Download Newton`s Laws Notetakers

Newton’s Laws of Motion
FORCE
Newton stated that the change in velocity of an object is caused by FORCES.
When the velocity of an object is constant, or if the object is at rest, it is said to be in equilibrium.
Contact forces: forces that result from physical contact between two objects.
Examples: ______________________________________________________________
Field forces: forces that can act at a distance.
Examples: ______________________________________________________________
FORCE DIAGRAMS
Force is a vector.
Force diagrams – show forces vectors as arrows
Ex:
Free body diagrams – shows only the forces acting on a single object
Ex:
Newon’s First Law (____________________________):
A body at rest will remain at rest, a body in motion will remain in motion, traveling with a constant velocity in a
straight line, unless an unbalanced force acts on it.
INERTIA = a measure of a body’s ability to resist changes in velocity.
(the greater the mass of a body, the less it will accelerate under the action of an applied force)
Why is it easier to push a Volkswagen then a Mack truck?
Newton’s Second and Third Laws
Newton’s Second Law: ______________
The acceleration of an object is directly proportional to the resultant force acting on it and inversely proportional
to its mass.
The direction of the acceleration is in the direction of the resultant force.
The SI unit of force is the NEWTON.
1N = 1 kg•m/s2
Weight: w = ______
Any acceleration requires a force. An acceleration can be ___________________________________.
Example: An elevator accelerates upwards. If Bart steps on the scale, what will it read?
Example: Now the elevator travels upward with a constant velocity. If Bart steps on the scale, what will it read?
Newton’s Third Law
If two bodies interact, the force exerted on a body 1 by body 2 is equal in magnitude, but opposite in direction to
the force exerted on body 2 by 1.
OR…
Action Reaction Pairs:
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Implicit in these laws are the following ideas:
 If ___________________________ acts on a body, its acceleration must be __________.
o An unbalanced force is one whose vector sum does not equal zero.
 If an unbalanced force acts on a body, it must _______________. It will continue to accelerate for as
long as the force(s) are unbalanced.
 If a body has no acceleration, the vector sum of all the forces acting on it must be zero.
Everyday Forces
The ___________ of an object is the force of ____________ acting of the _________ of that object.
USA: Pounds; Everywhere else (and in this class): ______________
Weight =
or
(mass) x (9.8m/s2)
force: the upward force acting on an object at rest on a surface; the normal force is perpendicular
to the surface on which the object is sitting.
Picture:
__________________ (T) is a force acting on a rope. When a rope is taught there is tension in the rope.
________________ (Ff) is a resistive force that opposes motion.
Multi-Object Force Diagrams:


Draw all objects as boxes.
Create a table of interactions within the system. (+ means interaction, - means no interaction 0 means
irrelevant)
 Using the table of interactions, draw the force pairs acting on all objects. (dashed arrows for field forces)
Example 1: Car hits a wall -
Car Wall Earth
Car
Wall
Earth
Example 2: Elephant vs. Man –
Eleph Rope Man
Earth
Ceiling Rope Box
Earth
Eleph
Rope
Man
Earth
Example 3 – Hanging Block
Ceiling
Rope
Box
Earth
How to solve problems with Newton’s Laws:
 Draw the picture or Free Body Diagram
 Label ALL forces (in x and y direction)
 Pick +x (or +y) to be in direction of
acceleration
 Sum the forces in each direction
 Solve
How to draw a free body diagram (FBD):
 All the forces on one object are draw as vectors
(arrows)
 The tail of the arrow starts on the object and point
off in the direction of the force.
 If the force is at an angle, include the angle.
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Example: A car engine will push a 2500 kg car forward with 800. N of force on flat ground. If the air resistance
is 300. N, what is the acceleration of the car?
F = ma
Draw FBD:
Example: What is the tension in the cable of an elevator with a weight of 8800 N that ascends with an
acceleration of 1.30 m/s2?
Example: A coach pulls a 35 kg bag of soccer balls across the field with a force of 84 N directed at an angle of
35o above the horizontal. If the bag travels at a constant velocity.
(a) how strong are the resistive forces from the field on the bag
(b) how strong is the normal force on the bag?
Example: A child holds a sled at rest on a frictionless snow-covered hill. If the sled weighs 100. N, find the
force the child must exert on the rope and the force the hill exerts on the sled.
Forces of Friction
The term friction refers to the resistive forces that arise to oppose the motion of a body past another with which
it is in contact.
____________friction (kinetic friction) is the frictional resistance a body in __________ experiences.
___________ friction is the frictional resistance a _________________ body must overcome in order to be set
in motion.
Where  = coefficient of friction
N = normal force
The magnitude of  depends on the nature of the surfaces in contact
NOTE: friction does NOT depend on the area of contact!
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Example: You need to move a box of books into your dormitory room. To do so, you attach a rope to the box
and pull on it with a force of 90.0 N at an angle of 30.0°. The box of books has a mass of 20.0 kg, and the
coefficient of friction between the bottom of the box and the hallway surface is 0.500. Find the acceleration of
the box.
Air Resistance and Terminal Velocity:
 Air resistance is similar to friction in that it ____________________ the direction of motion.
 Air resistance is the resistance of the air that the object is falling through.
o When the object is _____________________, it is not moving through much air so the air
resistance is ______________.
o The ________________ the object moves, the more air it travels through and the
______________ the air resistance is.
 When the forces are balanced, acceleration stops.
Common Misconceptions:
 When a ball has been thrown, the force of the hand that threw it remains on it for awhile.
o NO! The force of the hand is a ____________ force; therefore, once contact is broken, the
____________________________ exerted.
 Even if no force acts on a moving object, it will eventually stop
 _____________________ are two names for the same thing
 Air does not exert a force.
o NO. _________________________, but because it is balanced on all sides, it usually exerts
__________________ unless an object is moving.
Adding vectors in real life…
To Add Vectors
Step 1: Draw a vector diagram
Step 2: Create data table holding x and y components of each vector and the total x and y components of the
resultant vector.
Step 3: add the vectors along each axis and put in data table
Step 4: draw a vector diagram showing only the vector axis sums from step 3
Step 5: use the Pythagorean Theorem (a2 + b2 = c2) to find the magnitude of the resultant vector.
Step 6a: Use a trig function (usually tan) to find the angle.
Step 6b: specify both magnitude and direction of the vector.
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