Unit 3 Forces & Motion

Forces

Force- an action applied to an object to change its motion(push or pull)

Units of lb, N (equal to kg.m/sec2)If forces are balanced then the object won’t accelerate and it is said to be in equilibriumUnbalanced forces=acceleration

Forces always occur in pairs!!!!!

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:

Unbalanced 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.

3.3 Newton’s Third Law

Newton’s 3rd Law: For every action force there is a reaction force equal in strength & opposite in directionCan be positive or negativeExamples: rockets, stepping into a boat, throw a ball when on a skate board

Net Forces

Net Force - Total sum of all the forces that act on an objectIn the same direction – add forces togetherIn opposite directions –subtract forces

largest vector wins when forces act in opposite directions

At right angles (a2 + b2 = c2)When net Force = 0, there is no acceleration. This means the object is:

Not moving ORMoving at a constant velocity

Gravity, Weight & MassGravity-

Force pulling toward mass of planetDepends 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

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 (uniform increases in velocity) when in free fall!

Terminal Velocity- Highest velocity reached by a falling

object When an object stops accelerating,

but continues to fall, terminal velocity is achieved.

Occurs when force of air resistance balances the weight of the object.

Law of Universal Gravitation

force of attraction directly related to masses & indirectly related to distanceF=G m1m2

r2

F= Force of attraction G= Gravitational constant m1 = mass object 1 m2 = mass object 2 r = radius btw the objects

Falling without air resistance – feather and elephant

Falling with air resistance

Skydiver

Friction: Reduces accelerationWorks against motionOccurs when surfaces move against one 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?

Rules of Friction

If an object is slowing down, 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 speeding up, the frictional force (Ff) is less than the pulling force. Fp=pulling force = Fa=applied force

Fp or FaFf

Static Friction (Ffs) Sliding Friction (Ffk) The frictional force that must be overcome to get an object moving (Ffs)

μs = Ffs

FN

μ=coefficient of friction (ratio of frictional forces to the normal force)

Fw: Weight Force= mass X gravity

FN: Normal Force Often = to the Weight Force (Fw)

The frictional force that exists once an object is in motion (Ffk)

AKA kinetic frictionμk = Ffk

FN

Free-body diagrams• It is a diagram that visually

represents the various forces being applied on an object: • Fstatic = friction force (Ff)

• Fpush = applied force (FA)• N = the normal or support

force (FN); perpendicular to surface• W = weight of object (FW)

The sum of all of the forces involved is called the FNET

μ=coefficient of friction (ratio of frictional forces to the normal force)

FN= Normal Force (support force)Often = to the Weight Force (Fw)

Ff = μ FN

Coefficient of Friction

Sample Problem

A 10-kg box is being pushed at a constant speed along a tile floor with a frictional force of 20 N.1. Draw a free-body diagram.2. Determine the value for the

weight force, applied force and normal force.

3. What is the coefficient of friction between the tile and the box?

Sample Problem

A 5-kg box is sitting on a tile floor that has a coefficient of friction of .32. 1. Draw a free-body diagram.2. Determine the value for the

weight force, friction force and normal force.

3. What is the minimum applied force needed to move the box?