FRAMES OF REFERENCE
• Velocity measurements differ in
different frames of reference.
• Observers using different frames of
reference would generally not agree
on some features of motion.
• Write down all information that was
given in the problem and that you
want to know in the forms of
velocities with some kind of
EXAMPLE PG 108 SAMPLE 3F
A boat heading north crosses a wide river
with a velocity of 10 km/hr relative to the
water. The river has a uniform velocity of
5.0 km/hr due east. Determine the boat’s
velocity with respect to an observer on the
(pg 109 #1)
A passenger at the rear of a train traveling
at 15 m/s relative to Earth throws a baseball
with a speed of 15 m/s in the direction
opposite the motion of the train. What is
the velocity of the baseball relative to Earth
as it leaves the throwers hand?
4.1: CHANGES IN MOTION
• Force: a push or pull exerted on an
• Forces cause a change in velocity.
• Force is a VECTOR QUANTITY.
• SI unit of force is the Newton (N).
• Forces can act through contact or at a
CONTACT VS FIELD FORCES
• Contact Forces – result from physical
contact between two objects
• Usually easy to identify
• A physical push or a pull
• Field Forces – do not involve physical
contact between two objects
• Example: Electric Force, GRAVITY!!!
FBD: FREE BODY DIAGRAMS
• Helps analyze a situation
• Isolate an object and the forces acting
on that object.
• FBDs are used to show all the external
forces acting on an object.
HOW TO FBD
1. Draw your object as single point. (forces are
assumed to act on a single point at the center of an object)
2. Draw and label vector arrows representing
all external forces acting on the object.
3. Make sure you are only drawing the forces
acting on the object and NOT the forces that
the object acts on other things.
4. When finished, an FBD can be used to find
the net external force on an object.
• In the 1630s, GALILEO – yes not Newton
– realized that a block sliding on a
perfectly smooth surface would slide
forever in the absence of an applied
• In 1687, Newton further developed the
concepts that were initially developed by
Galileo which has now come to be known
as Newton’s 1st Law of Motion
NEWTON’S FIRST LAW
An object at rest remains at
rest, and an object in
motion stays in motion,
unless acted upon by an
LAW OF INERTIA
• Newton’s 1st Law came to be known as
the “Law of Inertia”
• Inertia = tendency of an object to not
• The more mass something has, the
harder it is to …
• Slow it down …if it is already moving
• Get it to start moving … if it is at rest
WEIGHT VS. MASS
• Mass = the amount of matter something
• Weight = the magnitude of the force of
gravity acting on an object.
**You need mass to calculate weight.
MASS ≠ WEIGHT
THE FORCE OF GRAVITY
• The force of gravity as we talk about it in
everyday life is called WEIGHT.
• We will call the force of gravity Fg.
• We can calculate the force of gravity
using the following formula.
Fg= Force of Gravity, measured in Newtons
m= mass, measured in Kilograms
g= acceleration due to gravity, -9.8 m/s2
A bag of sugar has a mass of 2.26 kg. What
is its weight?
If a the mass of Mickey Mouse is 24 kg. Calculate
Mickey’s weight on Earth.
THE NORMAL FORCE
• The Normal Force= a force exerted by
one object on another in a direction
perpendicular to the surface of contact.
A block of mass 26 kg is sitting stationary on a 1 m high
table. Calculate the block’s weight and normal force.
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