Download CH 3—Forces

CH 3—Forces
Force, Mass, and Acceleration
• For any object, the greater the force is
that’s applied to it, the greater its
acceleration will be.
• The acceleration of an object depends on
its mass, as well as the force exerted on it.
Newton’s 2nd Law of Motion
• The net force acting on an object causes
the object to accelerate in the direction of
the net force.
F
• Force = mass x acceleration
F = ma
• SI unit—kg m/s2 = Newton (N)
m x a
• You are pushing a friend on a sled. You
push with a force of 40 N. Your friend and
the sled together have a mass of 80 kg.
Ignoring friction, what is the acceleration of
your friend the sled?
0.5 m/s2
• A tennis ball with a mass of 0.06 kg had an
acceleration of 5,500 m/s2 as it raced
across the court. How much force would
the tennis racket have to exert to give the
ball this acceleration?
330 N
Friction
• friction—force that opposes motion
between two surfaces that are touching
each other
• The amount of friction between two
surfaces depends on two factors:
– kind of surfaces
– force pressing the surfaces together
Air Resistance
• When an object falls toward Earth, it is
pulled downward by the force of gravity.
Another force, called air resistance, acts in
the upward direction on all falling objects.
• The amount of air resistance depends on
the speed, size, and shape of the object.
Terminal Velocity
• The force of air resistance increases until it
becomes large enough to cancel the force of
gravity.
• The forces on the falling objects are balanced,
so the object no longer accelerates.
• It then falls at a constant speed called the
terminal velocity (highest velocity a falling object
will reach).
Section 2—Gravity
• According to the law of gravitation, any
two masses exert an attractive force on
each other.
• The attractive force depends on:
– Mass of two objects
– Distance between two objects
Acceleration Due to Gravity
• Near Earth’s surface, the gravitational attraction
of the Earth causes all falling objects to have an
acceleration of 9.8 m/s2.
• acceleration due to gravity = 9.8 m/s2
• velocity of a falling object = acceleration due to
gravity x time
• v=axt
• An acorn falls from a tree and hits the
ground in 1.5 s. What is the acorn’s
velocity?
–v=axt
– (9.8 m/s2) (1.5 s)
– 14.7 m/s
• Josh is climbing on the rafters. Brittany
distracts him and he falls to the ground in
0.5 s. What is Josh’s velocity when he hits
the ground?
– 4.9 m/s
Weight
• Whether you are standing, jumping, or
falling, Earth exerts a gravitational force on
you.
• weight—gravitational force exerted on an
object
• You can determine the weight of anything
by using Newton’s 2nd Law:
– F=mxa W=mxg
• How much does a person weigh on Earth
if he has a mass of 70 kg?
– (70 kg) (9.8 m/s2) = 686 N
• Gravity on the moon is 1/6 what it is on
Earth, so your weight on the moon would
be 1/6 what it is on Earth.
– A person weighs 480 N on Earth. What does
he weigh on the moon?
– w = (480 N)(1/6) = 80 N
Weight and Mass
• Weight and mass are not the same.
Weight is a force. Mass is the amount of
matter in an object.
• The mass of an object remains the same
on any planet and with or without gravity.
Projectile Motion
• projectile—anything that is thrown or shot
through the air
• Because of Earth’s gravitational pull and
their own inertia, projectiles follow a
curved path.
• horizontal velocity—velocity parallel to
Earth’s surface
• vertical velocity—velocity perpendicular to
Earth’s surface
• Figure 16 page 80
Centripetal Force
• centripetal acceleration—acceleration
toward the center of a curved or circular
path
• centripetal force—force that causes
acceleration toward the center of the circle
– In the case of a car rounding the curve, the
centripetal force is the friction between the
tires and the road surface.
Section 3—Newton’s 3rd Law of Motion
• Newton’s 3rd Law of Motion—When one
object exerts a force on a second object,
the second object exerts a force on the
first that is equal in size but opposite in
direction.
• “To every action, there is an equal and
opposite reaction.”
• Describe a person walking on a sidewalk
using Newton’s 3rd Law of Motion.
– A person exerts a force on the sidewalk and
the sidewalk pushes back on the person,
moving the person forward.
• Describe a fish swimming in the ocean
using Newton’s 3rd Law of Motion.
– A fish pushes on the water with his fins and
the reaction force of the water on the fish
propels the fish forward.
Momentum
• A moving object has a property called
momentum that is related to how much
force is needed to change its motion.
• momentum = mass x velocity
p=mxv
• SI unit: kg m/s
Law of Conservation of Momentum
• The momentum of an object does not
change unless its mass, velocity or both
change. Momentum can, however, be
transferred from one object to another.