Physical Science Chapter 3 Notes
Forces and Newton’s Laws
SECTION 1 (FORCES)
1. Force--is a push or pull that one body exerts on another.
a) Balanced forces—are forces that are equal in size and opposite in direction.
b) Unbalanced forces-- tend to cause an object to accelerate due to some net force.
c) Net Force—is the sum of all forces acting on and object.
i) (symbol is F, unit is the Newton (N) where 1 N = kgm2/s2) F = ma
ii) Newton--is the SI unit for force. One Newton is the force required to move a 1 kg object at a velocity of 1 m/s.
2. Friction - the force that opposes motion between surfaces. The amount of friction between two surfaces depends on how hard
the surfaces are forced together, the materials that make up the surfaces, and the smoothness of the surfaces.
a) Static friction - the friction between two surfaces that are not moving past each other.
b) Sliding friction - the force that opposes the motion of two surfaces sliding past each other.
c) Rolling friction - the friction between a rolling object and the surface it rolls on.
i) Coefficient of Friction--is a unit less number between 0 and 1.
(1) Coefficient is zero when the two surfaces slip easily past one another and 1 when two surfaces those almost stick
together.
(2) (Symbol μ) Equation that we use is μ =Ff/Fn.
Friction exists with liquids and gases as well as with solids. Friction between a solid surface and a liquid or gas is called
drag. When the gas is air, however, the friction is usually called air resistance
3. Gravity—is an attractive force between any two objects that depends on the masses of the objects and the distance between
them.
a) Universal law of gravitation—the force of attraction between two bodies. Every mass in the universe is attracted to
every other mass in the universe. The attractive force depends upon the mass of the two objects and the distance between
the two objects.
b) Gravitational Field—A field is a region of space where the density of the lines of flux (field are the same).
4.
Weight – The acceleration due to gravity pulling one object toward the center of another object. Because weight is a force it
is measured in Newton’s.
a) Weight and mass are not the same thing. Weight is a force and mass is a measure of the amount of matter an object has.
Weight is a special case of Newton's second law:
force = ( mass )( acceleration )
f = ma
Weight = ( mass )( gravity )
W = mg Where g is a special acceleration--the acceleration due to gravity.
SECTION 2 (NEWTON’S LAWS OF MOTION)
1) Newton’s First Law—an object at rest will remain at rest and an object in motion will remain in motion unless acted on by an
outside force.
a) Inertia—is the tendency of an object to resist change in its motion.
2) Newton's Second Law of Motion – acceleration varies directly as force and inversely as mass.
a) This las describes how force, mass, and acceleration are connected. The net force acting on an object causes the object to
accelerate in the direction of the net force.
Force = ( mass )( acceleration )
F = ma
net force
F
, or: a=
mass
m
N
kgm
m
Units for acceleration are ⇒
⇒ 2 ⇒ 2
kg
s kg
s
acceleration =
3) Newton's Third Law of Motion - when an object exerts a force on a second object, t he second object exerts a force on the
first that is equal in size and opposite in direction. In other words for every action there is an equal and opposite reaction.
a) Action-reaction forces are acting different objects, so even if the forces are equal they are not balanced because they act
on different objects.
SECTION 3 (USING NEWTON’S LAWS)
1) Air Resistance—is a friction-like force that opposes the motion of object as they move through the air.
2) Terminal velocity –is the maximum speed an object will reach when falling through a substance, such as air.
3) Projectile Motion - the curved path of an object caused by the inertia of the object and the acceleration of gravity.
a) The inertia of the object causes it to travel in a straight line.
Physical Science Chapter 3 Notes
Forces and Newton’s Laws
b) The acceleration due to gravity pulls the object toward the center of the Earth
4) Centripetal Acceleration—is the acceleration toward the center of a curved or circular path.
5) Centripetal force - an unbalanced force causing an object to accelerate away the center of a curved path. This is a fictitious
force and is caused by the inertia of an object set in a circular path with the tendency of the object to move in a line that is
tangent to the circle.
6) Momentum - related to how much force is needed to change the motion of an object. The momentum of an object is the
product of its mass and velocity.
a) Where: p = momentum, m = mass, and v = velocity
b) The Law of Conservation of Momentum: Momentum can be transferred from one object to another but cannot change
in total amount.
Momentum = ( mass )(Velocity )
P = mV
Chapter 3 Equations
Distance
D=Vt
Momentum P = m V
Conservation of Momentum
p1 = m1V1
p 2 = m 2V 2
if p1 =p 2 ,then:
m 1V1 =m 2V2
Force
F = ma
Weight W = mg
Where a =
kgm
s2
1 Hour = 3600 seconds
1 km=1000 meters
1kg= 1000 grams
t f − ti
∴ F =
Ff
Fn
m ΔV
Δt
m
s2
⎧Where μ is coefficient of frict ion
⎪
⎨ Where F f is the force of friction
⎪ Where F is the force of the normal
n
⎩
Where g ( Gravity ) = 9.8
Coefficient of friction μ =
1 Newton = 1
V f − Vi