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Applications of Forces
AP Physics 1
Apparent Weight
 The weight of an object is the force of
gravity on that object.
 Your sensation of weight is due to
contact forces supporting you.
 This force could either be a normal force
or a tension, depending on the situation.
 The most common example is your
apparent weight while standing in an
elevator.
 Sometimes you feel heavy in an elevator,
and sometimes you feel light. That
feeling is the normal force that pushes
up on you.
Apparent Weight
 If you are moving upwards and speeding up, your
acceleration points up. This means the net force
on you must also point up, which means that the
normal force must be larger than gravity. This is
why you feel heavy!
𝐹𝑛𝑒𝑡 = 𝑚𝑎
𝑁 − 𝑚𝑔 = 𝑚𝑎
𝑁 = 𝑚𝑎 + 𝑚𝑔 = 𝑚(𝑎 + 𝑔)
 If your acceleration points down, then it is
negative. This would mean that you feel lighter
than you are because normal force is smaller than
gravity.
Example
Brian’s mass is 70 kg. He is standing on a scale in an elevator that is moving at 5.0 m/s.
As the elevator stops, the scale reads 750 N.
Before it stopped, was the elevator moving up or down?
What is the magnitude of Brian’s acceleration?
How long did the elevator take to come to rest?
Suspended Objects
 Another common example in forces is an
object being suspended from multiple ropes.
 The key to solving these problems is to break
forces into horizontal and vertical components
and use Newton’s second law in both
directions.
 The net force on the object is equal to zero!
Example
If the suspended mass weighs 50 N,
calculate the tension in the horizontal
rope.
Inclined Planes
 A common force problem in physics
involves boxes on inclined planes.
 In these kinds of problems, we have to
break forces into components and use
Newton’s second law to calculate forces
and accelerations that are parallel and
perpendicular to the ramp.
Example
A 46 g domino slides down a 30 degree incline at a constant speed. What is
the coefficient of friction?
Example
A boy and his sled have a combined mass of 65 kg. What is their acceleration
as they start down an icy 22.6 degree incline with a coefficient of friction equal
to 0.10?
The boy is then pulled back to the top of the hill at a constant speed by a tow
rope. What is the tension in the rope?
Atwood Machines
 Atwood machines are devices that use pulleys and ropes to raise or lower
masses.
 The keys to solving Atwood problems:
 The tension in a single rope is the same everywhere (if the rope is
massless!).
 The magnitude of acceleration (if there is any) is the same for all objects in
the system.
 FBD on each object in the system!
Example
A 200 kg set used in a play is stored in the loft
above the stage. The rope holding the set passes
up and over a pulley, then is tied backstage. The
director tells a 100 kg stagehand to lower the set.
When he unties the rope, the set falls and the
unfortunate man is hoisted into the loft. What is
the stagehand’s acceleration?
Example
A mass, m1 = 3.00kg, is resting on a frictionless horizontal table is connected to a
cable that passes over a pulley and then is fastened to a hanging mass, m2 = 11.0 kg
as shown below. Find the acceleration of each mass and the tension in the cable.