AP Physics 1: Newton's 1st Law & FBDsAP Physics 1: Newton's 1st Law & FBDs

APPHYSICS1 · UNIT2 · F O R C E S A N D N E W T O N ' S L AW S

Newton's First Law and Free-Body

Diagrams

Forces, inertia, and how to draw an FBD

Big idea. An object keeps doing what it is doing (staying at rest or moving at constant velocity) unless an unbalanced net force acts on it. Free-body diagrams are how you turn a picture into physics. Every force problem in Unit 2 starts with one.

Newton's three laws at a glance

Law Name Statement Equation

1st Inertia Objects at rest stay at rest and objects in motion stay in motion at

constant velocity unless acted on by an unbalanced force

If ΣF = 0, then

a = 0

2nd Acceleration Acceleration is directly proportional to net force and inversely proportional to mass

ΣF = ma

3rd Action and

reaction

Forces come in pairs that are equal in size, opposite in direction, and

act on different objects

FA on B = −FB

on A

Inertia and mass

Inertia is the tendency of an object to resist a change in its motion.

Mass (kg) is the amount of “stuff” in an object and is the measure of its inertia. Mass is the same everywhere in the universe.

Weight is the force of gravity on an object: Fg = mg. It changes with location. On Earth g = 9.8 m/s² (about 10 for quick estimates).

Constant velocity does not need a net force. A force is only needed to change velocity (speed up, slow down, or turn).

Test trap. “Moving means there is a net force” is wrong. A car cruising at constant speed has zero net force. Its engine force exactly cancels drag and friction.

What is a force?

A force is a push or a pull that can accelerate an object. It is an interaction between two objects, so you should always be able to name both. An object cannot exert a force on itself (standard AP wording, 3.A.3.2).

Types of forces to know

Force Symbol Direction and facts

Gravity

(weight)

Fg =

mg

Always straight down toward the Earth’s center

Normal FN Perpendicular to and away from the surface. Keeps objects from “crashing through” surfaces. Not always equal to mg

Tension FT Along the rope, pulling on the object. A rope can only pull, never push Friction Ff Parallel to the surface, opposes motion (or attempted motion)

Applied FA A push or pull from a person or object

Drag / air

resistance

FD Like friction but in a fluid, opposes motion through it

Spring FS Push or pull from a spring toward its natural length

How to draw a free-body diagram (FBD)

1. Draw a dot for the object. Every object in the problem gets its own FBD, as if it were alone in empty space (a “free body”).

2. Draw an arrow away from the dot for every force acting on that object. Do not include forces the object exerts on others.

3. The arrow points the way the force pushes or pulls. Longer arrow means bigger force. Label each arrow clearly (FN, Fg, and so on).

4. Choose axes. Pick the direction of acceleration as positive when you can.

Rule. Do NOT draw “ma” or “velocity” as an arrow on an FBD. Those are results, not forces. Worked FBDs

Situation Forces on the object Reasoning

Person sitting in a chair FN up, Fg down At rest, so FN = Fg

Car at constant speed, to the

right

Fg down, FN up, Fthrust

right, FD left

Constant velocity, so FN = Fg and Fthrust = FD

Book sliding right on a table,

slowing down

Fg down, FN up, Ff left Only Ff is unbalanced, so acceleration points

left

Box hanging from a rope at rest Fg down, FT up FT = mg

Ball thrown upward, at the top

(ignore air)

Fg down only Velocity is momentarily zero but the force is

not. a = g downward

Test trap. No “force of the throw” or “force of motion” on a ball in the air. After it leaves your hand, only gravity (and air drag) act on it.

Equilibrium

An object is in equilibrium when ΣF = 0. That means it is at rest or moving at constant velocity. Check each axis separately: ΣFx = 0 and ΣFy = 0.

Practice

1. State Newton's first law in your own words.

2. What is the measure of an object's inertia?

3. Find the weight of a 6.0 kg object on Earth (g = 9.8 m/s²).

4. A crate slides at constant velocity on a floor. What can you say about the net force? 5. List all forces on a skydiver falling at terminal velocity.

6. Draw the FBD for a box at rest on a table. Which two forces balance?

7. Can an object exert a force on itself? Explain.

8. Why does a passenger lurch forward when a car brakes suddenly?

9. A ball is thrown straight up. At the very top, what is the net force?

10. A 2 kg book rests on a table. What is FN?

Answers

1. An object keeps its velocity (rest or constant speed in a straight line) unless an unbalanced net force acts on it.

2. Its mass (kg).

3. Fg = mg = 6.0x9.8 = 58.8 N.

4. Zero. Applied force equals friction, and FN equals Fg.

5. Gravity (down) and drag (up). They are equal, so net force is zero.

6. Fg down and FN up, with FN = Fg.

7. No. A force is an interaction between two different objects (AP standard 3.A.3.2). 8. Inertia. The passenger tends to keep moving at the original velocity while the car slows. 9. Fg = mg downward. The velocity is zero but the net force is not.

10. 19.6 N upward, since the book is in equilibrium.