Online Physics Classroom Textbook Notes
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Lesson 1: Newton's First Law of Motion
A. Newton's First Law
• Isaac Newton created a three laws to explain why objects move and don’t move.
• His first law is referred to as the law of inertia
• Newton's first Law- An object at rest stays at rest and an object in motion stays in motion with the same speed and in the same direction unless acted upon by an unbalanced force.
• Two Clauses and a Condition
○ If an object is at rest it will stay at rest
○ If an object is in motion, it will stay in motion unless acted upon by an unbalanced force
○ Resist changes
• Everyday Applications of Newton's First Law
○ An unbalanced motion is like a seat belt. It stops a person with force and brings them back to a state of rest.
B. Inertia and Mass
• Inertia- the resistance an object has to a change in its state of motion
• Galileo and the Concept of Inertia
○ Said moving objects eventually stop because of friction
○ He wanted to eliminate friction
○ He wanted to see that if friction was eliminated, then the ball would reach the same height it started
○ Forces Don't Keep Objects Moving
○ Force of friction- the presence of force of friction brings things to rest
§ If there was no force of friction in an object, than the object would continue moving at the same speed and direction forever
§ Mass as a Measure of the Amount of Inertia
§ Mass- a quantity that only relies upon the inertia of an object. The more inertia the object has the more mass it has
§ An object with more mass will resist change
C. State of Motion
• Inertia's motion that it is resisting is velocity. But if there is 0 velocity, inertia is resisting acceleration
D. Balanced and Unbalanced Forces
• Balanced Forces
○ Balanced force: when two forces act upon an object with equal magnitude and opposite directions so they balance the object. The object is at equilibrium maintaining its state of motion.
○ Ex. A person is standing on the floor. The floor is pushing the person up with a normal force and the gravity is pulling the person down with a force of gravity creating equilibrium.
○ Unbalanced Force: causes a state of motion. This is when an object accelerates after coming in contact with another object. There is also friction causing the object to slow down.
○ Ex. A book is traveling across the table. The gravity pushing the book downwards and the table pulling the book upwards creates equilibrium. However, there is friction between the book and table causing it to slow down.
Lesson 2: Force and Its Representation
A. Meaning of Force
• Force- a push or pull on an object as a result of an interaction with another object. When forces are unbalanced there is a change in velocity
• Contact versus Action-at-a-Distance Forces
○ Contact forces: when two objects interact by physically contacting each other.
§ Ex. Applied force, normal force, friction force, air resistance force, tension force, and spring force
○ At a Distance forces: when two objects can exert a push or pull but don’t have to interact physically by contacting each other.
§ Ex. Gravitational Force: planets rotating
§ Electric forces: the electronic pull of the protons in the nucleus of the atom
§ Magnetic force: two magnets having a magnetic pull on each other
○ The Newton
○ Abbreviated using N
○ 1 Newton = 1 kg • m/s2
○ Force is a Vector Quantity
○ Force needs to be described using direction and magnitude.
§ Amount of force and which direction the force is being exerted
§ The normal force and force of gravity has equilibrium (both have 20N). However the book has an unbalanced force with a friction of 5N to the left of the book causing it to slow down.
B. Types of Forces
○ Applied Force, Fap: a force applied to an object by a person or another object
○ Force of Gravity, Fgrav: force of attraction earth has on any two objects. Fgrav = m * g
○ Normal Force, Fnorm: is a support force that is in contact with another stable object
○ Friction Force, Ffrict: the force exerted by a surface
○ Air Resistance Force, Fair: a frictional force that acts upon objects as they travel through the air
○ Tension Force, Ftens: is a force exerted through a rope, cable , or wire when pulled tightly by a force acting upon it
○ Spring Force, Fspring: a force exerted by a compressed or stretched spring upon any object that is attached to it
○ Confusion of Mass and Weight
○ Weight: the force of gravity acting on an object, the pull of the earth
§ Measured in Newton's
§ Depends on gravity (earth vs. moon)
○ Mass: the matter in an object, the matter of an object will be the same no matter the universe
C. Drawing Free Body Diagrams
• They show magnitude and direction of forces
• Length of arrow shows magnitude, and direction of arrow shows direction of force
D. Determining the Net Force-D
• Net Force: the vector sum of all of the forces acting on an object
○ Net force and acceleration are directly proportional
Lesson 3: Newton's Second Law of Motion
A. Newton's Second Law
• Inertia- when forces acting on an object are balanced they have an acceleration of 0m/s/s-> equilibrium (objects that do not accelerate)
• Newton's second law states that an object accelerates when there is a net or unbalanced force acting on the object
○ This changes the objects speed and/or direction.
• Acceleration depends directly on the objects net force- as an objects net force increases, the acceleration will increase (vice versa)
• Acceleration depends indirectly on the objects mass- as mass increases, acceleration will decrease (vice versa)
• The BIG Equation
○ Unbalanced forces cause objects to accelerate with an acceleration that is directly proportional to the net force and inversely proportional to the mass.
○ This is represented in an equation: a=fnet/m (or fnet=m x a)
○ Net Force is a vector sum of all forces
○ 1 Newton = 1 kg x m/s2 : this means that 1 Newton is the amount of force needed to give a 1kg of mass an acceleration of 1m/s2
B. The Big Misconception
• Learning ≠ Storing
○ There are many misconceptions to motion when coming into physics
○ Sustaining motion does not require a continued force
• Are You Infected with the Misconception?
○ Forces cause acceleration, not motion
○ If balanced objects are in motion, they will continue to stay in motion
• Newton's First Law-Revisited
○ Force is not needed to keep an object in motion
○ A book being pushed across a table stops because of friction. Friction brings the book to a rest. If there was no force of friction, the book would continue moving with a constant velocity
C. Finding Acceleration
• Equations for force
○ Equation for Net Force: Fnet= m x a
○ Equation for Gravitational Force: Fgrav= m x g
○ Equation for Frictional Force: Ffrict= μ • Fnorm
D. Finding Individual Forces
• Need two knowns to find third unknown (ex. Must have mass and Fnet to find acceleration, a= Fnet/m
Lesson 4: Newton's Third Law of Motion
A. Newton's Third Law
• Newton's Third Law: for every action, there is an equal and opposite and reaction
• The force of one object will equal the second object
• The direction of force of the first object is opposite to the direction of the force of the second object
• Forces come in pairs- forces coming in contact
• Examples of Interaction Force Pairs
○ A fish swimming in water- force of water equals force of fish moving fins, direction of water is backwards and direction of fish is forwards (opposite)
○ A bird flying- bird moving wings pushing air down, while air is pushing bird upwards (air and bird are balanced)
○ Car moving- wheels push road backwards, and the road is pushing car forwards (opposite direction)
B. Identifying Action and Reaction Force Pairs
• Forces come in pairs- called action-reaction force pairs
○ Who is pushing whom in what direction?
• Hitting a ball with a bat-
○ Baseball forces bat to left, bat forces ball to right
○ Action-reaction force pairs