Forces in Motion Lab Packet
PhET Interactive Physics Lab
Forces in Motion: Virtual Lab
Guided Investigation with Introductory Science Readings
Name:
Date: Period:
Mission: Read each scientific introduction, then test the concepts in the PhET simulation.
PhET → Forces and Motion: Basics
Concept Briefing 1: Forces, Vectors, and Net Force (\(\Sigma F\))
A force is simply a push or a pull acting upon an object as a result of its interaction with another object. Forces are vectors, meaning they have both a magnitude (strength measured in Newtons, \(\text{N}\)) and a specific direction. When multiple forces act on an object simultaneously, we calculate the Net Force (\(\Sigma F\)), which is the overall resultant force. If equal forces pull in opposite directions, they cancel out (\(\Sigma F = 0\text{ N}\)) and the forces are balanced, resulting in zero change in motion. When opposing forces are unequal, the forces are unbalanced, producing a net force that accelerates the object in the direction of the stronger pull.
1
Investigation 1: Tug-of-War in the Net Force Tab
~12 Min
Open the Net Force tab. Turn on all 3 checkboxes: [✓] Sum of Forces (\(\Sigma F\)), [✓] Values, and [✓] Speed.
Large Puller: 150 N
Medium Puller: 100 N
Small Puller: 50 N
| Tug Setup | Left Force | Right Force | Net Force (\(\Sigma F\)) | Cart Motion Result |
|---|
| 1 Large Blue vs 1 Large Red | ______ N | ______ N | ______ N | □ Left □ Right □ Balanced (No move) |
| 1 Large + 1 Small Blue vs 1 Med Red | ______ N | ______ N | ______ N | □ Left □ Right □ Balanced (No move) |
| 1 Medium Blue vs 2 Small Red | ______ N | ______ N | ______ N | □ Left □ Right □ Balanced (No move) |
| Your Choice: ________________ | ______ N | ______ N | ______ N | □ Left □ Right □ Balanced (No move) |
1. Based on your reading and data, write a mathematical rule for finding Net Force when forces act in opposite directions:
2. Rope Knot Position Test: Does moving a puller closer to or farther from the cart change the pulling force or outcome? What did you discover?
3. In-Motion Challenge: Get the cart rolling right. While rolling, make the forces balanced (\(\Sigma F = 0\)). Does the cart stop immediately, or keep rolling? Explain why.
Forces in Motion Lab • PhET Basics Page 1 of 4
Part 2: Motion, Inertia, and Newton's First Law
Physics Lab Packet
Concept Briefing 2: Inertia & Newton's First Law of Motion
In everyday life, if you slide a textbook across a floor, it quickly skids to a halt. For centuries, thinkers believed objects naturally "wanted" to be at rest and needed continuous force to keep moving. However, Galileo Galilei and Sir Isaac Newton discovered this was an illusion caused by invisible resistive forces like friction and air drag.
Newton's First Law of Motion (The Law of Inertia) states: An object at rest stays at rest, and an object in motion stays in motion with the same speed and in the same direction, unless acted upon by an unbalanced force. Inertia is the natural resistance of any physical object to any change in its velocity. The amount of inertia an object possesses depends solely on its mass (\(m\))—the more mass an object has, the harder it is to start moving, stop, or turn.
2
Investigation 2: Skateboard on a Frictionless Track
~10 Min
Switch to the Motion tab. Check [✓] Force, [✓] Values, [✓] Masses, and [✓] Speed. Crucial Note: In this tab, there is zero friction!
Trial A: The Coasting Test (Zero Net Force)
Place the 50 kg crate on the skateboard. Push with +100 N until speed reaches ~10 m/s, then immediately release the pusher (Applied Force = 0 N). Observe the speedometer needle closely.
What happens to the speed of the crate after you let go? Does it slow down, speed up, or stay constant? Explain why using Newton's First Law:
Trial B: Mass vs. Inertia (50 kg Crate vs. 200 kg Refrigerator)
Reset the sim. Test pushing the 50 kg crate with +200 N for 5 seconds. Next, place the 200 kg refrigerator on the skateboard and push with +200 N for 5 seconds.
Which object gained speed more rapidly? How does the mass of an object relate to its inertia and resistance to acceleration?
Trial C: Thought Question — Stopping Without Friction
If an object is gliding across this frictionless track at 15 m/s, what must you do to bring it back to a complete stop (speed = 0 m/s)?
Forces in Motion Lab • PhET Basics Page 2 of 4
Part 3: The Science of Surface Friction
Physics Lab Packet
Concept Briefing 3: Opposing Forces — Static vs. Kinetic Friction
Friction is a contact force that opposes the relative motion between two surfaces touching one another. At a microscopic level, even smooth-looking surfaces possess jagged peaks and valleys. When two objects press together, these microscopic ridges interlock and form temporary bonds.
There are two main types of surface friction:
• Static Friction: Acts on objects that are stationary (not yet sliding). It dynamically matches whatever force you apply up to a maximum breakaway limit, keeping the net force zero.
• Kinetic (Sliding) Friction: Acts once the object breaks free and begins sliding. Kinetic friction always points in the direction exactly opposite to the direction of motion, acting as an unbalanced force that slows sliding objects to a stop unless continuously propelled.
3
Investigation 3: Static Breakaway and Sliding Resistance
~12 Min
Open the Friction tab. Check all 5 boxes: [✓] Forces, [✓] Sum of Forces, [✓] Values, [✓] Masses, and [✓] Speed. Keep the friction slider at the default "Medium" setting.
Static Friction Breakaway Test (50 kg Crate)
Use the single arrow (>) on Applied Force to test each force increment on the stationary crate:
Push: 50 N
Friction: _____ N
Moves? Y / N
Push: 100 N
Friction: _____ N
Moves? Y / N
Push: 125 N
Friction: _____ N
Moves? Y / N
Breakthrough!
Min Force: _____ N
Starts Sliding
1. What happened to the static friction force as you increased your push from 50 N to 100 N? Why did the crate remain completely stationary?
2. Once the crate is sliding, release your applied force to 0 N. In what direction does the friction force point relative to the crate's motion, and what does it cause the crate to do?
3. Slide the friction control all the way to "Lots". What changes about the force required to break the crate free? Explain why rougher surfaces create more friction.
Forces in Motion Lab • PhET Basics Page 3 of 4
Part 4: Newton's Second Law & Final Synthesis
\(F_{\text{net}} = m \cdot a\)
Concept Briefing 4: Newton's Second Law of Motion (\(F = ma\))
While Newton's First Law describes what happens when forces are balanced (\(\Sigma F = 0\)), Newton's Second Law explains exactly what occurs when an unbalanced net force acts on an object. An unbalanced force causes an object to accelerate—meaning it speeds up, slows down, or changes direction.
Mathematically: \(a = \frac{F_{\text{net}}}{m}\) or \(F_{\text{net}} = m \cdot a\).
This fundamental relationship reveals two vital principles:
1. Direct Proportionality: The greater the net force applied, the greater the acceleration.
2. Inverse Proportionality: The greater the object's mass, the less it will accelerate from that same force.
4
Investigation 4: Quantifying Acceleration & Mystery Mass
~10 Min
Open the Acceleration tab. Turn on all 6 checkboxes (including [✓] Acceleration). Set the friction slider to "None" so that \(F_{\text{applied}} = F_{\text{net}}\).
| Object & Mass (\(m\)) | Applied Force (\(F_{\text{net}}\)) | Predicted \(a = \frac{F}{m}\) | Sim Dial \(a\) (\(m/s^2\)) | Match? |
|---|
| Crate (50 kg) | 100 N | ______ \(m/s^2\) | ______ \(m/s^2\) | □ Yes □ No |
| Bucket of Water (100 kg) | 150 N | ______ \(m/s^2\) | ______ \(m/s^2\) | □ Yes □ No |
| Girl (40 kg) | 200 N | ______ \(m/s^2\) | ______ \(m/s^2\) | □ Yes □ No |
| Mystery Gift Box [ ? ] | 150 N | Solve for \(m\) below | ______ \(m/s^2\) | □ Solved! |
Mystery Mass Calculation: What is the mass of the Mystery Box [ ? ]?
Rearrange Newton's formula (\(m = \frac{F_{\text{net}}}{a}\)) and solve:
Final Synthesis & Real-World Application
1. A space probe coasts at 30,000 km/h in deep space (no friction, no air). How much engine thrust (force) is needed to maintain this speed?
2. If a 1,000 kg compact car and a 3,000 kg truck receive an identical 3,000 N net force, which accelerates faster and by how much?
Forces in Motion Lab • PhET Basics Page 4 of 4
Forces in Motion Answer Key
Teacher Answer Key & Scoring Guide
Forces in Motion: Lab Key
Complete Worked Solutions • 4-Part Guided Simulation
Total Lab Score: 32 Pts
8 Pts per Section / Page
Answers highlighted in green indicate key data or acceptable variations in student wording.
Part 1 of 4 Key
1
Part 1: Net Force & Vector Addition Solutions
8 Points
| Tug Setup | Left Force | Right Force | Net Force (\(\Sigma F\)) | Cart Motion Result |
|---|
| 1 Large Blue vs 1 Large Red | 150 N | 150 N | 0 N | ✓ Balanced (No move) |
| 1 Large + 1 Small Blue vs 1 Med Red | 200 N | 100 N | 100 N Left | ✓ Moves Left |
| 1 Medium Blue vs 2 Small Red | 100 N | 100 N | 0 N | ✓ Balanced (No move) |
| Sample Student: 1 Lg Red vs 1 Sm Blue | 50 N | 150 N | 100 N Right | ✓ Moves Right |
1. Mathematical rule for finding Net Force in opposite directions:
Expected Rule: Subtract the smaller opposing force from the larger force: \(F_{\text{net}} = F_{\text{larger}} - F_{\text{smaller}}\). The direction of the net force points toward the larger force. (If equal, \(F_{\text{net}} = 0\text{ N}\)).
2. Rope Knot Position Test:
Expected Observation: Placing figures closer to or farther from the cart makes zero difference. The tension and force applied along a straight rope do not depend on knot position.
3. In-Motion Challenge (Balanced forces while moving):
Expected Observation: The cart keeps rolling at a constant speed! It does not stop. Balanced forces mean zero net acceleration, so whatever motion the cart already had continues unchanged.
Forces in Motion Answer Key • PhET Basics Page 1 of 4
Part 2: Motion, Inertia, and Newton's First Law Solutions
8 Points
Trial A: The Coasting Test (Zero Net Force on Frictionless Track)
3 Pts
Sample Response: When the pusher releases the crate and applied force drops to 0 N, the speed remains completely constant (locked at 10 m/s). It does not slow down at all.
Because there is no friction or opposing force, the net force is zero (\(\Sigma F = 0\)). An object in motion has inertia and continues moving at constant velocity indefinitely until an unbalanced external force acts on it.
