Physics · 9th Grade · Kinematics and Linear Motion · Weeks 1-9

Free Fall and Gravity

Investigating the motion of objects acting solely under the influence of Earth's gravity.

• Common Core State Standards
• HS-PS2-1
• HS-ESS1-4

About This Topic

Free fall describes the motion of any object accelerating solely under the influence of gravity, with no other forces acting on it. On Earth, this acceleration is 9.8 m/s² downward, regardless of the object's mass. This topic connects directly to HS-PS2-1 and HS-ESS1-4, as students examine how gravitational acceleration links terrestrial motion to the behavior of objects in space. US physics curricula typically use this topic to bridge the kinematic equations from Unit 1 with the concept of gravitational force introduced in dynamics.

Galileo's insight that mass does not affect free fall remains counterintuitive for most students because they regularly observe a feather and a bowling ball falling at different rates in the atmosphere. Air resistance is the variable students must learn to separate from pure gravitational acceleration. Discussing vacuum demonstrations and NASA footage of a hammer and feather falling identically on the Moon gives students the visual evidence they need to accept the underlying physics.

Active learning works particularly well here because the concept is simple in a vacuum but nuanced in practice. When students predict fall times, test those predictions with actual drops, and then examine slow-motion video together, they build a lasting mental model of constant gravitational acceleration that far outlasts a textbook reading.

Key Questions

1. Why do all objects fall with the same acceleration in a vacuum regardless of mass?
2. How does air resistance affect the terminal velocity of a skydiver?
3. How can we calculate the height of a bridge by timing a falling stone?

Learning Objectives

• Calculate the final velocity of an object dropped from a specific height using kinematic equations.
• Compare the acceleration of objects in free fall with and without air resistance, explaining the difference.
• Analyze video footage of celestial bodies to identify instances of free fall and estimate gravitational acceleration.
• Design and conduct an experiment to measure the acceleration due to gravity, accounting for potential sources of error.

Before You Start

Introduction to Kinematics

Why: Students need to be familiar with basic kinematic equations (e.g., d = v₀t + ½at², v = v₀ + at) to solve for motion variables in free fall.

Vectors and Scalars

Why: Understanding the difference between vector quantities (like velocity and acceleration, which have direction) and scalar quantities is crucial for correctly applying physics principles to motion.

Key Vocabulary

Free Fall	The motion of an object where gravity is the only force acting upon it. Other forces like air resistance are considered negligible.
Acceleration due to Gravity (g)	The constant rate at which the velocity of an object in free fall increases. On Earth, this is approximately 9.8 m/s² downwards.
Air Resistance	A type of friction, or drag, that opposes the motion of an object through the air. It depends on the object's shape, size, and speed.
Terminal Velocity	The constant speed that a freely falling object eventually reaches when the resistance of the medium through which it is falling prevents further acceleration.

Watch Out for These Misconceptions

Common Misconception

Heavier objects fall faster than lighter ones.

What to Teach Instead

In a vacuum, all objects accelerate at exactly 9.8 m/s² regardless of mass. The difference students observe outdoors is entirely due to air resistance. Side-by-side slow-motion video of a coin and a flat sheet of paper, then the coin and a crumpled ball of paper, makes this distinction visible in a single class period.

Common Misconception

At the peak of its flight, a thrown ball briefly has zero acceleration.

What to Teach Instead

Gravity acts at a constant 9.8 m/s² downward at every point in the flight, including the instant the ball's velocity is zero at the top. The acceleration never pauses. Having students sketch velocity-time graphs for a full throw-and-catch cycle in groups, checking that the slope stays constant throughout, is the most direct way to address this.

Active Learning Ideas

Inquiry Circle: Reaction Time Drop

One student holds a ruler vertically while a partner positions their fingers just below the zero mark. The holder drops the ruler without warning, and the catcher records the catch position. Groups use the free-fall equation to calculate reaction time and compare results across the class.

25 min·Pairs

Think-Pair-Share: Air Resistance and Terminal Velocity

Pairs compare a single coffee filter and a stack of four dropped from the same height, observing which reaches terminal velocity sooner. They must use net force reasoning to explain why greater weight at the same cross-sectional area produces a higher terminal velocity.

20 min·Pairs

Stations Rotation: Free Fall Calculations

Three stations present different scenarios: a cliff drop, a ball thrown vertically upward, and a skydiver approaching terminal velocity. Groups rotate through each station applying kinematic equations and then check the next group's work when they rotate in.

40 min·Small Groups

Real-World Connections

• Engineers designing parachutes for spacecraft reentry must precisely calculate terminal velocity to ensure safe landings. They account for atmospheric density and parachute deployment to control descent speed.
• Athletes in sports like pole vaulting or high jump experience free fall after leaving the ground. Understanding the physics of their trajectory and acceleration helps coaches optimize performance.
• Astronauts on the Moon demonstrated free fall by dropping a hammer and a feather simultaneously. This experiment, conducted in a vacuum, visually confirmed that gravity accelerates all objects equally, regardless of mass.

Assessment Ideas

Quick Check

Present students with three scenarios: a dropped ball, a skydiver, and a satellite in orbit. Ask them to identify which scenarios primarily involve free fall and to briefly explain why, considering the forces acting on each.

Exit Ticket

Provide students with the height of a building (e.g., 50 meters). Ask them to calculate the time it would take for a stone dropped from the top to hit the ground, assuming no air resistance. They should show their formula and calculations.

Discussion Prompt

Pose the question: 'If a bowling ball and a ping pong ball are dropped from the same height in a room with air, which hits the ground first and why?' Facilitate a discussion that leads students to differentiate between gravitational acceleration and the effect of air resistance.

Frequently Asked Questions

What is the value of g on Earth and does it change?

The standard value is 9.8 m/s², though it varies slightly by location, ranging from about 9.78 m/s² at the equator to 9.83 m/s² at the poles, due to Earth's shape and rotation. For all 9th grade calculations, 9.8 m/s² is used as the accepted constant. On the Moon it is 1.6 m/s², and on Mars approximately 3.7 m/s².

How can you calculate the height of a bridge by timing a falling stone?

Drop the stone from the top and measure the time until you hear it hit the water below using d = ½gt². For example, a 3-second fall gives d = ½(9.8)(3²) = 44.1 meters. Reaction time adds uncertainty, so multiple trials and averaging improve accuracy.

Why does a skydiver reach a terminal velocity instead of accelerating indefinitely?

As the skydiver's speed increases, the upward force of air resistance also increases proportionally. Terminal velocity is reached when air resistance exactly equals the downward gravitational pull. At that point the net force is zero, acceleration stops, and the skydiver falls at a steady speed until deploying a parachute changes the balance.

How does active learning help students understand free fall?

When students predict a fall time, run the experiment, and compare their calculation to the measured result, the agreement is far more memorable than a worked example on a board. Collaborative frame-by-frame video analysis, where students mark ball positions at equal time intervals and measure the increasing gaps, makes constant acceleration visible rather than assumed.