Physics Review Unit IV
Name_______________________
What are they in your words?
Newton I “Laziness Principle
Newton III “Bully Principle”
Vocabulary
inertia
weight
newton
net force
action/reaction forces
Explain the difference between force pairs and opposite and equal forces on an object?
Explain why force pairs don’t cancel each other?
1.
Determine tension in each cable. (Hint: There is more than one way to define the
system.)
2. The box on the frictionless ramp is held at rest by the tension force. The mass of the box is
65 kg.
T
What is the value of the tension force?
What is the value of the normal force?
3. A man pulls a 45 kg box at constant speed across the floor. He applies a 168 N force at an
angle of 30°.
a.
b.
Sum the forces in the x-direction. What is the value of the frictional force
Sum the forces in the y-direction. What is the value of the normal force?
Remember dry ice.....
Draw force diagram for dry ice sitting still on table.
Draw force diagram for dry ice moving down table a few seconds after it is pushed.
Are they the same?
Why/why not?
Draw force diagram for dry ice while Priest applies a constant force in the direction it is moving.
Solving force problems:
1. Draw force diagram with each force identified
How?
*only one non-contact force = Fg
* “walk” around object to find other forces - all must be touching object
*the support force(like ground) may provide 2 forces if friction is present - Fn & Fk or s
*all forces are recorded in the exact direction they are applied to the object
*if the object is on a ramp it can be rotated - easiest to rotate so Fn is vertical
*figure new angle of gravity, easiest to measure from 0. (Priest’s rules)
*diagonal forces must be broken into their components, vertical(y) part & horizontal(x)
Use sin and cos.
2. Write net force equations
*if object is stopped or going at constant speed EQ = 0
*if object is accelerating EQ = ma
*be very careful to determine if acceleration is positive or negative
(Object speeding up, accel in same direction as motion)
(Object is slowing down, accel in opposite direction as motion)
*Fg = mass x 9.8 m/s2 (can use 10 m/s2)
3. Substitute values into EQ and solve.
Newton’s III
Always involves 2 objects and forces between them.
The first object CANNOT push any harder on the second than it pushes back.
Opposite & equal forces between 2 objects