Unit 2 Forces & Motion

Forces

Force- Ability to change motion(push or pull)

Units of lb, N=kg.m/sec2

If forces are balanced then the object won’t move and it is said to be in equilibriumForces cause an object’s velocity to change & can therefore cause acceleration.Unbalanced forces=move

Forces always occur in pairs!!!!!

Categories of Forces

Contact forces- force exerted from an external source

Applied force FrictionSupport force

Field forces- forces exerted without contact

MagneticElectricalGravitational

Newton’s Laws of MotionNewton’s First Law:

“AKA” the Law of InertiaInertia- property of an object to resist change in state of motion

an object will stay at rest until acted on by an unbalanced forceIn other words, things tend to keep on doing what they were doing in the first place unless you apply a force.

Newton’s Laws of MotionNewton’s Second Law:

force causes an object to accelerate while the object’s mass resists accelerationFnet=ma•a=acceleration (m/sec2),

F= force (N), m=mass (kg)• Fnet= Right – Left

• Fnet= Top - Bottom

EX. Lighter cars go faster than heavier ones pushed with equal force. Lighter cars resist the force of acceleration less allowing them to move faster than heavier ones.

Net Force

Net Force - Vector sum of all the forces that act on an object

In the same direction - addIn opposite directions – subtract

largest vector wins directionAt right angles (a2 + b2 = c2)When net Force = 0, there is no acceleration

3.3 Newton’s Third Law

Newton’s 3rd Law: For every action there is a reaction force equal in strength & opposite in directionFor every action there is an equal & opposite reactionCan be + (move to right) or – (move to left)Examples: rockets, stepping into a boat, throw a ball on a a skate board

Gravity, Weight & MassGravity-

The downward pull on you by the earthDepends on massMass- amount of matter in an object compared to a standard

Changes on other planetsOn Earth= -9.8m/s2

Weight- force created by gravity depends on mass

It is a force acting on object in units of lb or N

•Fw=mg

•Fw= weight force (N)

•m=mass (kg)

•g=acceleration due to gravity (-9.8 m/sec2)

•Fw=mg

•Fw= weight force (N)

•m=mass (kg)

•g=acceleration due to gravity (-9.8 m/sec2)

Fw

m g

Drag ForceDrag force – the force exerted by a fluid (liquid or

gas) on an object moving thru the fluid.- We refer to this as air resistance when objects

move thru the airThe faster an object goes the greater the drag

force.- When the drag force equals the force of gravity

there is no acceleration. - A constant velocity – known as terminal velocity.- Large surface areas have a lower terminal

velocity- Small surface areas have a higher terminal

velocity

Law of Universal GravitationSpeaking of gravity…There’s earthly gravity (the earth & objects are attracted to each other)There’s universal gravity (attraction between heavenly bodies like the Sun and moon)No matter what kind of gravity you speak, two variables influence the strength of this attractive force:

Object’s massDistance between objectsF = G m1 m2

-------- r 2

Free Fall- • Drop straight down in the absence of air

resistance.• Acceleration due to gravity -9.8 m/sec2

• Object’s will experience uniform acceleration and uniform increases in velocity when in free fall!

Terminal Velocity- Highest velocity reached by a falling objectWhen an object stops accelerating, but continues to fall terminal velocity is achieved.When air resistance balances the weight of the object pushing down on it.

Friction: A force that…Reduces accelerationWorks against motionOccurs when surfaces move against on anotherCauses wear on parts

Examples of friction:Air friction- (air resistance)- aka – “drag”Sliding friction (rub hands together)Viscous friction- (oil in car engines & joint fluid)Rolling friction- (wheel on road, ball bearings)

What is friction?

Static Friction (Ffs) Sliding Friction (Ffk) The frictional force that must be overcome to get an object movingμs = Ffs μ=coefficient of friction

FN

Fw: Weight Force= mass(gravity)

FN: Normal Force Is always = to the Weight Force (Fw)

μs is always greater than μk

The frictional force that exists once an object is in motion.AKA kinetic frictionμk = Ffk

FN

Rules of Friction

If an object is not moving, then the frictional force (Ff) is greater than the pulling forceIf an object is moving at a constant speed, then the frictional force (Ff) is equal to the pulling forceIf an object is accelerating, the frictional force (Ff) is less than the pulling force. Fp=pulling force = Fa=applied force

Fp or FaFf