Top examples of 3 practical examples of measuring velocity using photogates

Core examples of 3 practical examples of measuring velocity using photogates

When teachers ask for the best examples of 3 practical examples of measuring velocity using photogates, three classics always show up: a dynamics cart on a track, a free-fall experiment, and an Atwood machine. These aren’t just tradition; they’re reliable, repeatable, and easy to align with AP Physics, IB, and introductory college syllabi.

Let’s walk through these three in detail, then expand to more real examples that make your photogates earn their keep.

Example 1: Cart on a track – instantaneous and average velocity

A low-friction cart on an aluminum track is probably the most familiar example of measuring velocity with a photogate. It’s also one of the cleanest ways to introduce the difference between instantaneous and average velocity.

Basic setup

You place a photogate at some point along the track, connect it to an interface (Pasco, Vernier, or similar), and attach a picket fence or a single flag to the cart. The flag has a known width, say 5.0 cm (0.050 m). As the cart rolls through the photogate, the beam is blocked for a time interval Δt measured by the interface.

The software then calculates the velocity:

\[ v = \frac{\text{width of flag}}{\Delta t} \]

That’s an instantaneous velocity at the position of the gate.

Turning one gate into a full kinematics lab

With just one photogate, students can:

• Release the cart from different starting positions up a ramp.
• Measure the instantaneous velocity at the same gate position each time.
• Plot \( v^2 \) versus distance down the ramp to test the kinematic relation \( v^2 = v_0^2 + 2 a \Delta x \).

This is a textbook example of 3 practical examples of measuring velocity using photogates because it ties directly into energy conservation and constant-acceleration motion. Many high school and college lab manuals, including those from MIT OpenCourseWare and Harvard’s introductory physics labs (access varies), use some version of this setup.

Upgrading to two-gate average velocity

To push this example farther, add a second photogate a known distance \( L \) downstream. Now you can:

• Use the time between gates to find average velocity: \( v_\text{avg} = L / \Delta t_{12} \).
• Compare that to the instantaneous velocities measured at each gate.
• Estimate acceleration: \( a \approx (v_2 - v_1) / \Delta t_{12} \).

Here, the cart-on-track experiment becomes one of the best examples of how photogates can bridge the gap between “plug-and-chug” kinematics and actual data analysis.

Example 2: Free-fall measurement of g using a photogate

The second of our examples of 3 practical examples of measuring velocity using photogates focuses on gravity. Instead of dropping a ball and trying to time it by hand (which is painful and inaccurate), you drop a picket fence—a clear plastic strip with equally spaced black bars—through a single photogate.

How it works

As the picket fence falls, each black bar blocks the beam for a short time. The interface logs a sequence of times when the light is blocked and unblocked. From that, the software extracts:

• Instantaneous velocities at each bar.
• The acceleration due to gravity, \( g \), from the slope of a velocity–time graph.

Students can then:

• Compare their measured \( g \) to the accepted value of 9.8 m/s² reported by standards bodies like NIST.
• Discuss air resistance and why the values might differ slightly.

This free-fall setup is a perfect example of 3 practical examples of measuring velocity using photogates because it showcases how a single gate, plus clever design of the falling object, can generate rich data. In 2024–2025, you’ll still see this exact experiment in many AP Physics 1 labs, even in schools that have moved heavily into video tracking.

Modern twist: comparing photogates to video analysis

A nice 2024-style extension is to record the drop with a smartphone at 240 fps and analyze the motion using free software like Tracker. Students can then compare:

• Velocity from photogate timing.
• Velocity from frame-by-frame video.

The comparison turns into a conversation about resolution, uncertainty, and when you’d choose one method over the other.

Example 3: Atwood machine – constant acceleration with two masses

The third of the classic examples of 3 practical examples of measuring velocity using photogates is the Atwood machine: two masses connected by a string over a pulley.

Photogate placement

You mount a photogate so that one of the masses passes through it as the system accelerates. Attach a flag of known width to that mass. When you release the system:

• The mass accelerates downward through the gate.
• The photogate measures the time the flag blocks the beam.
• You calculate the instantaneous velocity using \( v = w / \Delta t \), where \( w \) is the flag width.

By measuring the time it takes the system to move a known distance and the velocity at a point, students can:

• Determine the acceleration experimentally.
• Compare it to the theoretical acceleration \( a = (m_2 - m_1) g / (m_1 + m_2) \).

This is a strong example of 3 practical examples of measuring velocity using photogates because it connects kinematics (velocity, acceleration) with dynamics (net force, mass). It also gives students a hands-on test of Newton’s second law.

Extension for 2024–2025 classrooms

In newer lab interfaces, you can log position, velocity, and acceleration simultaneously. Many systems allow exporting data to spreadsheets or Python notebooks, so students can:

• Fit velocity–time data with a linear regression.
• Extract acceleration from the slope.
• Quantify uncertainty more rigorously than older, purely manual methods.

This aligns well with current trends in STEM education that emphasize coding and data literacy, as supported by resources from organizations like the National Science Teaching Association.

More real examples of measuring velocity using photogates in the lab

Those three classics are the backbone, but the best examples of photogate use don’t stop there. Here are several more real examples that broaden how students see velocity and acceleration.

Horizontal launch: projectile velocity off the table

One powerful example of using photogates is to measure the launch speed of a cart or ball that leaves a table and lands on the floor.

Procedure in practice

• A cart is launched off a horizontal track at table height.
• A photogate near the end of the track measures the cart’s velocity using a flag.
• The cart then leaves the table and lands on the floor at some horizontal distance \( x \).

Students can then:

• Use \( v_x \) from the photogate and the measured time of flight (from vertical motion) to predict the landing point.
• Check predictions against actual landing spots.

This example of a photogate experiment ties directly into 2D projectile motion, a staple of any kinematics unit.

Collision experiments: before-and-after velocities

Another set of real examples include using two photogates on a track to study collisions.

Typical setup

• Place one photogate before the collision region and one after.
• A cart with a flag passes through the first gate, collides with another cart, then passes through the second gate.

Students can measure:

• Initial velocity before collision.
• Final velocity after collision.

From there, they can:

• Investigate conservation of momentum.
• Compare elastic vs. inelastic collisions.

In 2024–2025, this kind of experiment lines up well with inquiry-based approaches promoted by many university physics departments, such as those you’ll find through AAPT and major teaching-focused conferences.

Measuring terminal velocity in air (simplified)

For a more advanced class, you can adapt photogates to approximate terminal velocity of falling objects.

How it works

• Drop a small object with a flag through a tall vertical track or tube.
• Place multiple photogates at different heights.

If the velocities measured by the lower gates are nearly the same as those measured by the upper gates, students can argue the object is approaching terminal velocity.

This is one of the more creative examples of measuring velocity using photogates and opens the door to discussions about drag forces, Reynolds number, and real-world applications like skydiving or raindrop motion.

Constant-speed motorized cart: checking uniform motion

Not every lab has to be about acceleration. A motorized cart advertised as “constant speed” is a nice, low-drama test case.

Photogate use

• Place two or three photogates along a straight path.
• Measure the time to travel between each pair.

If the average velocities are consistent, the cart really is moving at constant speed. If not, you’ve created a natural conversation about experimental claims and manufacturer specs.

This example of photogate use is simple but surprisingly effective at teaching students to question equipment and data, not just accept them.

Why photogate velocity labs still matter in 2024–2025

With video tracking apps, smartphone sensors, and all-in-one motion detectors, it’s fair to ask: why keep going back to these examples of 3 practical examples of measuring velocity using photogates?

A few reasons keep photogates relevant:

• Timing precision: Photogates routinely resolve times in the microsecond range, making them ideal for fast events where video frame rates struggle.
• Simple modeling: The relationship between blocked time, flag width, and velocity is straightforward, which keeps the focus on physics, not software quirks.
• Low cognitive load: Students can see cause and effect—object passes, light breaks, time is recorded—without wrestling with complex interfaces.

Many modern lab curricula blend photogates with other sensors. A typical 2024 lab might use photogates for high-precision velocity measurements, then validate results with video or smartphone-based tools. That mix prepares students for the kind of multi-instrument workflows they’ll see in engineering and research.

For background reading on kinematics and motion concepts that underlie these experiments, resources from universities such as MIT and public science education sites like NASA offer accessible explanations and context.

FAQ: common questions about examples of measuring velocity with photogates

What are some examples of 3 practical examples of measuring velocity using photogates?

Three core examples include:

• A dynamics cart on a track, where a flag passes through a photogate to measure instantaneous velocity.
• A free-fall experiment using a picket fence to determine \( g \).
• An Atwood machine, where one mass passes through a gate and velocity is used to infer acceleration and test Newton’s laws.

These three form the backbone of many high school and college kinematics labs.

Can you give an example of using two photogates to measure acceleration?

Yes. Place two photogates a known distance apart on a track. As a cart accelerates down the track, each gate measures an instantaneous velocity. Using the time between gates, students can estimate acceleration from the change in velocity over that time, and compare it to predictions from kinematic equations.

Are photogates still better than video for velocity measurements?

They’re better for some tasks, not all. For fast, one-dimensional motion with a clear path—like carts, Atwood machines, or picket fences—photogates usually offer higher timing precision and cleaner data. Video analysis shines when motion is two-dimensional, complex, or when you want visual context. Many 2024–2025 labs use both and have students compare results.

What are examples of errors when measuring velocity with photogates?

Common issues include:

• Mis-measured flag width, which directly skews velocity.
• Misalignment so the object only partially blocks the beam.
• Wobbling or rotating objects that change the effective blocking width.
• Incorrect mode settings in the interface (e.g., not set to “picket fence” mode).

Discussing these errors is part of what makes these examples of photogate experiments so valuable for teaching scientific practice.

Where can I find more detailed lab guides for these experiments?

Look for lab resources from university physics departments and teaching organizations. For instance:

• MIT OpenCourseWare’s classical mechanics materials
• Harvard and other major universities’ introductory physics lab pages (where publicly available)
• Teaching guides and position statements from the American Association of Physics Teachers (AAPT)

These sources provide detailed write-ups that align well with the examples of 3 practical examples of measuring velocity using photogates described here.