Chapter 4
The Laws of Motion
Classical Mechanics
• Describes the relationship between
the motion of objects in our everyday
world and the forces acting on them
• Conditions when Classical Mechanics
does not apply
– very tiny objects (< atomic sizes)
– objects moving near the speed of light
Forces
• Force = a push or pull
• Vector quantity
• May be contact or field force
Contact and
Field Forces
Contact: Touching
Field: gravity,
charges, magnets
4.1 The 4 Fundamental Forces of Nature
– 1.
– 2.
– 3.
– 4.
Strong nuclear force
Electromagnetic force
Weak nuclear force
Gravity
– (FYI all 4 are field forces)
1. Strong Nuclear Force
• Strongest force
• Attractive force that holds the nuclei of atoms
together.
2. Electromagnetic Force
• Ex: electrical charges or magnet poles.
• Long-ranged but weak
• Attractive or repulsive force between objects
carrying electrical charge
3. Weak Nuclear Force
•Very short range and very weak.
•Responsible for radioactive decay.
4. Gravitational Force
• Always attractive, and acts between all matter
in the Universe.
• Weak, but very long ranged.
• It’s the dominant force in the universe for
shaping galaxies and stars.
• The forces on the two masses are equal in size
but opposite in direction.
Newton’s 3 Laws of Motion
Inertia
Is the tendency of an object to
continue in its original motion
(or resist a change in motion).
Mass is a measure of inertia.
 Scalar quantity
 SI units are kg

4.2 Newton’s 1st Law of Motion
• “Law of Inertia” = An object at rest stays at rest
and object in motion stays in motion unless
acted upon by a net external force.
• Net Force = the sum of all forces on an object
• Note: Force is a vector, so net force is calculated
by vector addition!
External force
– any force between the object and its environment
*Alternative statement of Newton’s First Law*
– When there are no external forces acting on an
object, the acceleration of the object is zero.
1st Law Implications
• Space objects will move
forever once set in motion
• Friction and air resistance
are the net forces that
usually slow objects.
Newton’s Second Law
(don’t copy this slide!)
The acceleration of an object is directly proportional
to the net force acting on it and inversely proportional
to its mass.

 F
aα
m
But we like to write it (see next slide):
4.3 Newton’s 2nd Law of Motion


F

m
a

If ΣF (net force) = 0, then there is
NO ACCELERATION.
If a constant force is applied to an object,
it will ACCELERATE!
(NOT move at a constant speed)
Force Units
• SI Force unit: Newton
• F = ma =
(kg)(m/s2)
=
FYI:
1 dyne = 1 g cm/s2
100,000 dyne = 1 N
kg= m
1N
2
s
Common
Forces
f
n
T
W