best fit to make time predictions about given heights. 3. Discover that gravity is a constant force acting on all objects on earth at all times. 4. Find that, ignoring air resistance, the only force acting on a flying object is the constant pull of gravity.

## Context for Use

For use with a high school physics course. Goes along well with units on gravity or ballistics.

Time needed for activity and data interpretation: about 2 hours.

Special Equipment needed: "Nerf-type guns" that shoot out foam balls, measuring tape, timers, gym bleachers.

Previously-learned Skills necessary: none (except a very basic understanding of Excel); this can be used to introduce a gravity unit or it can be used at any point in the unit.

This activity can be easily adapted to other settings—the main requirement is accessible high points from which to drop the balls from.

**Subject**: Physics:Classical Mechanics:Gravity

**Resource Type**: Activities:Field Activity, Lab Activity

**Special Interest**: Field-Based Teaching and Learning

**Grade Level**: High School (9-12)

## Description and Teaching Materials

This activity is introduced with a discussion about gravity. Specific points to discuss include that gravity is a constant force that is ALWAYS acting on objects on earth. Also, discuss with the class what will hit the ground first, a bullet that is dropped, or a bullet that is shot horizontally from a gun. This activity is best done in a gym with bleachers. Students are brought to the gym and broken down into groups of 3-4. Each group is given a "Nerf gun" with a foam ball, 2-3 timers, and a yard stick. The group starts by measuring the height from the ground of 5 different bleacher seats. The group then collects height and time data in their lab notebook from four trials at each seat. Two of the trials involve dropping the ball and seeing how long it takes to hit the ground (average the results). The other two trials involve shooting the ball horizontally from the gun and seeing how long it takes to hit the ground (average the results). The same group member must drop or shoot all the balls at a given step, and they must drop or shoot the balls from the same height on their person each time (like from their waist-level). Groups must remember to add the height that the person holds the ball above the step to the total height for that trial.

At the end of the data-collecting phase, the group will have 5 height vs. time data points for dropped balls, and 5 height vs. time data points for shot balls.

Groups will be allowed time in a computer lab for data analysis. Groups should create two graphs, one a height vs. time graph for dropped balls, and one a height vs. time graph for shot balls. Each group should then create a line of best fit with an equation for each of the graphs. Each group is given a height, and they must use their equations to determine as accurately as possible, with error bars, how long it will take a dropped ball and a shot ball to hit the ground from the given height.

This lesson is concluded the next day of class, when the teacher drops and shoots a ball from the given height. Students will be able to see how close their predictions were. A discussion can follow about the lab and the forces involved with the dropped balls and the shot balls.

## Teaching Notes and Tips

This will be my first year teaching physics, so I have not done this activity before. The more teaching experience I get, the more inquiry activities I want to incorporate into my classes. I would anticipate that students may want to goof around with the "Nerf guns"; therefore, clear expectations must be laid out ahead of time regarding the use of the guns. I would also anticipate that students might have trouble creating trendlines and equations in Excel. The teacher could guide the students through an example Excel spreadsheet beforehand that would give the students some foundational Excel knowledge before they have to use Excel themselves.

## Assessment

Throughout the whole process the teacher should monitor students and ask students formal and informal questions in order to make sure that the learning objectives are being met. The teacher will collect and grade lab notebooks, graphs, and predicted times from the students. Questions regarding the force of gravity acting on projectiles can be asked on additional assignments, quizzes, and/or tests in order for the teacher to determine if students have internalized the material.

## Standards

Physical Science D.2, The effect of gravity and friction on the motion of an object. Physical Science E.2, forces present in an interaction.

## References and Resources