Transcript Forces

Chapter 3
Forces
Forces
Forces, Mass, and Acceleration
• the greater the force applied on an object, the
greater the acceleration
• the greater the mass of an object, the less
acceleration
Forces
• Newton’s Second Law of Motion – the net
force acting on an object causes the object to
accelerate in the direction of the net force
– Formula for force:
F = ma
– F = force
m = mass
a = acceleration
Forces
• Newton’s Second Law of Motion – the net
force acting on an object causes the object to
accelerate in the direction of the net force
F = ma
– units for force: Newtons
æ
kg · m ö
çN or
÷
2
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s ø
Forces
Sample Problems
• example: A person that weighs 70 kg jumps
out of a plane at 9.8 m/s2. What is the force
that gravity is exerting on the person?
Forces
Sample Problems
• If gravity is exerting a force of 98 Newtons on
an object in air, and the acceleration due to
gravity is 9.8 m/s2, what is the object’s mass?
Forces
Sample Problems
• If an object weighs 10 kg and has a force of
200 Newtons acting on it, what is the
acceleration?
Forces
• friction – the force that opposes motion
between two surfaces that are touching each
other
– Newton’s first law would say that if you push
a skateboard, it would continue to move in a
straight line at constant speed. We know
however, that the skateboard slows down due
to friction
Forces
Types of Friction
1.static friction – the friction between two
surfaces that are not moving past each other
– example: when a box is so heavy you try and
push it but it doesn’t move
NOT
Moving
Forces
Types of Friction
2.sliding friction – the force that opposes the
motion of two surfaces sliding past each other
• example: when you push a box it slides across
the floor but the sliding friction makes it hard to
push (but it DOES move)
Forces
Types of Friction
3.rolling friction – the friction between a rolling
object (ex. tire) and the surface that it rolls on
– example: a car’s tire rolling over the
pavement or a train rolling on the rails
– usually much less friction than static or sliding
friction
Forces
• air resistance - a force that acts opposite to
the motion of an object
– example: running against the wind or a
parachute falling to the ground
– air resistance depends on the speed, size,
and shape of the object
– if no air resistance exists, two objects will fall
at the same rate (even a feather and apple)
Forces
Gravity
• law of gravitation – states that any two
masses exert an attractive force on each other
– the force of gravity increases when the mass
of either object increases
– the force of gravity increases when the two
objects move closer together
Gravity
Gravity
Gravitational Acceleration
• the gravitational attraction of Earth causes
falling objects to have an acceleration of
9.8 m/s2
• remember that F = ma so the force of
gravity on an object near Earth’s surface is:
F = m x 9.8
2
m/s
Gravity
• weight (W) – the gravitational force exerted
on an object
Weight = mass x 9.8
2
m/s
• measured in newtons (N)
• weight and mass are NOT the same because
mass does not change based on location but
your weight does
Gravity
• example: How much does a person with a
mass of 70 kg weigh on Earth?
• on the moon, the acceleration due to gravity is
only 1.6 m/s2 so you will weigh less on the
moon
Gravity
projectile motion
• the motion of anything tossed, thrown, shot,
etc. will travel in a curved path
• the object (projectile) will follow a curved path
because of Earth’s gravitational pull
Forces
• centripetal force – an unbalanced force that acts
in the direction toward the center of motion
– centripetal acceleration then, is the acceleration
of an object toward the center of a curved or
circular path
Forces
• centripetal force – an unbalanced force that acts
in the direction toward the center of motion
– centripetal acceleration then, is the acceleration
of an object toward the center of a curved or
circular path
– acceleration occurs during a curve because the
direction is changing therefore making velocity
change
– example: centripetal force (friction between the
tires and road surface) causes a car on a curve
to stay inward while the car’s inertia forces it
outward
Motion
• Newton’s third law of motion – describes
action-reaction pairs by stating that every action
force has an equal and opposite reaction force.
– “for every action, there is an equal and opposite
reaction”
– example: if an object hits water, water splashes
back up
Motion
• A rocket works by igniting fuel which causes
the gases to exert a downward force, resulting
in air being pushed downward exerting a force
upward, pushing the rocket up
Motion
• momentum (p) - property that a moving
object has that is related to how much force is
needed to change its motion
momentum = mass x velocity
p=mxv
• the unit for momentum is kilogram meters per
second
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ç
÷
è s ø
Motion
• momentum (p) - property that a moving
object has that is related to how much force is
needed to change its motion
momentum = mass x velocity
• two trucks might have the same velocity but
the bigger truck has more momentum
• example: Compare the momentum of a 50-kg
dolphin swimming at 10.4 m/s and a 6,300-kg
elephant walking 0.11 m/s.
Forces
Law of Conservation of Momentum
• If no other force acts on bodies in motion,
the momentum before collision is equal to
momentum after collision
• momentum is not lost or created – it is
conserved
• total momentum is zero.