Projectile Motion Virtual Lab: Video Tracking to Measure g

This projectile motion virtual lab is a video-tracking practical, like the chronophotography activity of the French lycée course or a free-fall measurement of g. The simulation records a launched ball as a strobe photograph, one image every Δt, with a scale bar. Students tap each image to build x and y against t, find vₓ from the x–t graph and g from the y–t and vy–t graphs, test Newton's second law with Δv arrows, then measure g on a mystery planet. Every value below was read from the simulation.

Curriculum links

• French lycée physique-chimie: Seconde, Mouvement et interactions (chronophotography, velocity vectors, Δv and forces); Première spécialité (mechanical energy); Terminale spécialité (motion in a uniform field, Newton's second law).
• AQA A-level Physics, required practical 3: determination of g by a free-fall method (see Extension).
• AP Physics 1, Unit 1 Kinematics: projectile motion.
• NGSS HS-PS2-1: analyze data to support Newton's second law.

Simulic is not affiliated with or endorsed by the French Ministry of Education, AQA or the College Board.

Before the lab (5 min)

Ask students to commit to a prediction, on paper or as question 1 of the class link:

"A ball is launched at 45° and photographed every 0.10 s. How are the images spaced?"

Many students expect equal spacing along the curve.

Method in the simulation

1. Click the Tracking tab. Keep Launch height 0.0 m, Launch angle 45°, Initial speed 15 m/s, Motion: Launch, Place: Earth (9.81), Strobe interval Δt 0.10 s and Data set no. 1.
2. Press Fire and wait until all 22 images are on the screen.
3. Tap the centre of every image: the point you tap is your measurement. A laptop or a tablet in landscape is more precise than a phone. Short of time? Auto-mark marks every image with a small reading error.
4. Copy t, x, y and vy for images 0, 5, 10, 15 and 20 into the table, or press CSV data for the full table.
5. Set Graph to x against t, then y against t, then vy against t. Record vₓ and both values of g from the results table.
6. Tick Δv. Note the direction of the purple arrows and the row a = Δv/(2Δt).
7. Set Place to Mystery planet (keep data set 1), press Fire, mark the images again and find g from the vy–t gradient. Press Reveal g only at the end.

Image	t (s)	x (m)	y (m)	vy (m/s)
0	0.00
5	0.50
10	1.00
15	1.50
20	2.00

Expected results

With Auto-mark on data set 1:

Image	0	5	10	15	20
x (m)	−0.01	5.29	10.65	15.93	21.22
y (m)	−0.02	4.06	5.67	4.88	1.66
vy (m/s)	—	5.80	0.67	−4.22	−9.36

• Horizontally, the images are about 1.06 m apart: vₓ = 10.60 m/s (accepted 10.61).
• Vertically, the gaps shrink by about g·Δt² = 0.098 m per image on the way up (1.04, 0.91, 0.78 m…) and grow on the way down.
• g: 9.77 m/s² from the y–t parabola and 9.78 m/s² from the vy–t gradient (−0.4%).
• Δv points straight down on every image: a = (0.02 ; −9.82) m/s², and m·|a| = 4.91 N equals the weight mg.
• By hand: with taps scattered by about 3 pixels, g came within about 2% on a laptop and 6% on a phone.
• Mystery planet, data set 1: g = 14.97 m/s² (y–t) and 15.01 m/s² (vy–t). Reveal g shows 15.00.

Questions for students

1. (Prediction, asked again after the lab) How are the images spaced?
2. To find g from y against t, which are the independent and dependent variables?
3. What is the horizontal velocity vₓ from the x–t graph?
4. What is g on the mystery planet (data set 1), from the vy–t gradient?
5. What do the Δv arrows tell you about the forces on the ball?

Answers for teachers: (1) Equal horizontal gaps; vertical gaps shrink going up and grow coming down. (2) Independent: time t; dependent: height y. (3) Accept 10.3–10.9 m/s (10.60 with Auto-mark). (4) Accept 14.0–16.0 m/s² (15.01 with Auto-mark; true 15.00). (5) Every Δv points straight down with the same size: a constant, downward acceleration g. The only force is the weight, so vₓ stays constant.

Common misconceptions

• "The ball slows down horizontally." The images stay about 1.06 m apart horizontally.
• "At the top the acceleration is zero." At image 11 vy is only 0.25 m/s, but Δv still points down with the same length.

Extension

• Free-fall method of g: set Motion: Drop from rest (the launch height jumps to 10 m). Auto-mark gives 15 images and g = 9.80 m/s² (y–t) and 9.81 m/s² (vy–t).

FAQ

Can the class link open on the Tracking screen?

Yes. In the link's starting values, set Screen to Tracking. Keep Place (Tracking screen) on Earth, Strobe interval Δt at 0.1 and Data set number (auto-mark error, mystery planet) at 1, so the answers match.

What if groups use different data set numbers?

Each data set number gives a different mystery planet, with g between 3 and 25 m/s² (data set 2: 21.8; data set 3: 8.8). Questions 1, 2, 3 and 5 still work, but the range for question 4 only fits data set 1.

Should students tap the images or use Auto-mark?

Tapping is the real skill; the ranges allow careful tapping on a phone. Auto-mark suits a short lesson: it lands within about 1% of the accepted values.

Related simulations and guides

Measuring g with a simple pendulum Free fall and fall with air resistance

For range, angle and air resistance, see the projectile motion lesson plan. For another way to measure g, see the simple pendulum lesson plan.