Physics 213
General Physics
Chapter 5
Laws of Physics
True or False?
• All things in nature obey the laws of
physics
• Most things in nature obey the laws of
physics
• Some things in nature obey the laws of
physics
Laws of Physics
• Man-made
• Based on our observation of God’s creation
Newton’s 1st Law
• Objects at rest tend to remain at rest
unless acted upon by an external force
• Objects in motion tend to remain in motion
unless acted upon by an external force
Newton’s 1st Law
• Simply put; objects tend to resist changes
in their state of motion
• Putting it another way, objects tend to
resist acceleration
• This tendency to resist changes in their
motion is called inertia
Force
A force is required to cause a
change in motion
Force
A force is required to cause an
change in motion acceleration
Force
A force is required to cause an
change in motion acceleration
Newton’s 1st Law
Types of Forces:
•
•
•
•
•
•
•
Gravitational
Electrical
Magnetic
Normal (support)
Friction
Tension
Elastic (spring force)
Newton’s 2nd Law
• Newton observed that if a force is exerted
on an object, the object accelerates
• By measuring displacements and times,
he was able to calculate the corresponding
acceleration
Newton’s 2nd Law
F → a
Newton’s 2nd Law
• Newton observed that an increase in force
results in an increase in acceleration
• Likewise a decrease in force results in a
decrease in acceleration
• The acceleration is directly proportional to
the force causing the acceleration
• By changing the magnitude of the force
applied to the same object, the
acceleration changed proportionally
Newton’s 2nd Law
F a
Newton’s 2nd Law
F a
How do you turn the ≈ into = ?
Newton’s 2nd Law
F a
How do you turn the ≈ into = ?
How do you turn the proportionality into an equation?
Newton’s 2nd Law
F  ma
Newton’s 2nd Law
F

ma

Newton’s 2nd Law


F

m
a

Newton’s 2nd Law
Examples 1, 2, 3
Newton’s 3rd Law
When an object exerts a force on a second
object, the second object exerts an equal
and opposite force on the first object
If I push on the wall with a force of 10 lbs,
the wall pushes back on me with a force of
10 lbs
Newton’s 3rd Law
What force causes an automobile to move?
What force causes a propeller-driven airplane to
move?
What force causes a rowboat to move?
What force causes you to move when you walk?
If a small sports car collides head-on with a
massive truck, which vehicle experiences the
greater impact force? Which vehicle
experiences the greater acceleration due to
the impact?
Newton’s 3rd Law
When a car collides with a mosquito, both feel
the same force. Why does the mosquito
stop while the car continues, seemingly
unaffected?
Summary of Newton’s Laws
1st
Objects tend to resist changes in motion
2nd
An object’s acceleration is proportional to the
net force applied to it
3rd
Every force has an equal and opposite force