4 4 Everyday Forces 4 Everyday Forces

4 ­ 4 Everyday Forces

Weight
• Weight: the magnitude of the force of gravity
acting on an object (scalar)
• W=mg (Weight = mass x free­fall acceleration)
• Force of gravity, F g , is a vector

Practice
• A 4 kg book is dropped on Earth
• What is its acceleration?
• What is its weight?
• What should it weigh on a planet where g is 3.5
times Earth’s gravity?
• A 5 kg book is at rest on a table
• What is its acceleration?
• What is its weight?

Normal Force
• Normal force: a contact force exerted by one
object on another in a direction perpendicular
to the surface of contact
• F n or N
• F n is NOT always equal to F g
• F n and F g are NOT action­reaction pairs

Force of Friction
• Friction opposes the applied force
• Friction opposes motion
• Friction depends on surface contact
• Air resistance is a form of friction
• F R is proportional to speed
• Terminal velocity: free­fall where F R =F g

Static Friction
• Static friction: the force exerted on a motionless
body by its environment to resist an external
force
• As the applied force increases, the force of static
friction also increases until it reaches its
maximum value
• F s ≤ μ s F n (Static force of friction ≤ coefficient of
static friction x force normal)
• F s,max = μ s F n (Maximum static force of friction =
coefficient of static friction x force normal)

Kinetic Friction
• Kinetic Friction: the force exerted on a moving
object
• Kinetic friction is less than static friction
• F k = μ k F n (Force of friction = coefficient of
kinetic friction x force normal)

Practice
• It takes a force of 80 N to set a 22 kg crate in
motion. Find the coefficient of static friction.
• If it takes only 65 N to keep the same crate
moving at a constant velocity, what is the
coefficient of kinetic friction.

Practice
• A museum curator moves artifacts into place.
Find F s,max and F k to move a 120 kg aluminum
statue across a steel floor if μ s =0.61 and
μ k =0.47.
• Note: a table of coefficients can be found on
page 144

Practice
• A box weighing 400 N moves with a constant
velocity across the floor when it is pushed with
a force of 500 N exerted downward at an angle
of 35° below the horizontal. Find μ k between
the box and the floor.

Practice
• A student moves a box of books down the hall
by pulling on a rope attached to the box. The
student pulls with a force of 150 N at an angle
of 20° above the horizontal. The box has a
mass of 30 kg, and the μ k between the box and
floor is 0.25. Find the acceleration of the box.

Practice
• A 10 kg bag of groceries is in equilibrium on
an incline of angle Ө = 20°. Find the normal
force of the incline on the bag.

Practice
• A 80 kg box slides down a 20° ramp with an
acceleration of 1.5 m/s 2.
• Find μ k between the box and the ramp.
• Find the acceleration a 175 kg box would have on
this ramp.