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Unit 15: Forces
Key Terms
Forces
force
newton
net force
friction
static friction
sliding friction
rolling friction
fluid friction
air resistance
gravity
terminal velocity
projectile motion
Newton’s 1st & 2nd Laws
inertia
mass
weight
Newton’s 3rd Law
momentum
law of conservation of
momentum
Universal Forces
electromagnetic forces
strong nuclear forces
weak nuclear forces
gravitational forces
centripetal force
Enduring Understanding: The effects of forces on the motion of a mass are described by Newton’s three laws of motion
Essential Questions
 What is a force?
Force – an action exerted on a body in order to change the body’s state of rest or motion
- a push or pull that acts on an object.
- force has a magnitude and direction, so it is a vector quantity.
- it can cause a resting object to move, it can accelerate a moving object by changing the object’s speed
or direction.
measuring force: easy to measure. Spring scale at the grocery store to measure food.
units of force: measured in newtons, abbreviated as N.
1 newton is the force that causes 1kg mass to accelerate at a rate of 1m/s each second (m/s2)
1N = 1kg·m/s2
representing force: you can use an arrow to represent the direction and strength of a force. The direction
of the arrow represents the direction of the force. The length of the arrow represents the strength,
or magnitude, of the force.

What are examples of balanced and unbalanced forces?
Combining Forces: You can combine the force arrows to show how forces combine. Forces in the same direction add
together and forces in opposite directions subtract from one another. The net force is the overall force acting on an
object after all the forces are combined.
Balanced Forces – the net force is zero and there is no change in the objects motion
Unbalanced Forces – a force that results when the net force acting on an object is not zero. When an unbalanced
force acts on an object, the object accelerates.

What does Newton’s first law of motion tell us about bodies with no unbalanced force acting upon them?
Newton’s First Law of Motion – the state of motion of an object does not change as long as the net force acting on the
object is zero. (also, the Law of Inertia)
-unless an unbalanced force acts, an object at rest will stay at rest, and an object in motion will remain in motion
with the same speed and direction.

Why is Newton’s first law called the law of inertia?
- inertia – the tendency of an object to resist a change in its motion.
- an object at rest remains at rest and an object in motion will maintain its velocity unless it experiences
an unbalanced force.
EXAMPLE: a soccer ball rests on the field motionless until a force applied to it in the form of a
kick. The kicked ball begins to roll. Because friction between the grass and the ball acts on the
ball as it rolls, the ball slows. The force of friction slows the ball and brings it to a stop. As the
soccer ball sat motionless in the grass, the forces acting on it were balanced. The ball remained at
rest until an unbalanced force acted on it. The ball has inertia.
EXAMPLE: front end car collision. The car stopped suddenly. You kept moving forward inside
of the car. Because you have inertia, you continue moving forward.

What does Newton’s second law of motion tell us about bodies with unbalanced force acting upon them?
Mass- a measure of inertia of an object and depends on the amount of matter the object contains.
Newton’s Second Law of Motion- the acceleration of an object is equal to the net force acting on it divided by the
object’s mass. (also, the Law of Acceleration)
- the acceleration of an object is directly proportional to the unbalanced force on the object and inversely
proportional to its mass
- thus, doubling the mass of an object cuts its acceleration in half.
a = acceleration (m/s2)
F = the unbalanced force (N)
m = the mass of the object (kg)
- the acceleration of an object is always in the same direction as the net force.
-note, this also applies when a net force acts in the direction opposite to the object’s motion. (deceleration)

What are resistive forces?
Resistant forces – forces that act only in opposition motion (friction and fluid resistance)

What are fluid resistive forces and how do they work?
Fluid friction – opposes the motion of an object through a fluid (air or water). Fluid friction forces get
stronger as the speed with which the body moves through the fluid increases.
Example: water: submarine, stirring cake batter
air resistance – fluid friction acting on an object moving through the air
- bicyclists and speed skaters often wear slick racing suits to reduce air resistance.

What is friction and how does it work?
Friction – a force that opposes the motion of objects that touch as they move past each other.
- friction acts at the surface where the objects are in contact.
- note that “in contact” includes solid objects that are directly touching one another as well as objects moving
through a liquid or a gas.
static friction – the friction force that acts on objects that are not moving. It always acts in the directions
opposite to that of the applied force.
sliding friction – a force that opposes the direction of motion of an object as it slides over a surface.
Sliding friction is less than static friction so less force is needed to keep the object moving than
was needed to get it to initially move.
rolling friction – the friction force that acts on rolling objects. The force of rolling friction is 100-1000x
less than forces of static or sliding friction. That is why movers often use dollies. Ball bearings
are often used to reduce friction in machines.
When is friction useful: starting, stopping, turning when talking running, driving a car
When is friction a problem: friction between moving parts in a motor

What does Newton’s third law of motion tell us about the forces acting on a body?
Newton’s Third Law of Motion – whenever one object exerts a force on a second object, the second object exerts an
equal and opposite force on the first object. These two forces are action and reaction forces. (also, the Law of Action and
Reaction)
EXAMPLE: the force your bumper car exerts on the other car is the action force. The force the other car exerts
on your car is the reaction force. These two forces are equal in size and opposite in direction.
Action-Reaction Forces do Not Cancel = because they do not act on the same body.
EXAPMLE: swimmer in a pool
Action: swimmer applies a on the water.
Reaction: the water allies a force on the swimmer.
-water and swimmer are not the same object.
Momentum – the product of an object’s mass and its velocity.
- an object has a large momentum if the product of the mass and velocity is large.
- the momentum of any object at rest is zero.
-EXAMPLE: space shuttle before launch = zero momentum, where a shot bullet has a very large momentum.
Momentum = Mass X Velocity
p = mv
kg·m/s
In physics, conservation means that something has a constant value.
EXAMPLE: two trains in a closed system. A closed system means other objects and forces cannot enter or leave
the system. Objects within the system, however, can exert forces on one another.
Conservation of Momentum – if not net force acts on a system, then the total momentum of the system does not
change.
- meaning momentum does not increase or decrease
- in a closed system, the loss of momentum of one object equals the gain in momentum of another object –
momentum is conserved.
Enduring Understanding: The law of Universal Gravitation states that gravitational forces act on all objects irrespective
of their size and posit
 Essential Questions
 How do mass & distance affect the gravitational force between 2 objects?
-gravitational force between two objects is proportional to their masses and decreases rapidly as the
distance between the masses increases. The greater the mass of the objects, the greater the pull between
them. Gravitations force decreases with the square of the distance between the objects.
F = G m1m2
G: (gravitational constant) 6.67 x 10-11N·m2/kg2
2
r

What is free fall?
free fall – the motion of a body when only the force of
gravity is acting on the body.
- it is the movement of an object toward the center
of Earth solely because of gravity.
- Gravity causes objects to accelerate downward,
whereas air resistance acts in the direction opposite
to the motion and reduces acceleration.
- Terminal velocity – is the constant velocity of a
falling object when the force of air resistance equals
the force of gravity.

What is free fall acceleration near the surface of the earth?
Objects falling near Earth’s surface accelerate downward at a rate of 9.8 m/s2.

How are weight and gravitational force related?
Weight is the measurement of gravities effect on an object.
W = mg
Weight = Mass X Acceleration due to gravity
F = ma
W = the weight of the object (N)
m = the mass of the object (kg)
g = free fall acceleration (9.8m/s2)
F = the unbalanced force (N)
m = the mass of the object (kg)
a = acceleration (m/s2)

What is the relationship between mass and weight?
Mass- a measure of inertia of an object and depends on the amount of matter the object contains.
Weight – the force of gravity acting on an object.
- An object’s weight is the product of the object’s mass and acceleration due to gravity acting on it.
Enduring Understanding: All forces in nature can be attributed to one of the four fundamental forces; gravitational,
electromagnetic, strong nuclear, and weak nuclear forces.
Essential Questions

What forces exist in nature?
All the universal forces act over a distance between particles of matter, which means that the particles need not
be in order to affect one another. In addition, each of these forces is affected by the distance between the
particles of matter.
1. electromagnetic forces – electric and magnetic forces are the two different aspects of the electromagnetic
forces. They are associated with charged particles.
- electric forces and magnetic forces are the only forces that can both attract and repel.
Electric Forces – they act between charged objects or particles such as electrons and protons.
static: cotton loses electrons easily while polyester gains electrons easily.
Magnetic Forces – act on certain metals, on the poles of magnets, and on moving charges.
2. strong nuclear – a powerful force of attraction that acts only on the neutrons and protons in the nucleus,
holding them together. The range over which the force acts is approximately equal to the diameter of the proton
(10-15m). Although this force acts over only an extremely short distance, it is 100 times stronger than the electric
force of repulsion at these distances.
3. weak nuclear – an attractive force that acts only over a short range. (10-18m)
4. gravitational – (weakest) an attractive force that acts between any two masses.
Newton’s Law of Universal Gravitation states that every object in the universe attracts every other
object.
-gravitational force between two objects is proportional to their masses and decreases rapidly as the
distance between the masses increases. The greater the mass of the objects, the greater the pull between them.
Gravitations force decreases with the square of the distance between the objects.
F=G
m1m2
r2
G: (gravitational constant) 6.67 x 10-11N·m2/kg2
Centripetal force – a center-directed force that continuously changes the direction of an object to make it move
around the center.


Over what range do they act?
How do they relate to one another in terms of relative strength?
Strong nuclear
Electromagnetic
Weak nuclear
Gravitational
Range
2x10-15m
infinity
10-18m
infinity
Acts on which particles?
neutron and protons
all particles
Relative Strength
1
10-3
10-16 weaker than the strong nuclear force
10-41 (weakest)