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Chapter 3
Forces
Section 3.1: Newton’s 2nd Law
 2nd Law of Motion: describes how
force, mass, and acceleration are
related
 Formula: force = mass x acceleration
F=mxa
 Newtons (N): unit for force
 Example Problem: How much force is
needed to accelerate a 1000 kg car 3
m/sec/sec?
2nd Law Practice Problems
1. How much force is needed to accelerate a 1400 kg
car 2 m/s/s?
2. If a 70 kg swimmer pushes off a wall with a force of
250 N, at what rate will the swimmer accelerate
from the wall?
3. A dancer lifts his partner above his head with an
acceleration of 2.5 m/s/s. The dancer exerts a
force of 200 N. What is the mass of the partner?
 Friction: force that opposes motion
 Microwelds: source of friction
 Static Friction: no movement
 Sliding Friction: solid objects slide
over each other
 Rolling Friction: friction between
rolling object and surface it’s rolling
on.
Air Resistance: opposes the force
of gravity
 Makes objects slow down
Terminal Velocity
Because forces are
balanced, skydiver has
reached terminal
velocity.
 The downward
force of gravity
equals the upward
force of air
resistance.
 When these forces
are balanced,
object no longer
accelerates, it falls
at a constant
speed.
Section 3.2: Gravity
 Newton’s Law of Gravitation: force of
attraction that exists between all
objects in the universe
 The size of the gravitational force
between 2 objects depends on…
 Masses
 Distances between them
Falling Objects
 All objects fall at the same rate:
9.8 m/s/s
 Example Problem: A student drops a
penny from the top of the school
building. If it takes 5 seconds for the
penny to hit the ground, what is the
speed of the penny just before
impact?
 Multiflash photography shows that
each ball has the same acceleration
downward (9.8 m/s/s) whether it’s
thrown or dropped.
Weight & Mass
 Weight: gravitational force exerted
on an object (N)
 Mass: measure of how much matter
is in an object (kg)
 Formula: weight(N) = mass (kg) x
acceleration due to gravity (m/s/s)
Example Problem
 What is the weight of a person with a
mass of 50 kg?
 Remember: Earth’s acceleration of
gravity is 9.8 m/s/s.
weight(N) = mass x gravity
1. A man has a mass of 75 kg on the
Earth. What is his weight?
2. Find the acceleration of gravity on a
planet if a person with a mass of 66
kg weighs 646.8 N on that planet.
3. A person weighs 500 N on the Earth.
What is the person’s mass?
Projectile Motion
 The path of a projectile is curved
because of gravity.
Centripetal Force
 When an object moves in a circular
path, it accelerates towards the
center of the circle because of
centripetal force.
Facts About Gravity
 The size of the gravitational force
between 2 objects depends on…
 Masses
 Distances between them
 As you go farther from earth, weight
decreases.
 With no air resistance, a penny & a
feather fall at the same rate.
 Objects in free fall seem to be
weightless.
Elevator is in free fall.
Scale shows
Scale shows boy’s
weight
zero weight
Lab:
Finding Acceleration Due to Gravity
Trial
Time for 20 swings
(sec)
1
54.62
2
54.25
3
54.48
Find average value of T
Length = 1.84 m
T
Time of 1 swing
Formula
Problem Set-Up
Answer + Label
Section 3.3: Newton’s 3rd Law
 3rd Law of Motion: every action has an
equal and opposite reaction
 All forces come in pairs.
 Examples:
 Walking: feet push floor, floor pushes back
 Flight of a bird: bird exerts force on air and air
pushes back
 Rocket: gases move down and rocket moves up
Momentum
 Formula: p = mass x velocity
 Unit for momentum: kg-m/s
 Law of Conservation of Momentum:
momentum isn’t created or destroyed
 Momentum can be transferred from one
object to another in a collision
Momentum Practice Problems
1. An athletic has a mass of 100 kg and a velocity
of 8 m/s. What’s the athlete’s momentum?
2. What is the mass of a person walking at a speed
of 1 m/s if their momentum is 60 kg-m/s?
3. What is the velocity of an object that has a mass
of 175 kg and a momentum of 1750 kg-m/s?