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?