Transcript Forces 12.1 Pg 356-362 - Physical Science 2014-2015

Forces
12.1 Pg 356-362
FORCE:
A push or a pull that acts on
an object
Can cause a resting object to
move, or it can accelerate a
moving object by changing
the object’s speed or
direction.
FORCE:
Unit of force: Newton (N)
1 Newton is the force that causes a
1 kg mass to accelerate at a rate of
1 m/s2
 1 N = 1 kgm/s2
kg = mass and m/s2 = acceleration
so…
Force = mass x acceleration
Representing Force
Example: Measuring
force with a spring scale
at the grocery store
When the vegetables are
in the basket the force
pushes the basket down
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To represent force use
an arrow in the direction
of the force.
Vectors can be used to represent
force because force has magnitude
and direction
Combining forces
Forces CAN be combined (vector
addition)
3 people pushing a car out of gas




Forces in the SAME direction you ADD
Forces in the OPPOSITE direction you
SUBTRACT
NET FORCE: overall force acting on an
object after all the forces are combined
Balanced forces:
When the forces on an object are
balanced, the net force is ZERO and
there is no change in the object’s
motion

0
The forces are balanced when they are
equal in size and opposite in direction
THINK equal push and pull
Unbalanced force
When an unbalanced force acts on
an object, the object accelerates
THINK TUG OF WAR
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FRICTION:
A force that opposes the motion of
objects that touch as they move past
each other
Friction makes
 Walking possible
 Food stay on your fork
 Tires on your car grip the road so the car moves
forward
 A feather slow down as it falls
There are four main types of friction:
static friction, sliding friction, rolling
friction, and fluid friction
Static Friction
 The friction force that acts on
objects that are not moving
 Always acts in the direction
opposite to the applied force
 Every time you take a step
and push off the ground you
are experiencing static
friction
It is not the same as static
electricity!!
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Sliding Friction
A force that opposes the direction of motion
of an object as it slides over a surface
Sliding friction is less than static friction,
therefore once you get something moving it is
easier to keep it moving
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Rolling Friction
 The friction force that acts on
rolling objects, occurs when
the floor and object are bent
slightly out of shape
 Rolling friction is is about 100
to 1000 times less than the
force of static or sliding friction
 Ball bearings are often used to
reduce friction in machines
Friction is greatly reduced
because it changes sliding friction
to rolling friction
Think in-line skates and skate
boards
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Fluid Friction
 The force that opposes
motion of an object as it
moves through a fluid
 Fluids are liquids (like water)
or mixtures of gases (like air)
 Fluid friction increases as as
the speed of the object
increases
 Air resistance is fluid friction
acting on an object moving
through air
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GRAVITY:
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Force that acts
between any two
masses
Acts downward
toward the center of
Earth
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Falling objects
Gravity causes objects to
accelerate downward,
whereas air resistance
acts in the direction
opposite to the motion and
reduces acceleration
Air resistance acts in the
direction OPPOSITE to
the motion
Gravity
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Air Resistance
Terminal Velocity
Terminal velocity of a
skydiver is approximately
 As speed increases air resistance
195 km/h (122 mph or 54
increases
m/s)
Gravity
 If an object falls for a long time
the upward force of air resistance
becomes equal to the downward
force of gravity
Then the object continues to fall at a
constant velocity (or constant speed)
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 Terminal velocity is the constant
velocity of a falling object when
the force of air resistance equals
the force of gravity
Air Resistance
Projectile Motion
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Both marbles fall at the
same rate even if one is
also moving forward
 The motion of a falling object
after it is given an initial
forward velocity
 The path of a projectile is
always a curve
 Air resistance and gravity are
the only forces acting on a
projectile
 The combination of an initial
forward velocity and the
downward force of gravity
causes the ball to follow a
curved path
Projectiles and Speed
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 In the diagram the marbles
roll off the table and fall to the
floor
 The marbles continue to
move forward while they fall
 Therefore, as the marble’s
velocity increases the
distance away it travels will
increase
 All marbles hit the ground at
the same time no matter how
fast they travel forward
Projectile Motion Problem
 How fast would the person have to run in order to land in the water safely
(land 2.5m away)?
 Given info:
 Height of cliff = 9 m
 Distance forward = 2.5 m
 Gravity = 9.8 m/s2
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 Find Velocity (v) and Time (t)
2.5m
 First find time
t = √2(h)
t = √2(9m)
9.8m/s2
t = √18m
9.8m/s2
= √1.836734694s2 = 1.4s
9.8m/s2
 Now find the velocity (speed)
v=d
t
v = 2.5m
v = 1.8 m/s
1.4s
 So the person would have to run at a velocty of 1.8 m/s in order to land at
least 2.5 m away from the cliff. If not he could be seriously injured on the
rocks below.