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MR. SURRETTE
VAN NUYS HIGH SCHOOL
CHAPTER 7: MOTION
CLASS NOTES
MECHANICS
Simply put, the study of motion is the study of anything that moves. In physics, the study of motion is
called mechanics.
BRANCHES OF MECHANICS
Mechanics is divided into two branches: kinematics and dynamics. Essentially, kinematics is the “how
of motion” (descriptive) and dynamics is the “why of motion” (explanatory).
KINEMATICS
Kinematics includes the measurement of distance, speed (velocity), and acceleration.
SPEED
Any object in motion moves a certain distance in a given time. Speed is a measure of how fast
something is moving:
s=d/t
(Speed [m/s] = Distance [m] / Time [s])
TYPES OF SPEED
The speed an object has at any given moment is called instantaneous speed. It is the speed shown by
the speedometer of a car. It is rare that a car or any other moving body travels an entire distance at the
same speed. This is why we usually measure the average speed of the entire trip.
Example 1. What is the average speed if a car travels 320 kilometers in 4 hours?
1A.
(1) Speed = Distance / Time
(2) s = d / t
(3) s = 320 km / 4 hours
(4) s = 80 km / hour
VELOCITY
Velocity is speed combined with the direction of motion. When a car travels 60 kilometers per hour,
that is the speed of the car. If a car travels 60 kilometers per hour north on the highway, that is its
velocity.
VELOCITY EQUATION
v = d / t
(Velocity [m/s] = Distance [m] / Time [s])
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MR. SURRETTE
VAN NUYS HIGH SCHOOL
ACCELERATION
When the velocity of an object changes, the object accelerates. Acceleration is the rate of change in
velocity:
a = v / t
(Acceleration [m/s2] = Velocity [m/s] / Time [s])
Example 2. A car goes from rest to 20 m/s in
6 s. What is its acceleration?
2A.
(1) a = v / t
(2) a = 20 m/s / 6 s
(3) a = 3.33 m/s2
FREE-FALL VELOCITY
Objects fall to the ground because of Earth’s gravity. When an object falls without friction, it is in freefall. The influence of Earth’s gravity on freely falling objects is represented by the letter g. The value of
g is 9.8 meters per second, every second. This means:
g = 9.8 m/s2
FREE-FALL VELOCITY
For an object falling from rest, the velocity v at time t can be expressed:
v = gt
Example 3. How fast does a rock in free-fall move after 6 seconds?
3A.
(1) v = gt
(2) v = (9.8 m/s2)(6 s)
(3) v = 58.8 m/s
FREE-FALL DISTANCE
The distance traveled by an object in free-fall starting from rest is:
d = ½ gt2
Example 4. How far does a rock in free-fall travel after 6 seconds?
4A.
(1) d = ½ gt2
(2) d = ½ (9.8 m/s2)(6 s)2
(3) d = 176.4 m
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MR. SURRETTE
VAN NUYS HIGH SCHOOL
DYNAMICS
Kinematics uses distance, velocity, and acceleration to describe motion. Dynamics analyzes motion in
terms of forces. The laws of motion were formulated by Isaac Newton three centuries ago.
NEWTON’S FIRST LAW
Newton’s first law is the Law of Inertia: “an object at rest will remain at rest and an object in motion
will remain in motion with a constant velocity unless acted on by a net external force” (force is later
defined in Newton’s Second Law).
MASS VERSUS WEIGHT
Mass is the same as inertia. The greater an object’s mass, the harder it is to change its motion. For
example, interstate trucks take much longer to speed up and slow down than motorcycles.
MASS VERSUS WEIGHT
Weight is a mathematical relationship:
w = mg
Example 1. Calculate the weight of a 10 kg object on earth:
1A.
(1) w = mg
(2) w = (10 kg)(9.8 m/s2)
(3) w = 98 N
(A “newton” is explained in Newton’s Second Law).
NEWTON’S SECOND LAW
Newton’s second law, the Law of Acceleration, states: “the acceleration of an object is directly
proportional to the resultant force acting on it and inversely proportional to its mass.”
NEWTON’S SECOND LAW
Newton’s second law is written mathematically as:
F = ma
Force [N] = (Mass [kg])(Acceleration[m/s2].
(One Newton is about 0.25 pounds.)
Example 2. How much force is needed to accelerate a 3 kg mass by 4 m/s2?
2A.
(1) F = ma
(2) F = (3 kg)(4 m/s2)
(3) F = 12 N
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MR. SURRETTE
VAN NUYS HIGH SCHOOL
Example 3. What acceleration is produced by a
10 N force pushing on a 3 kg mass?
3A.
(1) F = ma
(2) a = F / m
(3) a = 10 N / 3 kg
(4) a = 3.33 m/s2
FRICTION
Friction is a resistive force that opposes the motion of an object. For example, air resistance slows
down the motion of falling objects.
NEWTON’S THIRD LAW
Newton’s third law, the Law of Action-Reaction, states: “when two bodies interact, the force which
“A” exerts on body “B” (the action force) is equal in magnitude and opposite in direction to the force
which body “B” exerts on body “A” (the reaction force).”
NEWTON’S THIRD LAW
A consequence of the third law is that forces occur in pairs. Remember that the action force and the
reaction force act on different objects.
NEWTON’S THIRD LAW
Graphically, this can be represented as:
Note: Action and Reaction forces occur at the same time.
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