Transcript Forces - Weebly

What do you know about
forces?
 In your own words, define the term “force”.
 Give two examples of forces.
 Do you think a force affects the motion of
an object? If so, how?
 Describe a “real-life” situation in which you
encounter a force.
Forces
What is a force?
 A force is a push
or pull exerted
on an object.
• The object on which the force is
exerted is called the system.
• The world around the object
that exerts forces on it is called
the environment.
environment
system
environment
environment
Types of Forces
 Contact Force-a force
that acts on an object
only by touching it.
 Long Range Force- a
force that acts on an
object without
contact.
The force of gravity is
an example of a long
range force.
• Each force has a specific, identifiable,
immediate cause called the agent.
• The agent can be animate, such as a
person, or inanimate, such as a desk, or
a magnet.
The agent for the force
of gravity is the earth’s
mass.
Force is a vector quantity.
 A force has both magnitude and direction.
 A force is represented with an arrow that
points in the correct direction.
 The length of the arrow is proportional to
the size of the force.
 The tail of the force vector is always on the
object, even when the force is a push.
 The symbol F with a subscript label is
used to identify the agent.
Common Types of Forces
Force
Symbol
Friction
Ff
The contact force that acts to
oppose motion between
surfaces
Parallel to the surface and
opposite the direction of
the motion
Normal
FN
The contact force exerted by a
surface on an object.
Perpendicular to and away
from the surface
Spring
Fsp
A restoring force, that is, the
push of pull a spring exerts on
an object.
Opposite the
displacement of the object
at the end of a spring
Tension
FT
The pull exerted by a string or
rope when attached to an
object and pulled taut.
Away from the object and
parallel to the string or rope
at the point of contact.
Thrust
Fthrust
Forces that move objects such
as rockets, planes, cars, etc.
In the same direction as
the acceleration.
Weight
Fg
Definition
Direction
Long range force due to
Straight down toward the
gravitational attraction between center of the earth.
the earth and an object.
Net Force
 Because forces are vectors, the total force on an
object is the vector sum, or resultant, of all
forces exerted on an object.
 The vector sum of two or more forces on an
object is called the net force.
●
Fman
●
Fnet
Fwoman
Equilibrium
 If the net force on an object is zero, the
object is in equilibrium.
 If an object is in equilibrium, the forces are
balanced. (Equal in magnitude, but
opposite in direction.)
 An object in equilibrium will be either at
rest or moving at a constant velocity.
Drag Force
 When an object moves through any fluid,
such as air or water, the fluid exerts a drag
force on the moving object in the direction
opposite to its motion.
 As the speed of the object increases, so
does the magnitude of the drag force.
 The size and shape of the object also
affects the drag force, as well as the
temperature and viscosity of the fluid.
Terminal Velocity
gravity
 An object in free fall has very




little velocity at the start, and
therefore a small drag force.
The downward force of gravity
is much stronger than the
upward drag force so the
object accelerates downward.
As the ball’s velocity
increases, so does its drag
force.
Eventually the drag force
equals the force of gravity.
When this happens there is no
net force and no acceleration.
The constant velocity that is
reached when this state of
equilibrium is achieved is
called the terminal velocity.
Air resistance
gravity
Air resistance
Terminal velocity
Newton’s First Law of Motion
 An object that is at rest will remain at rest
or an object that is moving will continue to
move in a straight line with constant
speed, if the net force acting on that object
is zero.
 Newton’s first law is often called the Law
of Inertia.
 Inertia is the tendency of an object to
resist change.
Newton’s First Law
Newton’s Second Law of Motion
 If the net force on an object is not equal to zero,
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the motion of the object will change.
A change in motion is called acceleration.
Experiments show that the acceleration of an
object is proportional to the net force exerted on
the object and inversely proportional to the mass
of the object being accelerated.
This is a statement of Newton’s second law and
can be written as an equation.
Newton’s Second Law a = Fnet/m or F=ma
Measuring Force
 One unit of force causes a 1 kg mass to
accelerate at 1 m/s2.
 Because force is equal to mass times
acceleration, F=ma, one force unit has the
dimensions 1 kg●m/s2
 This unit in the SI system is known as the
newton, N.
Using Newton’s Laws
Consider a ball falling in midair:
 If it is touching nothing and air resistance is
neglected, the only force acting on it is Fg.
 The ball’s acceleration is -9.8 m/s2, or g.
 Newton’s 2nd law becomes F = mg.
 The magnitude of the object’s weight is equal to
its mass times the acceleration it would have if it
were falling freely.
Mass vs. Weight
 Mass, the amount of
matter an object
contains, does not
change. (Measured in
kilograms)
 Weight is dependent
upon the acceleration
due to gravity and will
vary from planet to
planet. (Measured in
Newtons)
What is being measured,
mass or weight?
 A bathroom scale contains springs.
 When you step on a scale, the scale exerts an upward
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force on you.
Because you are not accelerating, the net force is zero.
Therefore, that magnitude of Fsp = Fg.
A spring scale, therefore measures weight,
not mass.
If you were on another planet, the compression of the
spring would be different, and the scale’s reading would
be different.
Practice Problems
 On Earth, a scale shows that you weigh 585 N.
What is your mass?
 What would the scale read on the moon (g=1.60
m/s2)?
 Your new motorcycle has a mass of
250. kg. What is its weight in newtons?
 A 7.50 kg television is set on a spring scale. If
the scale reads 78.4 N, what is the acceleration
due to gravity at that location?
Apparent Weight
 If a bathroom scale supports you-it provides the
only upward force-then it reads your weight.
 How would the measurement of the scale
change if you push down on the bathroom
counter while standing on the scale? Push up
on the bathroom counter? Stand with one foot
on the scale and one foot off?
 In these cases, the force exerted by the scale is
the apparent weight.
Weight in an Elevator
 If you stand on a scale in
an elevator that is in
equilibrium, the scale
reads your weight.
 If the elevator accelerates
upward, then you feel
heavier and the scale
reads a larger force.
Why?
 If the elevator accelerates
downward, then you feel
lighter and the scale
reads less. Why?
If the cable holding the
elevator beaks, the
scale with you on it
accelerates with a = -g.
What would the scale
read?
Which exerts the greater force?
 You are driving down the highway and a
bug splatters on your windshield. Which is
greater: the force of the bug on the
windshield, or the force of the windshield
on the bug?
Newton’s Third Law of Motion
 All forces come in pairs. The two forces in
a pair act on different objects and are
equal in strength and opposite in direction.
 In other words; for every action force,
there is an equal but opposite reaction
force.
 FA on B = -F B on A
Which exerts the greater force?
 You are driving down the highway and a
bug splatters on your windshield. Which is
greater: the force of the bug on the
windshield, or the force of the windshield
on the bug?
 The forces are equal.
 The deceleration of the bug is greater due
to its smaller mass, but the forces are
equal.
Practice Problem
 A 50.0 kg bucket is being lifted by a rope.
The rope will not break if the tension is
525 N or less. The bucket started at rest,
and after being lifted 3.0 m, it is moving at
3.0 m/s. If the acceleration is constant, is
the rope in danger of breaking?
Practice Problem #2
 Five people are playing tug-of-war.
Anders and Alyson pull to the right with
45 N and 35 N respectively. Calid and
Marisol pull to the left with 53 N and 38 N,
respectively. With what force and in what
direction does Benito pull if the game is
tied?
Practice Problem #3
 A 7.25 g bullet is fired from a gun. The
muzzle velocity of the bullet is 223 m/s.
Assume that the bullet accelerates at a
constant rate along the barrel of the gun
before it emerges with constant speed.
The barrel of the gun is 0.203 m long.
What average force does the bullet exert
on the gun?