Download Notes: Forces and the Laws of Motion

Notes: Forces and the Laws of Motion (Ch. 4 in Text)
Name: ______________________________
What is a force? A
N or Newton’s
push or a pull
Units:
Represented with:
Force Diagrams:
“Net Force/Resultant Force”:
The vector sum of all forces

Remember your rules of vectors!
**Net forces are NEVER drawn on a free-body diagram**
*Balanced Forces – when a Net Force = 0 or Equilibrium:
Ex:
*Unbalanced Force - when a Net Force≠ 0 :
Ex:
Some Common Forces (section 4-4):
Weight (gravitational force)
Weight: the force of gravity acting downward on an object
Depends on where you are!!! Units: N
Mass: how much matter makes up an object
NEVER CHANGES - kg
Formula: W = mg (Weight = mass*gravity)
You try:
1. If the same astronaut stood on Mars where the acceleration due to
gravity is 3.7 m/s2, how much would the astronaut weigh?
2. Sally has a mass of 50 kg. What is her weight (in Newtons) on earth? On the moon? On mars?
Normal Force:
Example: Crate on ground
Example: Crate on Incline
Friction or Drag forces:
Tension :
Push and Pull:
HW: Section 4-1; 1-5 and Section 4-4, pg 143; 1,2,5 and Free Body exercises (use pencil!)
Notes: Newton’s Three Laws (section 4-2, 4-3):
Name: __________________________
Consider a satellite drifting through space to Pluto? What is pushing or pulling it along? Why does the spacecraft
continue to move?
Inertia:
Determined by:
Examples - Which object has more inertia?
1.
The Earth or a soccer ball?
2. A full shopping cart or an empty one?
3. A moving car or as still car?
Newton’s First Law states that:
In other words:
Practice Explaining using Newton’s Laws
Formula: 1. State the law “Newton’s First law says….”
2. Connect the objects in the situation to the law “ The objects on the table are at objects at rest….”
3. State what happens and why “Since there is no unbalanced force acting on the objects, only the table cloth,
the objects….”


Why does the table cloth trick work?
Why do we wear seatbelts?
Now, once a net force is exerted on an object, what determines how it will move?
Newton’s Second Law states that:
In other words:
Examples:
1. A boy pushes forward a cart of groceries with a total mass of 40.0 kg. What is the acceleration of the cart if the
net force on the cart is 60.0 N?
2. Ms. Andi is on a rolley chair on a nearly frictionless surface. Mr. Door pushes her to the right with 25N and Ms.
Brock pushes her to the left with a force of 45N. If Ms. Andi is 52kg, with what acceleration does she move?
(draw a picture, find the net force then set = to m*a)
3. A 525kg car travels from 0.0 m/s to 60.0 m/s in 3.0 s. What is the net force acting on it? (hint: find the
acceleration, then the force)
4. A marathon runner wants to accelerate toward the finish line for the last 50.0 m of the race. She is running at a
speed of 5.0 m/s and gets to a speed of 10.0 m/s just as she crosses the line. The amount of force needed to do
this is 57N. What is the runner’s mass?
Textbook Problems: pg. 132 Practice C
Newton’s Third Law:
Action Reaction Pairs:
Examples: For each situation described above, create a quick sketch and determine the action force and the
reaction force
1. You walk forward
4. Apple falls to the ground (on its way down!)
2. Book rests on table
5. Rocket accelerates in space
3. Thrown ball is caught by catcher
6. Sailboat sails forward
HW: Section
4-2 Review 1,2,5 only,
Section 4-3 Review 1,2,3,5 only
Friction
Types of Friction:
Example:
1. _____________________
2. _____________________
3. _____________________
4. _____________________
Calculating Sliding Friction:
Coefficient of Friction:
Static:
Kinetic:
Formula:
Examples:
A 24 kg crate initially at rest on a horizontal floor requires a 75 N horizontal force to set it in motion. Once it starts
moving, it requires a horizontal force of 55 N to keep it going. Find the coefficient of static friction and the coefficient of
kinetic friction between the crate and the floor.
HW: pg. 139 Practice D all
Problem Solving with Newton’s Laws:
1. Find the acceleration of the crate. This is a frictionless surface.
a)
b)
25 N
25 N
37
37
4.0 kg
4.0 kg
2. A traffic light weighing 100 N hangs from a cable tied to two other cables fastened
to a support as shown below. The upper cables make angles of 37º and 53º degrees
with the horizontal. Find the tension in the three cables.
3. A 6.0 kg mass placed on an inclined plane is held stationary by a rope attached to the wall as shown
a) Calculate the magnitude of the tension in the rope.
b) The rope breaks causing the mass to side down the inclined plane. Find the
acceleration of the block down the ramp.
37
4. The pulley in the figure is frictionless and the rope has negligible
the system is released the tension in the rope is 16 N.
a)
Find the size of the acceleration.
b)
m2=?
mass. When
m1=8.0 kg
Find the mass of m2.
5. Find the frictional force between the mass and the table that will keep the system in static equilibrium as shown.
37
2.0 kg
1.0 kg
k=?