Download Pearson Prentice Hall Physical Science: Concepts in Action

Chapter 12
Forces and Motion
PEARSON PRENTICE HALL
PHYSICAL SCIENCE: CONCEPTS
IN ACTION
12.1 Forces
 Objectives:
 1. Describe examples of force and identify
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appropriate SI units used to measure force
2. Explain how the motion of an object is
affected when balanced and unbalanced forces
act on it
3. Compare and contrast four kinds of friction
4. Describe how Earth’s gravity and air
resistance affect falling objects
5. Describe the path of a projectile and identify
the forces that produce projectile motion
Force and SI Units
 In its most basic form, a force is a push or a
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pull
A force can cause a resting object to move or
it can accelerate a moving object by changing
the object’s speed or direction
The SI unit of force is the Newton (N)
1 N causes a 1 kg mass to accelerate at a rate
of 1 m per second each second
1 N = 1 kg-m/s2
Use an arrow to represent the direction &
strength of a force
Balanced & Unbalanced Forces
 You can combine force arrows to show the result of
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how forces combine
Forces in the same direction add together while
opposing forces are subtracted
Definition: net force is the overall force acting on an
object after all forces are combined
When the forces on an object are balanced the net
force is zero & there is no change in the object’s
motion
When an unbalanced force acts on an object, the
object accelerates
Friction
 Definition: friction is a force that opposes the
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motion of objects that touch as they move past
each other
Four types of friction are static friction, sliding
friction, rolling friction and fluid friction
Definition: static friction is the friction force that
acts on objects that are not moving
Static friction acts in the direction opposite that
of the applied force
Definition: sliding friction opposes the direction
of motion of an object as it slides over a surface
 Sliding friction is less than static friction & less force
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is needed to keep an object moving or start it
moving
Definition: rolling friction is the force that acts on
rolling objects
When a round object rolls across a flat floor, both
the object and the floor are bent slightly out shape
Definition: fluid friction opposes the motion of an
object through fluid Ex: spoon through batter
Definition: a fluid is a mixture of water and gases
Definition: air resistance is fluid friction acting on a
moving object through the air
At high speed air resistance becomes important
This is why bicyclists, skaters and other athletes
wear slick racing suits
Gravity and Falling Objects
 Definition: gravity is a force that acts between
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any two masses
Earth’s gravity acts downward toward the center
of the earth
Gravity causes objects to accelerate downward
Air resistance acts in the direction opposite of
gravity, opposing the motion and reducing
acceleration
Definition: terminal velocity is the constant
velocity of a falling object when the force of air
resistance equals the force of gravity
Terminal velocity equals 9.8 m/s2
Projectile Motion
 Definition: projectile motion is the
motion of a falling object after it is
given an initial forward velocity
 The combination of an initial forward
velocity and the downward vertical
force of gravity causes the object to
follow a curved path
12.2 Newton’s First & Second
Laws of Motion
 Objectives:
 1. Describe Newton’s first law of
motion & its relation to inertia
 2. Describe Newton’s second law of
motion and use it to calculate
acceleration, force and mass values
 3. Relate the mass of an object to its
weight
Newton’s first law
 In the mid 1600s Isaac Newton expanded on the work
of Galileo (Galileo’s work from late 1500s to early
1600s)
 Newton’s first: the state of motion of an object does
not change if the net force acting on it is zero
 Put another way: an object at rest tends to stay at rest
& an object in motion tends to stay in motion with the
same direction and same speed
 Definition: inertia is the tendency of an object to resist
a change in its motion
 Newton’s first law is also called the Law of Inertia
 Q: How does a zero net force affect an object’s motion?
Newton’s Second Law
 Newton’s second: the acceleration of an object is equal
to the net force acting on it divided by the object’s
mass
 Acceleration = net force or a = F
Mass
m
 Cross multiplying the equation tells us that F=ma
 It is useful to know that N & m are equivalent
kg s2
 The acceleration of an object is always the same
direction as the net force but..
 The second law applies when net force acts in a
direction opposite to the object’s motion such as seat
belts opposing a passenger’s forward motion in a
collision (causing deceleration)
Let’s try a couple problems…
 Q: A 20 N net force acts on an object with a
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mass of 2.0 kg. What is the object’s
acceleration?
A: F=ma F=20 N m= 2.0 kg so 20=(2.0)(a)
Divide both sides by 2.0 20/2.0 = 2.0/2.0 (a)
A= 10 m/ s2
Q: What is the acceleration of a 1000 kg car
subject to a 550 N net force?
A: F=ma or a=F/m
550 N/ 1000 kg = 0.5 m/ s2
Mass and Weight
 Definition: weight is the force of gravity acting on an
object
 Weight formula: weight = mass x acceleration due to
gravity or W=mg
 F or W must be written in Newtons, acceleration in meters
per second squared and the mass in kilograms
 Acceleration due to gravity = 9.8 m/ s2
 Q: If an astronaut has a mass of 112 kg, what is his weight
on Earth:
 W= mg so 112 kg x 9.8 m/ s2 = 1100 N
 Mass is a measure of inertia of an object
 Weight is a measure of the force of gravity acting on an
object
12.3 Newton’s Third Law of
Motion and Momentum
 Objectives:
 1. Explain how action and reaction
forces are related according to
Newton’s third law of motion
 2. Calculate the momentum of an
object and describe what happens
when momentum is conserved during
a collision
Action and Reaction Forces
 Newton’s third law: Whenever one object exerts a
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force on a second object, the second object exerts an
equal and opposite force on the first object
Forces come in pairs: push and pull, action & reaction
Ex: swimmer pushes against the water (creating an
action force) & the water produces a reaction force
that pulls the swimmer along (action on the water,
reaction on the swimmer)
When action & reaction forces do not act on the same
object as in the water & swimmer, the net force is not
zero
When the action & reaction forces act on the same
object the net force is zero
Momentum
 Definition: momentum is the product of an
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object’s mass and its velocity
If the object has a lot of momentum, it is hard to
stop
An object has a lot of momentum if the product
of its mass and velocity is large
Momentum = mass x velocity
Law of Conservation of Momentum: if no net
force acts on a system, the total momentum of
the system does not change
In a closed system, the loss of momentum of
one object equals the gain in momentum of
another object & momentum is conserved
Let’s try a problem
 Q: Which has more momentum, a 0.046 kg
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golf ball travelling at 60 m/s, or a 7.0 kg
bowling ball going 6.0 m/s? **notice the
units for momentum are kg-m/s**
A. Golf ball = 0.046 kg x 60 m/s = 2.8 kg-m/s
Bowling ball= 7.0 kg x 6 m/s = 42 kg-m/s
So the bowling ball has more momentum
Q: If an eagle and a bumblebee are traveling
at 8 km/h, which has more momentum &
why?