Types of forces
Forces acting on an object
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All forces are measured Newtons.
Not all forces are present in every
situation.
Identify the forces that apply to each
particular situation.
Weight
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Weight = mass x gravity
The force of weight acts on every
mass that is attracted due to gravity.
What is the weight of a 5 kg object
here on Earth?
(5kg)(9.8 m/s2) = 49 N
Normal force
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The normal force is the opposite but
equal force of Newton’s 3rd Law.
An object resting on a desk exerts a
force downward on the desk due to
gravity. The desk exerts a force
upward on the object.
How do we know these forces are
equal in size?
Normal force
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The upward force in this example is the
normal force.
The normal force exists between any 2
objects in contact, and is always
perpendicular to the surfaces in contact.
For a rigid horizontal surface, the normal
force equals the weight (equal in size but
opposite in direction). N = W = mg
Normal force on inclined surface
•For
an object resting on an
inclined surface, the normal
force does NOT equal the
weight.
•By law of similar triangles,
the angles are equal (show
why).
•Then mg can be resolved
into parallel and
perpendicular components.
•The perpendicular
component of the weight is
the same size as the normal.
•You can see that N=mgcosθ
Normal force on inclined surface
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For an object resting on an inclined
surface, the normal force does NOT
equal the weight.
N = mgcosθ (equation used to find size
only)
θ is the angle between the surface and
the horizontal.
example
Normal force activity
Materials: wood mass, spring scale, protractor
Task:
1.
Gather and record enough data to calculate the
normal force felt by a wood mass resting on an
inclined plane.
2.
Then measure and record the actual normal force
(it will be the force to just perpendicularly pull the
mass off the inclined plane.
3.
Calculate percent difference between two values.
4.
Repeat for a plane inclined at a different angle.
5.
Use independent thought to write a paragraph
analyzing what was learned, what went wrong,
how to make it better, etc
Tension
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Tension is the force in a rope, cable, or
wire that is attached to an object.
Tension always points away from the
object along the rope, cable, or wire.
Tension drawings.
Upthrust
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Upthrust is the force experienced by a
body immersed in a fluid (a gas or a
liquid). Also known as the buoyant
force (FB).
Upthrust pushes up on the body. If
upthrust ≥ weight, the body will float.
The amount of upthrust is equal to the
weight of the fluid displaced by the
body.
Note the relative size of each arrow
Air Resistance
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Air resistance is the force that opposes
motion of bodies through the air.
The amount of air resistance depends
upon the speed, shape, and size of the
body.
When air resistance = weight, the body
attains terminal velocity and no longer
accelerates while falling.
A more spread-out shape means more
air resistance and a smaller terminal
velocity.
Free Body Diagrams
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In a free body diagram (FBD), the
object is drawn as a point.
All the forces acting on the object are
drawn as arrows radiating from the
point.
Using FBDs
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Steps to using FBD to solve a problem:
Step 1: Resolve each force into its x and y
components.
Step 2: Add all the x components together watch signs! If the object is at equilibrium
(not accelerating) – the sum of the forces is
equal to 0.
Step 3: Repeat step 2 for the y components.
Step 4: Solve simultaneous equations.
Equations used with FBDs
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Fx = Fcosθ
Fy = Fsinθ
∑ Fx = 0 if the object is not accelerating in
the x direction (ax = 0 m/s2)
∑ Fy = 0 if the object is not accelerating in
the y direction (ay = 0 m/s2)
Examples
Draw an FBD for each problem,
and an equation for ΣFx and ΣFy
1.
2.
3.
A 3.0-kg textbook rests on a horizontal
table.
A 3.0-kg textbook rests on a plane that is
inclined at 35° from the horizontal. There
must be a force holding the book in place –
draw that and label it Fapplied. Include it in
your equations.
A 3.0-kg textbook is suspended from the
ceiling by a string.
FBD activity
Normal force as a result of other forces
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Using FBD’s, we can see the result that an
applied force will have on the magnitude of
the normal force.
If an object has an upward force acting on it,
the normal force will be reduced.
If an object has a downward force acting on
it, the normal force will be increased.
examples
Friction
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Friction is a force that opposes motion
(points in the opposite direction as
motion).
Friction exists between 2 masses in
contact.
Friction tends to slow motion down.
2 kinds of friction
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Two kinds of friction:
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Static friction – the frictional force
between 2 objects that aren’t moving
with respect to each other.
Kinetic friction – the frictional force
between 2 objects that are moving with
respect to each other.
2 kinds of friction
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It is harder to get an object moving
than to keep it moving.
The force due to static friction is
always larger than the force due to
kinetic friction.
Friction equation
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Friction is fun!
f = μN
f is the force due to friction (in
Newtons).
μ is the coefficient of friction (unitless)
N is the normal force (in Newtons).
Coefficient of friction
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μ is a characteristic of the 2 surfaces in
contact.
A rougher, bumpier surface would
have more friction and a bigger μ. Ex
A smoother surface would have less
friction and a smaller μ. Ex
μ is unitless and always < 1.0.
2 coefficients of friction
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μs is the coefficient of static friction.
μk is the coefficient of kinetic friction.
Which one will be greater?
Coefficient of static friction is always
greater than kinetic friction.
Friction examples
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What is the frictional force acting on a
box if I am pushing on it with a force
on 4 N but the box isn’t moving?
4N
f smax
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f smax is the maximum static friction force
that must be overcome before the object
can begin to move.
f smax = μs N
Once the friction force is larger than f smax,
and the object begins to move, then the
object experiences kinetic friction.
Examples
Friction activity
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Find the frictional forces of stationary
and moving objects on horizontal and
inclined surfaces.