Download 4-4 Everyday forces

5.1 Forces in 2-D; Vectors
5.1 Summary
• When two vectors are at right angles, you can use
the Pythagorean theorem to determine the
magnitude of the resultant vector
• The law of cosines and the law of sines can be
used to find the magnitude of the resultant of
any two vectors
• The components of a vector are projections of
the component vectors
• Vectors can be summed by separately adding the
x- and y-components
5.2 Friction
Weight
• Fg: is the vector quantity directed toward the
center of Earth (called: weight)
• When mass and acceleration of gravity are
known weight can be calculated
• Fg = mg
The Normal Force
• A television is on a table: why doesn’t the TV
continue to fall to the center of the earth
• The TV is in equilibrium (not moving), the TV
has Fg, therefore it has a normal force (Fn) (in
the opposite direction that is equal to Fg)
Force Normal
• Use “normal” because the direction of the
contact force is perpendicular to the table
surface
• “normal” means perpendicular
• Normal force is always perpendicular to the
contact surface
Force of Friction
• Friction opposes the applied force
• Static friction (Fs) the resistive force that keeps
the jug from moving
• As long as the jug does not move,
Fs = -Fapplied
• When the applied force is as great as it can be
without causing the jug to move it has reached its
maximum Fs,max
Force of Friction
• When the applied force on the jug exceeds
Fs,max, the object moves.
• There is still a frictional force acting on the
object as it moves, but it is less than Fs,max
• Kinetic friction (Fk) the resistive force that
opposes the two contacting surfaces that are
moving past one another
Force of Friction
• When two surfaces are stationary with respect to
each other, the surfaces stick together somewhat
at the contact points
• Adhesion is caused by electrostatic forces
between molecules of the two surfaces
• Due to adhesion, the force required to cause a
stationary object to begin moving is usually
greater than what is necessary to keep it moving.
Demo 1: Static vs. Kinetic Friction
• Is Kinetic friction less than static friction?
• Procedure: Use the spring scale to measure the
force required to start the rectangular block
moving. Then use the spring scale to measure
the frictional force for constant velocity. Perform
several trials. Have students record all data and
find the average for each. (note: normal force
and the surfaces remain the same, the only
difference in the two average values is due to
motion)
Demo 2: Static vs. Kinetic Friction
• Does friction depend on the surface?
• Procedure: attach the hook to one of the two
uncovered sides of the block. Pull the block
across the table with the spring scale. Repeat the
demo. With each new surface of the cube
exposed to the table. Repeat with a different
cube and different surfaces (glass, carpeting,
sandpaper) Summarize results and reach a
conclusion
Coefficient of friction
• Coefficient of friction: the ratio between the
normal force and the force of friction between
two surfaces.
• Coefficient of friction: the ratio of the force of
kinetic friction to the normal force.