Physics
Section 2: Acceleration, Forces, FBD
and Newton’s Laws (~20% of Comp)
• Velocity is the rate at which your position changes.
• Acceleration is the rate at which your velocity changes. Acceleration occurs
whenever there is a change in speed, direction, or both. (Sometimes we calculate the
magnitude of acceleration from speed (which is a positive number without
direction) but the sign (+/-) of acceleration is still important. Look for words like
positive, negative, speeding up, slowing down, at rest or stop.)
_
s
• An object is in free fall if it is accelerating due to the force of gravity and no other
forces are acting on it.
o The letter “g” is used for the “free fall acceleration” which is often called
acceleration due to gravity.
• Forces (non-zero net force, unbalanced forces) can change motion.
o A force is what we call a push or a pull, or any action that has the ability
to change an object’s motion.
o Forces can be used to increase the speed of an object, decrease the speed of
an object, or change the direction in which an object is moving.
• Inertia is a term used to measure the ability of an object to resist a change in its
state of motion. Classically, it is a property of matter unchanged by position or
velocity. (Identical objects moving a different velocities have the same inertia.)
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Physics Comp Study Guide: 7th
▪ Action-reaction forces happen simultaneously thus the two objects both
experience an equal magnitude impulses in opposite directions. (The acceleration
of two objects depends upon the mass through F=ma. So, different size masses
experience action-reaction forces (assuming no other forces) will have different
acceleration and thus different final velocities.)
▪ Action-Reaction forces are NOT balanced forces by themselves because they act
on two different objects. (Other forces may balance one or both objects involved.)
▪ An Action force vector and a Reaction force vector are always diagramed on two
separate FBDs (free body diagrams). Each object requires its own FBD. (This
means when we draw one FBD for one object we only showing half of an Action-
Reaction pair.)
Newton’s Universal Law of Gravitation
All objects have mass. All masses share
gravity. Gravity is a mutually felt force between every
pair of object in the universe. Since gravity is an
inverse square law, small things that are far away
have a negligible amount of gravity. See equation ➔
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• A static problem generally means there is no motion. Objects are at rest.
• A dynamic problem involves at least one moving object.
• Equilibrium can be static or dynamic.
o In all equilibriums, there are no forces or balanced forces.
o Balanced force vectors cancel out leaving zero net force.
o The object with zero net force has zero acceleration.
o Zero acceleration means constant velocity.
• Friction results from relative motion between objects.
o Friction comes in two types
▪ Static friction tries to prevent sliding
▪ Kinetic friction opposes the direction of slide
• Newton (N): SI unit of force.
o 1N =
kg m
s2
▪ Net Force is the sum of all the forces acting on an object.
▪ Remember the two equations for Fnet
▪ Sum of all of the forces
▪
Fnet = F1 + F2 + F3
▪
Sometimes there is only one force! Lucky!
▪
Fnet = F1
▪ How the total mass is observed accelerating
▪ Fnet =mtotala
▪ Tension is a reaction force similar to normal force. When a rope or
wire is subject to an applied force or force of gravity of a hanging
object, the tension is simply equal and opposite. Without an applied
force there is no tension.
▪ Free body diagrams
▪ Make a square or circle to represent an object
▪ Label the mass of the object
▪ Define your coordinate system (right and up are positive)
▪ Draw and label all force vectors to scale (size matters)
▪ Write in known values and calculate unknown values
NLUG and Coulomb’s Law: What happens to force when you alter the distance (radius, r) in an inverse square law?
Take the reciprocal of the factor that changes distance and square it. Inverse-square.
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• The coefficient of friction is a ratio of the strength of sliding friction between
two surfaces compared to the force holding the surfaces together, called the normal
force.
o The normal force is the force perpendicular to two surfaces which are
moving relative to each other.
▪ The most common normal force is in response to gravity.
• In many problems, the normal force is the reaction in an action-reaction pair.
• The maximum net force that can be applied before an object starts sliding is called
the force of static friction.
• The coefficient of static friction (μs) relates the maximum force of static friction
to the normal force.
• If an object is on flat ground with balanced gravity and normal force, we know that
normal force is balanced with gravity. N = mg
µN = µmg = Ff