Types of Forces, Weight, and MassTypes of Forces, Weight, and Mass

APPHYSICS1 · UNIT2 : F O R C E A N D T R A N S L A T I O N A L D Y N A M I C S

Types of Forces, Weight, and Mass

AP Physics 1 Guide 2

Goal: Name the common forces, state their directions, and distinguish mass from weight. Study notes in original wording, organized around a Newton's Laws slide deck and standard AP Physics 1 content. Use g = 9.8 m/s² unless your teacher says to use 10.

Big Picture

A force is a push or a pull that can change an object's motion. Forces are vectors with magnitude and direction, measured in newtons (1 N = 1 kg·m/s²). An object cannot exert a net force on itself, so every force needs an outside agent.

Contact and Non-Contact Forces

Contact forces require touching: applied force, friction, tension, normal force, drag, spring force, buoyant force, thrust.

Non-contact (field) forces act at a distance: gravity, electric and magnetic forces. At this level you mostly use gravity.

Common Forces

Force Symbol Direction Key facts

Weight

(gravity)

F_g or

mg

Straight down, toward Earth's center F_g = mg

Normal F_N Perpendicular to the contacting surface,

pushing on the object

Needs contact; push only; not always equal to weight,

and not always upward

Tension F_T Along the rope, pulling away from the object Pull only; same everywhere in an ideal light rope Friction F_f Parallel to the surface, opposing sliding (or

attempted sliding)

Static or kinetic (Guide 5)

Applied F_A Direction of the push or pull Any push or pull from a person or object

Drag / air

resistance

F_D Opposite the object's motion through a fluid Like friction for fluids; grows with speed Thrust F_Th Direction the engine pushes the vehicle Force from engines or rockets

Spring F_s Toward the spring's relaxed (unstretched)

length

Magnitude kx; restoring force

Buoyant F_B Upward Upward force of a fluid on an object

Weight vs. Mass

Mass Weight

What it is Amount of matter; measure of inertia Force of gravity on the object

Unit kilogram (kg) newton (N)

Formula m F_g = mg

Changes with location? No Yes (depends on g)

Scalar or vector Scalar Vector (points down)

On Earth g ≈ 9.8 m/s². A 30 kg person has weight 30 × 9.8 = 294 N.

On the Moon g ≈ 1.6 m/s², so the same 30 kg person weighs about 49 N, but still has a mass of 30 kg. Do you have the same mass on the Moon as on Earth? Yes. Do you weigh the same? No.

Normal Force: Do Not Assume F_N = mg

Normal force is whatever the surface must supply to prevent the object from moving through it. On a flat table with nothing else pushing vertically, F_N = mg.

Pull upward on the object and F_N becomes smaller than mg; push down and it becomes larger. On an incline, F_N is perpendicular to the incline (not vertical) and equals mg cos θ when no other perpendicular forces act.

A wall can exert a horizontal normal force: it is perpendicular to the wall, not necessarily upward. Check Yourself

Q: What is the weight of a 5 kg object on Earth? A: 5 × 9.8 = 49 N.

Q: What is its mass on the Moon? A: 5 kg. Its weight there is about 5 × 1.6 = 8 N.

Q: Can tension push? A: No. A rope can only pull.

Q: Is a person standing still on level ground subject to F_N = mg? A: Yes, since the two forces balance vertically. Key Terms and Actors

Term / Actor What it is and why it matters

Force A push or pull; vector measured in newtons.

Weight (F_g) Gravitational force on an object, F_g = mg.

Normal force (F_N) Perpendicular contact force from a surface.

Tension (F_T) Pulling force transmitted through a rope or cable.

Friction (F_f) Contact force parallel to a surface that opposes sliding.

Drag (F_D) Resistive force from a fluid (like air).

Spring force (F_s) Restoring force from a stretched or compressed spring.

Buoyant force (F_B) Upward force from a fluid.

Making This Essay-Ready

Memory trick: Name each force by what is pushing or pulling: "the floor pushes up on the box" = normal force. Trap: Weight and normal force are separate forces on different interactions; they are not an action-reaction pair. AP Physics 1 Guide 2 · AP Physics 1 study notes