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?
Galileo argued that only when friction is present—as it usually is—is a force needed to keep an object moving.
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 ______.
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:
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 -
Example 2: Elephant vs. Man –
Example 3 – Hanging Block
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
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?
Draw FBD: SF = ma
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 m = coefficient of friction
N = normal force
The magnitude of m depends on the nature of the surfaces in contact
NOTE: friction does NOT depend on the area of contact!
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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