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Ch 4: Forces In One Dimension
4.1 Force and Motion
Q: What is a force, in most simplistic terminology?
A: A push or pull
Q: Does a force always result in motion of an object?
A: Nope; must be great enough to overcome any other forces acting on the object
Q: If an object is in motion, is there necessarily a force acting on it?
A: Nope; but there was once, most likely
Q: If an object experiences a change in motion, is there a force acting on it?
A: Yep. Right at the point of change.
Q: What do you call a change in motion?
A: acceleration
Q: What does net force mean?
A: The sum of all of the forces acting on an object
So really, if there is a net force acting on an object it will accelerate; if not, it
won’t.
DEF: Net Force = The sum of all the forces acting on an object
∑F
If ∑ F = 0, the object is not accelerating
Applied Forces
Net Force
Contact Forces versus Field Forces
The Fundamental Forces / Field Forces
The fundamental forces are all types of field forces which means “action at a distance”
forces, or no direct contact forces. What that means is that as objects enter a field, they
are subject to the effects of the field. In order of decreasing strength, they are:
1. Strong Nuclear Force = the attractive force that holds the subatomic particles of a
nucleus together.
2. Electromagnetic Force = the force between electrical charges. May be a force of
attraction or repulsion. It is assumed that their charges are equal in magnitude,
but opposite in direction.
3. Weak Nuclear Force = the force between subatomic particles during certain types
of radioactive decay.
Both the strong and weak nuclear forces operate only on a nuclear scale, and are thus only
viable within a range of 10-15 m. Beyond this range, they have no effect.
4. Gravitational Force = the force of mutual attraction between two objects.
5. Weight = a field force due to the gravitational attraction between two objects.
Usually, it’s between Earth and some object. This is a VERY important one when
we start using Newton’s 2nd Law (N2).
Fg or FW
Contact Forces
See Table 4-2, p 94.
Any force that involves direct contact between two or morr objects is a contact force.
Some examples of contact forces are:
1.
Friction = the force (or component of the force) that is parallel to the contact
surface and oppositely directed to the sliding motion of the object. Frictional forces
are response forces. Their magnitudes increase to a certain point in direct response
to the push / pull exerted on the object. We will cover this in greater detail soon! 
Ff
2.
Normal = the contact force exerted by a surface on an object.
support force.
FN. AKA the
3. Spring = A restoring force that pushes or pulls on an object. Because it is a
restoring force, it is opposite the original displacement, and is therefore, negative.
Morr with this later.  Fsp
4. Tension = the pull created by a string, spring, rope, or cable when attached to an
object with NO slack (pulled taut). FT
5. Thrust = A general term for the forces that move objects like rockets, planes, cars,
and even people.
Fthrust
Q: If an object doesn’t move, what is the magnitude and direction of the support force, in
relation to the weight of the object?
A: Equal and opposite to the weight. Keep this in mind – it is mui importante!
Now let’s take a look at Sir Isaac Newton, and Newton’s Laws!
N1 = Newton’s First Law = The Law of Inertia.
Originally, it translated this way: every object continues in its state of rest, or of uniform
motion in a straight line, unless it is compelled to change that state by forces impressed
upon it. Now, we state it like this:
An object in motion tends to stay in motion and an object at rest tends to stay at rest
unless acted upon by some outside force.
So, if there is a change in the motion, then there is a force present.
Q: Are mass and inertia the same thing?
A: Some will say yes, but I disagree. Inertia is a measure of something or someone’s
ability to resist a change in its motion. Mass is the quantification (putting a number to) of
the ability to do so.
The Equilibrium Rule
DEF: Tension = stretching force
DEF: Newton (N) = the SI unit of force
(The force needed to accelerate a mass of 1 kg to
1 m/s/s; morr with this later)
If ∑ F = 0, the object is not accelerating, and we say it is in mechanical equilibrium
DEF: Vector = an arrow whose length represents the magnitude of a quantity, and whose
direction represents the direction of the quantity
Anything in linear motion can be expressed as a vector quantity.
TRY THESE
1. If a gymnast is hanging from 2 rings, how would the scale readings on both ropes
compare to her weight?
2. Suppose she hangs with slightly more weight on the left ring, how would the scale
read.
3. What is the net force on a bathroom scale when a 150 lb person stands on it?
4. If you stand on 2 scales with your weight evenly divided, what will each scale read?
What about if you stand with more of your weight on one scale versus the other?
1. ½ of her weight. The sum of the 2 scales = her weight.
2. right side reads less, but the sum will always = her weight.
3. Zero; the scale reads the support force, which has the same magnitude as the
weight, not the net force
4. ½ of your weight. The sum must cancel out your weight, just like before.