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Forces
Forces - The Nature of Force
Combining Forces
The combination of all forces acting on an object is called the
net force.
Forces - The Nature of Force
Unbalanced Forces
Unbalanced forces acting on an object result in a net force
and cause a change in the object’s motion.
Forces - The Nature of Force
Balanced Forces
Balanced forces acting on an object do not change the
object’s motion.
Forces - The Nature of Force
Asking Questions
Before you read, preview the red headings. In a graphic
organizer like the one below, ask a what or how question for
each heading. As you read, write answers to your questions.
Question
Answer
What is a force?
A force is a push or pull.
What happens when forces
combine?
Forces combine to produce a
net force.
Forces - Friction and Gravity
Gravity
Two factors affect the gravitational attraction between
objects: mass and distance.
Forces - Friction and Gravity
Gravity
The force of gravity on a person or object at the surface of a
planet is known as weight.
Forces - Friction and Gravity
Forces - Friction and Gravity
Free Fall
Use the graph to answer the
following questions.
Forces - Friction and Gravity
Free Fall
Interpreting Graphs:
What variable is on the
horizontal axis? The vertical
axis?
Time is on the horizontal axis,
and speed is on the vertical
axis.
Forces - Friction and Gravity
Free Fall
Calculating:
Calculate the slope of the
graph. What does the slope
tell you about the object’s
motion?
The slope is 9.8. The speed
increases by 9.8 m/s each
second.
Forces - Friction and Gravity
Air Resistance
Falling objects with a greater surface area experience more
air resistance.
Forces - Friction and Gravity
Comparing and Contrasting
As you read, compare and contrast friction and gravity by
completing a table like the one below.
Friction
Gravity
Effect on motion
Opposes motion
Pulls objects toward one
another
Depends on
Types of surfaces
involved, how hard the
surfaces push together
Mass and distance
Measured in
Newtons
Newtons
Forces - Newton’s First and Second Laws
Calculating Force
A speedboat pulls a 55-kg water-skier. The force causes the skier
to accelerate at 2.0 m/s2. Calculate the net force that causes this
acceleration.
Read and Understand
What information have you been given?
Mass of the water-skier (m) = 55 kg
Acceleration of the water-skier (a) = 2.0 m/s2
F = ma
F = 55 kg x 2 m/s2 = 110 N
Forces - Newton’s First and Second Laws
Calculating Force
Practice Problem
What is the net force on a 1,000-kg object accelerating
at 3 m/s2?
3,000 N (1,000 kg X 3 m/s2)
Forces
Newton’s First Law
If no forces are exerted on an object, the object
continues in its original state of motion
Newton’s Second Law
The acceleration of an object is proportional to the
force acting on it and inversely proportional to the
mass of the object
F = ma
What if a=0 ?
Newton’s Third Law
If two objects interact they exert equal forces on
each other in opposite directions
Forces - Newton’s First and Second Laws
Newton’s First and Second Laws
I. Newton’s First Law of Motion
A. Inertia
B. Inertia Depends on Mass
II. The Second Law of Motion
A. Changes in Force and Mass
Forces - Newton’s Third Law
Calculating Momentum
Which has more momentum: a 3.0-kg sledgehammer swung at 1.5
m/s or a 4.0-kg sledgehammer swung at 0.9 m/s?
Read and Understand
What information have you been given?
Mass of smaller sledgehammer = 3.0 kg
Velocity of smaller sledgehammer = 1.5 m/s
Mass of larger sledgehammer = 4.0 kg
Velocity of larger sledgehammer = 0.9 m/s
Forces - Newton’s Third Law
Calculating Momentum
Momentum = Mass X Velocity
Perform the calculation.
Smaller sledgehammer = 3.0 kg X 1.5 m/s = 4.5 kg•m/s
Larger sledgehammer = 4.0 kg X 0.9 m/s = 3.6 kg•m/s
Forces - Newton’s Third Law
Calculating Momentum
Practice Problem
A golf ball travels at 16 m/s, while a baseball moves at 7
m/s. The mass of the golf ball is 0.045 kg and the mass
of the baseball is 0.14 kg. Which has the greater
momentum?
Golf ball: 0.045 kg X 16 m/s = 0.72 kg•m/s
Baseball: 0.14 kg X 7 m/s = 0.98 kg•m/s
The baseball has greater momentum.
Forces - Newton’s Third Law
Conservation of Momentum
In the absence of friction, momentum is conserved when two
train cars collide.
Forces - Rockets and Satellites
What Is a Satellite?
A projectile follows a curved path because the horizontal and
vertical motions combine.
Forces - Rockets and Satellites
What Is a Satellite?
The faster a projectile is thrown, the father it travels before it
hits the ground. A projectile with enough velocity moves in a
circular orbit.
Forces - Rockets and Satellites
What Is a Satellite?
Depending on their uses, artificial satellites orbit at different
heights.
Forces
Graphic Organizer
Type of Friction Occurs When
Example
Static
An object is not
moving
Friction between an
unmoving book and
desk
Sliding
Two solid surfaces
slide over each
other
Rubber pads on a
bicycle’s brakes
Rolling
An object rolls
across a surface
Ball bearings in
skateboard wheels
Fluid
A solid object moves
Air resistance
through a fluid