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Physics
“Forces and the Laws of
Motion”
"Forces"
 kinematics
- the study of motion without
regard to the forces causing the motion
 dynamics - the study of forces which cause
motion
 force - the product of a body's mass and
any acceleration acting on the body
Types of Forces
 1.
Electric Forces -- forces caused by the
interaction of electrons. Most forces we deal with
are electric. For example, when you push an object
you are creating electric forces between the
electrons in your hand and the electrons in the
object. Mechanical and frictional forces are
electrical forces.
 2. Gravitational forces -- The force of attraction
between masses. Gravitational forces are the
weakest of all forces. But gravitational forces act
over very long distances.
Types of Forces
 3.
Magnetic forces -- forces produced by
moving electric charges. Magnetic forces are
closely related to electric forces but the
relationship is not completely understood at
present.
 4. Nuclear forces -- forces within the nucleus
which hold particles together. Nuclear forces
are the strongest of the known forces but they
act over the shortest distances.
 5. Weak Interaction forces -- the forces
which is believed to cause atoms to break
apart.
Vector Addition: Graphical
Method
A vector requires magnitude and direction.
 Magnitude is indicated by the length of the arrow
representing the vector.
 Direction is simply the direction of the arrow.
 Vectors are always drawn head to tail.
 The result of a series of vectors is measured from
the tail of the first vector to the head of the last
vector drawn.

Sample Problem
A
plane flies 300km N, turns and flies
400km E. What is the displacement of the
plane from its starting position?
 Remember, displacement is a vector
requiring both magnitude and direction.
 Solve this problem graphically.
Solution
 1.
Consider the magnitude of the vectors and your
sketching area and develop a scale for drawing the
diagram.
 2. Draw the vectors using the " head to tail " method
being careful with the magnitude and direction.
 3. Measure the length ( Magnitude ) of the resultant and
use a protractor to find the angle between the initial vector
and the resultant.
 4. Express your answer indicating both magnitude and
direction. The direction should have both degrees and the
general direction.
Vector Operations
Mathematical Method
 c 2 = a2 + b 2
 sin x = opp/hyp
 cos x = adj/hyp
 tan x = opp/adj
 A man walks 3m N and 4m E. Find the
resultant vector of his walk.
Resolving Vectors Into
Components
 It
is possible to regard a single vector as two
perpendicular components; i.e., two vectors acting at a
900 angle to each other.
 This is useful in determining the most efficient angle
for force application in many areas such as pushing
lawn mowers, hanging signs, pulling sleds, etc.
 You will see as we work problems in this area that you
increase or decrease the angle of application of forces
automatically simply because it "feels" right.
Example
A
boy pulls on a sled with a force of 80N. If the angle
of the rope with the ground is 300, find the horizontal
component ( that force which is actually causing
horizontal motion).
 Find the vertical component of the applied force (that
force which is used to overcome friction). friction the opposition to motion
 What general statement can be made concerning the
angle of applied force?
 As the angle of the applied force increases, the
horizontal force _____________?
 Find the component velocities of a helicopter traveling
95km/h at an angle of 350 with the ground.
Adding Vectors That Are Not
Perpendicular
of cosines - c2 = a2 + b2 - 2ab cos C
 law of sines - a/Sin A = b/ Sin B = c/ SinC
 A hiker walks 25.5km from her base camp at 350
south of east. On the second day, she walks
41.0km in a direction 650 north of east, at which
point she discovers a forest ranger’s tower.
Determine the magnitude and direction of her
resultant displacement between the base camp
and the ranger’s tower.
 Page 121, 1-4
 law
Changes in Motion
 1.
A net force will change the state of
motion of an object.
 2. Forces can be exerted through long
distances.
 3. Forces always occur in pairs.
 4. In each pair of forces, one acts opposite
the other.
Units
System
Mass
Acceleration
Force
SI
kg
m/s2
N- kg m/s2
cgs
g
cm/s2
dyne-g cm/s2
Avoirdupois slug
ft/s2
lb-slug ft/s2
Forces are vectors, the product of a vector and a
scalar.
Newton's Laws of Motion
 Newton's
First Law of Motion ( Law of
Inertia )

An object at rest or in motion will
remain so until acted upon by an outside
(net) force.
 inertia - the resistance of a body to
change its state of motion.
Sample Problem
 Derek
left his physics book on top of a
drafting table. The table is inclined at a 350
angle. Find the net external force acting on
the book, and determine whether the book
will remain at rest in this position.
Newton's Second and Third Laws
of Motion
 Newton's
Second Law of Motion

When an unbalanced force acts on an
object, the object will be accelerated. The
acceleration will vary directly with the
applied force and inversely with the mass of
the object.
 Equation:
F = ma
Sample Problem
A
5.5kg watermelon is pushed across a table. If
the acceleration of the watermelon is 4.2m/s2 to the
right, find the net external force exerted on the
watermelon.
Newton's Third Law of Motion
 For
every action there is an equal and
opposite reaction.
 This is the law of interaction.
Everyday Forces
 Mass
depends on the amount of matter a body
possesses. Weight depends on the distance an
object is from the center of the earth.
 A question for thought; If an object fell down a
very deep hole, is there a point at which the object
would become weightless?

F = ma Fw = mg Fw = weight,
g = 9.8 m/s2 , etc.
Sample Problems
 Find
the weight of a 2 kg object.
 Find the mass of an object which weighs 49N.
Friction
 Friction
is the force which opposes the motion of
two surfaces in contact.
 Static friction is the force which opposes the start
of motion.
 Sliding friction is the force which opposes motion
in progress. Sliding friction is much less than static
friction.
 Rolling friction is the force which opposes motion
between a rolling object and the surface on which it
rolls.
Coefficient of Friction
 The
ratio of the static frictional force,( Fs ), to
the force between two objects in contact, (Fn),
is known as the coefficient of friction. The
normal force, ( Fn ), always acts perpendicular
to the surfaces in contact.

Fs

 = ------
Fn
Sample Problem
A
24kg crate initially at rest on a horizontal surface
requires a 75N horizontal force to set it in motion.
Find the coefficient of static friction between the
crate and the floor.
Sample Problem
A
student moves a box of books by attaching a
rope to the box and pulling with a force of 90.0N at
an angle of 30.00. The box of books has a mass of
20.0kg, and the coefficient of kinetic friction
between the bottom of the box and the sidewalk is
0.50. Find the acceleration of the box.
Experiment
 You
will be given a metal block and a
wooden board to use as an incline. Use this
material to determine the coefficient of
static friction between the metal and the
wood.
 Your group will write one ppoc and be
prepared to share the results with the class
by giving a presentation.
Vocabulary Review
Terms
 force
 inertia
 Newton’s 3 Laws of
Motion
 friction
 Truths about forces
