Forces Guided Notes Packet 2
Name:___________________
Friction

Friction _________________________________

Friction is dependent on the _____________________of the surfaces

Friction is dependent on __________________________
Equation for Friction
  has no units
 depends on _________________________________________
 Higher  = ______________________ surface / ____________________ friction
Note: Friction does NOT
depend on the surface area of
contact
The coefficient of friction is different for when an object is at rest and when it is moving.
•
μs = coefficient of ____________ friction (object at rest)
•
μk = coefficient of ____________________ friction (object moving)
Static friction is ________________ than kinetic friction - its harder to start an object moving than it is to keep it
moving.
We do problems1)
A 28 kg crate initially at rest on a horizontal floor requires a 75 N horizontal force to set it in motion. Find the
coefficient of static friction between the crate and the floor.
2) A force of 40.0 N accelerates a 5.0-kg block at 6.0 m/s2 along a horizontal surface.
a. How large is the frictional force?
b. What is the coefficient of friction?
3) A 12 kg suitcase is pushed with a force of 38 N to the left. If the coefficient of kinetic friction between the suitcase
and the floor is 0.3, how far will the suitcase move after 5 sec?
Checking your understanding
You Do Problems –
1) A 30 kg crate requires a 53 N force to keep it moving at 1 m/s. Find the coefficient of kinetic friction.
2) You need to move a 105-kg sofa to a different location in the room. It takes a 403-N force to start the sofa
moving. What is the coefficient of static friction between the sofa and the carpet?
Apparent Weight / Elevator Problems
Question: How does the weight of a person in an elevator depend on the motion of that elevator?
What will the scale show if the elevator is
1. at rest or moving with constant speed
2. speeding up
3. slowing down
Newton’s 3 law: Force with which the person acts on the scale (reading of the scale) is equal to the normal force on the
person. So, if we find normal force we know the reading of the scale, so called APPARENT WEIGHT.
You Do Apparent Weight
You are riding in an elevator holding a spring scale with a 1kg mass suspended from it. You look at the scale and see that
it reads 9.3 N. What, if anything, can you conclude about the elevator’s motion at that time?
Force at Angles & Inclined Planes Guided Notes
Forces applied at an angle
When forces are applied at angles other than 90o, we need to resolve the force into its component vectors.
Then, we find the net force in each direction, write our force equations (Fnet = ma), and solve.
Forces applied at an angle – we do
Luke Skywalker starts to pull a sled with Princess Leia across a large ice pond with the force of 100 N at an angle of 30.0°
with the horizontal. Find normal force and initial acceleration if the weight of sled and Princess Leia is 800 N and the
friction force is 40 N.
Forces applied at angles – You do
1. A box of books weighing 325 N moves with a constant velocity across the floor when it is pushed with a force of 425
N exerted downward at an angle of 35.2 degrees below the horizontal. Find the coefficient of friction between the box
and the floor.
2. Two forces act on a 4.5-kg block resting on a frictionless surface as shown. What is the magnitude of the horizontal
acceleration of the block?
Inclined Planes
Draw a free-body diagram for a block accelerating down a ramp.
Make x be ____________________ to
incline and y be _____________________.
Now, only ________________ has to be
resolved into components.
Motion and all the other forces will be in
the x or y direction.
Notice that the angle for resolving mg is the
same as the angle of the incline
Inclined Planes – We do
A cute bear, m = 60 kg, is sliding down an iced incline 300. The ice can support up to 550 N.
(a) Will the bear fall through the ice?
(b) If the coefficient of the friction is 0.115, what is the acceleration of the bear?
What’s my strategy??
1) Draw the free-body diagram
2) Choose a coordinate system with x parallel to incline
3) Resolve mg into components
4) Add vectors perpendicular to plane and set Fnet = ma = 0.
5) Add vectors parallel to plane and set Fnet = ma.
Inclined plane – you do
3. A block weighing 15.0 newtons is on a ramp inclined at 40.0° to the horizontal. A 3.0 Newton force of friction, Ff , acts
on the block as it is pulled up the ramp at constant velocity with force F, which is parallel to the ramp.
Find F.
4. A 75 kg box slides down a ramp inclined at 25O with an acceleration of 3.60 m/s2.
a) Find the coefficient of friction.
b) What acceleration would a 175 kg box have on this ramp?
Tension Guided Notes
Name______________________
Class______________
How to solve Tension problems
Follow the same procedure as other force problems, but keep in mind:
1) Draw a free body diagram for EACH object or for each junction in a rope.
2) Remember that the tension on opposite sides of a rope is equal and opposite.
3) Create a system of equations – write Fnet equations for each object / rope and set them equal to each other.
We do problems
1) Two blocks are connected by a string and pulley as shown. Assuming that the string and pulley are massless, find
a) the magnitude of the acceleration of each block
b) Tension force on the blocks
2) A 10-kg block is connected to a 40-kg block as shown in the figure. The surface on which the blocks slide is
frictionless. A force of 50 N pulls the blocks to the right.
a) What is the magnitude of the acceleration of the 40-kg block?
b) What is the magnitude of the tension T in the rope that connects the two blocks?
Check your understanding
You Do
1) Find the tension in each rope.
2) Find the tension in each cable and the acceleration of the blocks. M1 = 10 kg, and M2 = 5 kg
Universal Gravitation Guided Notes
Newton’s Law of Universal Gravitation
One of the most significant intellectual achievements in the history of thought.
•
Every object attracts every other object.
•
The force between two objects is ____________________________________ and
______________________________ to the square of the __________________ between their centers.
•
The force acts along the line joining the two objects.
m1m2
F=G 2
r
G = 6.67x10-11 Nm2/ kg2
“Universal gravitational constant”
How does universal gravitation relate to the force of gravity on Earth?
We Do Problems
1) Two satellites of equal mass orbit earth. Satellite A is 2 times farther from the center of earth than Satellite B.
How does the gravitational force from the Earth on Satellite A compare to that on Satellite B?
2) A 90 kg satellite orbits the earth at a distance of 300 km. The mass of the earth is 6.0 X 1024 kg and the radius of the
earth is approximately 6.4 X 106 m. What is the gravitational force between the satellite and the earth?
You Do Problems
1) Two satellites of orbit earth. Satellite A has twice as much mass as Satellite B but has ½ the orbital distance.
How does the gravitational force from the Earth on Satellite A compare to that on Satellite B?
2) Calculate the force of gravity between Earth (mass = 6.0 x 1024 kg) and the moon (mass = 7.4 x 1022 kg). The
average Earth—moon distance is 3.8 x 108 m.