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FORCES
Mechanics
KINEMATICS DYNAMICS
Describes motion. The forces that cause motion.
Forces
Elementary Definition:
Force – a push or a pull
Not totally accurate though.
Forces
True Physics Definition:
Force – any action that can cause a change of motion in an object
Forces
Vector quantity – Forces have magnitude and direction
May be a contact force or a field force Contact forces result from physical contact between
two objects Field forces act between objects that are not in
physical contact
Contact and Field Forces
Type of Forces
CONTACT FORCES FIELD FORCES
Elastic Force (Felas ) Frictional Force (Ffric) Applied Force (FA) Tensional Force (Ftens) Normal Force (FN ) Buoyant Force (FB )
Strong Nuclear Force Electromagnetic
Force Gravitational Force
(Fg ) Electric Force (Felec ) Magnetic Force (Fmag )
Contact Forces
Physical contact must be present during the exertion of the force.
Elastic Force (Felas )– due to stretching or compression of an elastic object (rubber band, springs, basketball bouncing)
Tensional Force (Ftens) - due to objects pulled by a rope or cable-like object
Frictional Force (FFr) – any force that opposes motion due to two types of matter coming in contact (air resistance, cleats on a grass field)
Contact Forces (cont.)
Physical contact must be present during the exertion of the force.
Applied Force (FA ) – generic term for any other type of force (boxer strikes another fighter, wrecking ball hits the building)
Normal Force (FN ) – force that acts perpendicularly when an object is on a surface
Buoyant Force (FB ) – upward force due to a fluid (liquid or gas) pushing on an object
Field Forces
From strongest to weakest …
Strong nuclear force – holds particles in nucleus together
Electromagnetic force – electric and magnetic fields. Holds particles together so that they can bend, stretch or shatter.
Gravitational – attractive force that exists between all objects
Which of the following is a field force?
Norm
al
Fricti
onal
Tensional
Electr
omagneti...
Applied
0% 0% 0%0%0%
1. Normal2. Frictional3. Tensional4. Electromagneti
c5. Applied
Which of the following is a contact force?
Stro
ng Nucle
ar
Magn
etic
Fricti
onal
Grav
itational
Electr
omagneti...
0% 0% 0%0%0%
1. Strong Nuclear2. Magnetic3. Frictional4. Gravitational5. Electromagneti
c
What type of force always opposes motion?
Norm
al
Applied
Fricti
onal
Tensional
Grav
itational
0% 0% 0%0%0%
1. Normal2. Applied3. Frictional4. Tensional5. Gravitational
What type of force takes the sum of all the forces present?
Norm
al N
et
Fricti
onal
Tensional
Summation
0% 0% 0%0%0%
1. Normal2. Net3. Frictional4. Tensional5. Summation
What type of force is always due to pulling and never due to pushing?
Norm
al
Magn
etic
Fricti
onal
Tensional
Applied
0% 0% 0%0%0%
1. Normal2. Magnetic3. Frictional4. Tensional5. Applied
What type of force is present when an object lies on a surface?
Norm
al
Electr
omagneti...
Fricti
onal
Tensional
Applied
0% 0% 0%0%0%
1. Normal2. Electromagneti
c3. Frictional4. Tensional5. Applied
Free Body Diagram
Must identify all the forces acting on the object of interest.
Choose an appropriate coordinate system.
If the free body diagram is incorrect, the solution will likely be incorrect.
Mass vs. Weight Mass is the quantity of matter contained
in an object SI unit of mass is the kilogram (kg)
Weight is the gravitational force exerted on an object This weight can change based on gravitational
fluctuations while mass stays constant.
Units of Force
SI unit of force is a Newton (N) – the amount of force needed to accelerate a 1kg object by 1 m/s each second
US Customary unit of force is a pound (lb)
1 N = 0.225 lb
2s
mkg1N1
Weight – Gravitational Force
Weight is considered a force – Fg
Fg = m ag
Fg = m x 9.8 because a = 9.8 m/s2
downward
Some Notes About Forces
Forces cause changes in motion Motion can occur in the absence of forces (if they are
already in motion) All the forces acting on an object are added as
vectors to find the net force acting on the object m is not a force itself
Newton’s Second Law is a vector equation
Sir Isaac Newton
1642 – 1727 Formulated basic
concepts and laws of mechanics
Universal Gravitation Calculus Light and optics
Newton’s First Law Law of Inertia
An object at rest will stay at rest and an object in motion will stay in motion unless a nonzero net force acts on it.
An object moves with a constant velocity unless acted upon by a net force (nonzero).
Examples of Newton's First Law - Fun in Space
Newton’s First Law
Also referred to as the Law of Inertia
Inertia – the tendency of an object to remain in its present state (at rest or in motion at constant velocity)
Inertia is directly proportional to mass. As the mass increases, the tendency to retain the present state of motion increases.
Commonly shortened to “F=ma”. Correctly, it is :
Only forces which act on that object affect the acceleration of the object.
Forces exert by the object on another object do not.
Newton’s Second Law
mm
FaaF
,
Newton’s Second Law
If mass is held constant, acceleration is directly proportional to the net force.
To produce the same amount of acceleration, as the mass increases, the force must also increase.
If the force is held constant, as the mass increases the acceleration will decrease.
Now draw a graph for each scenario.
Newton’s Third Law – when one object exerts a force on a second object, the second object exerts a force on the first that is equal in magnitude, but opposite in direction
Forces always occur in pairs. (action / reaction pairs)(ex: push on a door, we feel door push back)
Action Reaction pairs
Action-reaction forces – equal in magnitude, opposite in direction.
Also, action-reaction force pairs ALWAYS act on DIFFERENT objects.
A single isolated force cannot exist.
Friction
Where in real-life do we try to reduce weight to reduce required force?
Friction – the force that opposes the motion between two surfaces that are in contact.
Two Main Types of Friction
There are two main kinds of friction:static friction – the force that opposes the start of motion
sliding or kinetic friction – the force between surfaces while in motion
(Also rolling friction – but we won’t deal with this one.)
To calculate force of friction (FF), use the equation:
FFr = μFN or μ = FFr /F N
μ = coefficient of friction, constant for any two types of matter
μ = ratio of the frictional force to the normal force
Frictional force is directly proportional to the μ.
Ffr is always parallel to the surface and opposite the motion of the object.
FN is force normal, or force perpendicular to surface.
μ represents coefficient of friction, and changes with surface
Air Resistance
Air resistance is the net force of the air molecules striking a moving object- another source of friction
- without air resistance (like in a vacuum, all objects would fall unimpeded at the acceleration of gravity.
When force of air resistance equals the force of gravity, terminal velocity is reached.- velocity becomes constant
For example, terminal velocities:ping pong ball – 6 m/sskydiver - 60 m/sskydiver w/chute – 5 m/s
How does a parachute work?
How else could a diver change his/her terminal velocity?