AP Physics 1 - Forces Study Guide

This doesn't have center of mass or springs. Please find other flashcards for those topics.

Inertia

Tendency of an object to maintain constant velocity.

Weight

Force of gravity acting on an object. In Newtons.

Mass

Amount of matter an object has. In kilograms.

Force

A push or a pull. In Newtons.

Net force

Vector sum of all forces acting on an object. In Newtons.

Static equilibrium

The state where an object is at rest because all the forces and torques acting on it are balanced, resulting in zero net force and zero net torque. F_net = 0. Example: Object at rest on level ground.

Kinetic equilibrium

F_net = 0; at constant velocity and 0 acceleration.

Normal force

Force perpendicular to the surface.

Static friction

Friction when object does not move against contact surface. Example: When you push a lunch table and it doesn’t move.

Kinetic friction

Friction when object moves against contact surface. Example: You go down a slide wearing jeans and they slow you down.

Coefficient of friction (μ)

μ = F_f/F_n → the ratio that tells how large friction is compared to how hard the surfaces are pressed together

μ_s vs μ_k

μ_s = static (prevents motion), μ_k = kinetic (while sliding)

Static friction formula

f_s ≤ μ_s N (≤ is mandatory)

Kinetic friction formula

f_k = μ_k N (always equal)

μ_k compared to μ_s

Smaller than μ_s

Object moving at constant speed on flat ground → how to find μ_k

μ_k = F_f/F_n 

Object sliding at constant speed on incline with no other forces → value of μ

μ = tan θ (the angle θ is the incline angle)

Normal force for any object on an incline

F_n = mg cos θ (cos of the incline angle, not sin)

Static friction direction

Opposite the way it would move if no friction

Kinetic friction direction

Opposite the actual sliding direction

Action/reaction force pairs

Two forces that are always equal in size, always opposite in direction, and act on two different objects. They happen at the exact same time and are of the same exact type. Newton’s 3rd Law: No force exists alone. Example: you push on a wall, so the forces are you to wall and wall to you. There are two forces. Not only one.

Terminal velocity

Free fall when force of air resistance equals force of gravity.

Newton’s First Law

No change in speed (a=0) only if F_net = 0

Newton’s Second Law

When a ≠ 0, F_net = ma.

Newton’s Third Law

Forces exist in action/reaction pairs.

Draw free body diagram of parked car on level ground.

Draw free body diagram of driving car on level ground at constant velocity.

Draw free body diagram of car accelerating on level ground.

Draw free body diagram of car parked on a hill.

Draw free body diagram of car accelerating up a hill.

Draw free body diagram of a car accelerating down a hill.

Why does a feather fall slower than a bowling ball in air, but same speed in vacuum?

In air: feather has large air resistance relative to weight → net force smaller → smaller acceleration.

In vacuum: no air resistance → only gravity → F = mg = ma → a = g for both (mass cancels).

Action-reaction forces (3rd law pair) are equal/opposite — why don’t they cancel?

You push wall (forward on wall), wall pushes you (backward on you) → forces don’t cancel because they’re on separate bodies. One force is on you (you push the wall) and one force is on the wall (the wall pushes you) so they’re different systems.

Find mass when given F_net and a (acceleration)

Fₙₑₜ = ma → m = Fₙₑₜ / a (MAKE SURE TO DRAW A FREE-BODY DIAGRAM AND SHOW ALL YOUR STEPS INCLUDING HOW YOU GOT “m = Fₙₑₜ / a”!!!)

Find acceleration when there are multiple horizontal forces (push + friction)

F_netx = Fₐ − f = ma → a = (Fₐ − f) / m

Object sliding down ramp at constant speed → what does that tell you?

F_net parallel = 0 and F_net perpendicular = 0. Component of weight down ramp exactly equals friction (or pulling force)

Pulling object up incline at constant speed → how to find friction force?

f = F_applied − mg sinθ

Find μ_k when moving at constant speed on incline

μ_k = f / (mg cosθ). f comes from constant speed balance

Rope over pulley fixed to floor/ceiling → tension when lifting load

Tension = full weight of load (you pull with full weight, not half)

Person can lift 1600 N but pail is 1200 N in fixed-pulley setup → can she lift it?

No. She pulls with the full 1200 N (mechanical advantage = 1)

Frictionless table + hanging mass pulley system → acceleration formula

a = (m_hanging g) / (m_cart + m_hanging)

Two strings at angles supporting hanging object → how to solve tensions

ΣF_x = 0 and ΣF_y = 0. Split each tension into components

Apparent weight when elevator accelerates downward

Apparent weight = m(g − a) → feels lighter

Apparent weight when elevator moves at constant speed (up or down)

Apparent weight = mg → feels normal

Force applied at angle θ above horizontal, constant speed on level ground → normal force

F_net = mg − F sinθ

Object moving at constant velocity (any situation) → net force

F_net = 0 in every direction

Constant speed down incline, no other forces → friction vs weight component

f = mg sinθ (parallel forces balance)

Pushing downward at angle reduces friction → why?

Downward component of push reduces N → reduces f = μN

Tension in massless frictionless rope over pulley

Same magnitude throughout entire rope

Free-body diagram → three things you NEVER draw

Net force arrow, motion/velocity arrows, forces the object exerts on others

Apparent weight definition

The reading on a scale → normal force (or tension) supporting you

Elevator accelerating upward → apparent weight

m(g + a) → feels heavier

Elevator in free fall (a = g down) → apparent weight

0 N → weightless

Constant speed on incline → two parallel forces

Component of weight down ramp = friction (or applied force)

Hanging mass accelerating horizontal cart → only driving force

m_hanging g (tension pulls cart, gravity pulls hanging mass)

g

acceleration due to gravity (9.8 m/s²)

a

acceleration of the object

m

mass (kg)

F

any force (Newtons)

Fₙₑₜ or ΣF

net force / sum of forces

F_g or mg

weight / gravitational force

F_N or N

normal force

f

friction force (general)

f_k

kinetic friction

f_s

static friction

μ

coefficient of friction (general)

μ_k

coefficient of kinetic friction

μ_s

coefficient of static friction

T

tension in rope/string

F_netx

sum of forces in x-direction (horizontal)

F_nety

sum of forces in y-direction (vertical)

F_a

applied / pushing / pulling force

m/s²

meters per second squared (unit of acc