Chapter 12- Forces

Physical Science

12.1 Forces• 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 1st 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 2nd 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

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

Mercury

Venus Earth Mars Jupiter Saturn Uranus NeptuneGravity

at equator (Earth=1)

0.38g 0.9g 1g 0.38g 2.64g 0.93g 0.89g 1.12g

.16 .3810

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 3rd 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 & 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

*Stuck in Space??

*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

*Astronauts & Reaction Forces

*How do objects move? What pushes them?

*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