Download 4.4 Physics Section 4.4

Physics Section 4.4
Describe various types of forces
Weight is a measure of the
gravitational force exerted
on an object. It depends
upon the objects mass and
its location in a
gravitational field.
To calculate the weight of an object
Fg = mg
Fg = objects weight (N)
m = objects mass (kg)
g= acceleration due to gravity
Example:
Calculate the weight of an object on the Earth’s surface,
whose mass is 15 kg.
A normal force is a force that acts perpendicular to a
surface.
• The normal force ,Fn , is
perpendicular to the surface of
the plane but not always
opposite to the weight Fg . The
normal is opposite to the
component of the weight that is
perpendicular to the contact
surface.
The normal force can be
calculated by:
Fn = mg cos Θ
example
• A box with a mass of 25 kg sits on an inclined
plane at a 35 degree angle to the horizontal.
Find the normal force.
A box on an inclined plane might not slide down the plane if
the angle of the plane is not great enough. A component of
the gravitational force should cause it to slide. What force
counteracts the downhill force?
Friction forces
• Static friction, Fs , is a force
that resists the initiation of
sliding motion between two
surfaces that are in contact
and at rest. As long as the
box does not move, the static
friction is equal to and
opposite in direction to a
force applied parallel to the
surface.
Fs = - Fapplied
When the applied force
exceeds the maximum static
friction force Fs,max, the box
begins to slide.
Kinetic friction, Fk, is the force that opposes the
movement of two surfaces that are in contact and are
sliding over each other.
1.
2.
3.
4.
5.
Properties of friction.
Fk is less than or equal to Fs .
The force of friction (both Fk
and Fs ) are proportional to the
normal force.
Both Fs and Fk are parallel to
the surfaces in contact.
If Fa = Fk ,the box moves at a
constant velocity.
If Fa > Fk ,the box accelerates.
The coefficient of friction is the ratio of the magnitude
of the force of friction between two objects in contact
to the magnitude of the normal force with which the
objects press against each other.
Coefficient of kinetic friction
Coefficient of static friction
µk = Fk
Fn
µs = Fs,max
Fn
example
• A 32 kg box initially at rest on a horizontal floor
requires a 65 N horizontal force to set it in motion.
Find the coefficient of static friction. After it starts to
move, a horizontal force of 45 N will keep it moving
at a constant velocity. Find the coefficient of kinetic
friction.
example
• A student pulls a 30 kg box with a force of 85 N
with a rope that makes a 25 degree angle with
the ground. If the coefficient of kinetic friction
is .22, find the acceleration of the box.
Student practice problems: practice D, 2,3a
practice E 1,3
Fluid friction is the friction exerted on an object as it
moves through a fluid. (gas or liquid)
At relatively low
velocities, the magnitude
of the fluid force is
proportional to the
magnitude of the velocity
As a free falling object accelerates, its
velocity increases as does the air
resistance force, until the force due to
gravity is equal to the fluid resistance
force. At this point the object no longer
accelerates and has reached terminal
velocity. http://www.youtube.com/watch?v=1ukf2vntU44
Four Fundamental Forces
1.
2.
3.
4.
Electromagnetic – friction, magnetic, electrical
Gravitational
Strong Nuclear
Weak Nuclear
Assignment
• Page 143
• Problems 1,2,3,4,5