B. Dynamics – the Study of Forces

1. Basic Force Definition - “a push or a pull”

a) Fundamental Forces

i) Gravitational Force (Fg) – every object with mass has an attraction to

every other mass in the universe

ii) Electromagnetic Force – (Fe) – the force that charged particles have on

each other

iii) Strong Nuclear Force – an attractive force that holds protons and

neutrons in the nucleus of the atom acting in the 10-15 m range

iv) Weak Nuclear Force – the force exerted between all subatomic particles

enabling a conversion of one type of quark into another

b) Named Forces – (for everyday use in high school physics)

i) Gravitational Force (Fg) – as above

ii) Frictional Force – (Ff) – the force that opposes motion

iii) Normal Force – (FN) – the force that acts perpendicular to a surface

iv) Applied Force – (FA) – forces exerted by objects or people

v) Tension Force – (FT) – the force exerted by a rope or string caused by objects or people

Assignment: Free Body Diagrams – See HO (first side)

c) Net Force

The net force (Fnet) is not a separate force on its own. It is the vector ______of all forces acting on an object. When the Fnet = 0 the object is moving at ______, (c.v.). (This includes a velocity of 0). Conversely, if an object has a c.v. then the net force acting on it is ______. We say that the forces are ______. Objects will only move in the ______of the net force. Recall that Newton’s second law is F = ma. The force in this equation is the ______force. An object will always move in the direction of the ______. We now rewrite the equation thus:

d) Newton’s Laws Revisited – Memorize word for word!

Law 1: Objects will remain at rest or in uniform straight line motion unless acted upon by a net force.

Keyword: Inertia - ______

Law 2: Acceleration is directly proportional to force and inversely proportional to mass.

Keyword: ΣF = ma

Law 3: For every action there is an equal and opposite reaction but not on the same

object.

Keyword: FA on B = - FB on A

e) The Newton (N)

From this equation calculate the units for force.

Units for force:

“A Newton is defined as the amount of force required to

accelerate a 1 kg mass at a rate of 1 m/s2”

f) Mass and Weight

If you are going to buy some apples they are likely sold by the pound or ____. The cashier will ‘______’ the apples and charge you the appropriate amount. In our everyday experience we do not distinguish between mass and weight but in science the two terms are distinct from each other. ______is the amount of matter that an object possesses. ______on the other hand is the force of gravity on an object. The SI units for mass are ____ and the SI units for weight are ___ (Newtons – a combination of 3 SI units: m, s and kg). So we have weight as a vector quantity namely ______. And the direction is towards the ______or c.o.m.

Weight is dependent on ______. No matter where you are in the universe your mass will not ______. However, your weight will change depending on your distance from the center of the planet you are near or on.

To calculate your weight you are going to use a slightly modified ΣF = ma. Remember that the F in this equation is the net force or vector sum of all the forces acting on you. Your weight is equal to the force of gravity on your body.

ΣF = ma

ΣF = Fg

Fg = ma

a = g (gravitational constant near the surface of the Earth)

Fg = mg where g = 9.81 m/s2

Fg = mg

A person with a mass of 80.0 kg would have a weight of:

This same person on the moon would have the same mass but a different weight. The acceleration due to gravity on the surface of the moon is 1.60 m/s2. So the weight of this person on the moon would be:

The acceleration due to gravity is different on each planet because of its individual mass and radius.

Planetary Data

Name / Average radius
(m) / Mass (kg) / Mean distance from sun (m)
Sun / 6.960 x 108 / 1.991 x 1030 / ------
Mercury / 2.43 x 106 / 3.2 x 1023 / 5.80 x 1010
Venus / 6.073 x 106 / 4.88 x 1024 / 1.081 x 1011
Earth / 6.3713 x 106 / 5.979 x 1024 / 1.4957 x 1011
Mars / 3.38 x 106 / 6.42 x 1023 / 2.278 x 1011
Jupiter / 6.98 x 107 / 1.901 x 1027 / 7.781 x 1011
Saturn / 5.82 x 107 / 5.68 x 1026 / 1.427 x 1012
Uranus / 2.35 x 107 / 8.68 x 1025 / 2.870 x 1012
Neptune / 2.27 x 107 / 1.03 x 1026 / 4.500 x 1012