Chapter 12- Forces
Physical Science
12.1 Forces
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Force,
Units
Measuring
Force vectors
Combining forces
Balanced Forces – Equilibrium
Unbalanced forces – acceleration
Friction…. Static, sliding, rolling, fluid
Gravity – acceleration
Terminal velocity – Air Resistance
Projectile Motion
Force
• Force (F) - A push or a pull on an object
• An ‘influence’ or ‘interaction’
• Net Force (Fnet) – Total Force on an object
• Forces in opp directions will subtract, forces in same direction will add
Combining Forces – Net Force
• Multiple forces may be
acting on an object at
the same time, the Net
Force is the
combination (resultant)
of all these forces.
Balanced Forces
• Forces in each direction cancel each other
• Net force of ZERO
• If forces are balanced, object is in equilibrium
• Object is either AT REST, or AT A CONSTANT VELOCITY
Unbalanced Forces
• Net force is NOT Zero
• Unbalanced Forces (net force) causes acceleration
Friction
• Static Friction
• Resistance to the start of motion
• Sliding Friction
• Resistance to one object sliding over another object
• Rolling Friction
• Resistance to one object rolling over another object
• About 100-1000 times less than Static/Sliding
•Static > Sliding >
Rolling
• Fluid Friction
• Air Resistance
• Moving through water
Gravity
• Force that acts between any two masses
• Only significant when one of the objects is
very large (Earth, Sun, etc.)
• Pulls objects together (attractive force)
• Force of Gravity is also called weight
• Acts downward toward center of Earth
• Used balanced by some other force
• Support force from ground, chair, etc.
• asdf
Falling Objects
• Gravity accelerates objects downward
• Air resistance acts opposite to gravity and slows down
the acceleration
• Air resistance gets greater with velocity
• Eventually if falling so long that air resistance balances
gravity you will reach terminal velocity
• Terminal Velocity – constant velocity of a falling object
when the force of air resistance equals force of gravity
Terminal Velocity
Projectile Motion
• Any object that is only under the influence of
gravity and air resistance is a projectile
• If given some initial forward velocity,
projectiles will follow a curved path
• Combination of initial forward velocity and
downward vertical force of gravity causes it
to follow curved path.
12.2 Newton’s 1st & 2nd Laws of Motion
• Is a force required for motion?
• Aristotle (300 BC)
• Proposed that force is required to keep an object moving at a constant velocity
• Galileo & Newton (1500’s-1600’s)
• Moving objects are that are not experiencing friction or any other force will continue to
move indefinitely
• Would keep moving forever, if there is no friction
• Objects have a natural tendency to stay in motion or at rest
Newton’s
st
1
Law of Motion
• Objects at rest will stay at rest, objects in motion will stay in motion
unless acted upon by an outside force
• Objects have a natural tendency to resist changes in their motion
• Inertia – property of matter that resists changes in its motion
• Mass is a measure of inertia….. More mass, more inertia
Examples of Inertia
• Seatbelts
• Rollercoaster
• Car dashboard
• Tennis ball/string
• Earth’s rotation
• Helicopter
Newton’s
nd
2
Law of Motion
• Acceleration of an object is equal to the net fore
acting on it divided by the object’s mass
• Acceleration is proportional to net force
• More force, more acceleration
• Acceleration is inversely proportional to net force
• More mass, less acceleration
•𝑎 =
𝐹𝑛𝑒𝑡
𝑚
Weight & Mass
• Weight – Force of gravity acting on an object
• Mass – How much matter is in an object
• A measure of how much inertia an object has
• Weight = Mass x acceleration due to gravity
•W = mg
•Weight – in Newtons
•Mass – In Kilograms (kg)
•Acc of gravity – in m/s2
Gravitational force differs on other planets
Mercur Venus
y
Gravity
at
0.38g
equator
(Earth=1)
0.9g
Earth
Mars
Jupiter Saturn Uranus Neptun
e
1g
0.38g
2.64g
0.93g
0.89g
1.12g
.16
.38
10
2.64
Why do astronauts float around inside the
international space station?
• They are falling with the space station around Earth
• Continuously falling without ever running into anything.
• Gravity is still pulling them
Why do satellites orbit Earth?
• Satellites are forced to circle the Earth by the pull of Earth’s gravity.
• To orbit a satellite must be travelling at a very fast speed!
• Around 17,000 mph or faster
• Satellites are falling around the Earth
• Objects in orbit feel weightless
• Not actually weightless
12.3 Newton’s
rd
3
Law
• Newton’s 3rd Law
• For every action force there is an equal and
opposite reaction force
• Equal in magnitude, opposite in direction
• Forces only exist in pairs (Action/Reaction)
• Can’t have an action without a reaction
*
*Action forces produce the interaction, and
the reaction force must be in existence at
the same time, have the same size, and be
opposite in direction
*You apply a 10 N eastward force to wall,
wall applies a 10 N westward force to you
*
* An astronaut in space…not moving with just a spacesuit on and he’s not tethered to the
ship. If he is only a few meters away from his ship and there are no other people to
help him how could you get back to his spaceship?
Oh No!!
What’s really pushing you?
• To accelerate you need an outside force…. You cannot apply a net
force to yourself!
• Something outside of you needs to push you in order to
move/accelerate
• The astronaut in the previous slide need his tool to push him.
Should always be able to identify each
action/reaction
•x
•s
*
*
* Car??
* Rocket??
* Person??
Are these forces equal?
• Bug/windshield
• Ball/Bat
• Small car/truck
Momentum (p)
• Inertia in motion’
• How much an object in motion wants to stay in motion
• How much motion an object has
• Momentum = mass x velocity
•P = mv
• Vector quantity
Example Questions
• A 100 kg cart is moving with a velocity of 5 m/s, what is its
momentum?
• 500 kg*m/s
• A 2 kg bowling ball is rolling with a speed of 5 m/s, what is
its momentum?
• 10 kg*m/s
•f
•f
Law of Conservation of Momentum
• The total amount of momentum in a system must stay constant
unless an outside net force acts
• Internal forces cannot change momentum
• Collisions
• If no outside forces…. Momentum is only exchanged between objects never
lost
• Recoil from gun
•g
12.4 Universal Forces
• There are four fundamental forces that exist in the universe that can act
across a distance (push/pull things without touching)
• Strong Nuclear Force
• Acts on neutrons and protons in the nucleus of an atom to hold them together
• Electromagnetic Force
• Holds electrons to the positively charged nucleus
• Weak Nuclear Force
• Responsible for radioactive decay
• Gravity
• Attractive force that acts between any two masses
• Every object in the universe attracts every other object