Forces

Force a push or pull acting on an object typically measured in Newtons (kg•m/s2) is a vector can be combined to predict motion net

force Soccer Simulation

Types of Forces

Contact Forces Applied Normal Friction Air Resistance Tension Spring

Non-Contact Forces Gravity Electromagnetic

any push or pull on an object created from another source (person, animal, another object, etc.)

Applied Force

Normal Force the support force exerted on an object directly

related to weight (gravity) consequence of Newton’s 3rd Law is always perpendicular to the surfaces in

contact

Box

Gravity

NormalForce

900 Box

Gravity

Frictio

n

900

Normal

Force

exerted by a surface as an object moves across it or attempts to move across it

opposes the motion of an object depends on the type of surfaces and the normal

force (weight) Types

Kinetic Static

Friction

Motion

Friction

In which directionis the force (friction)

vector pointing?

Air Resistance friction due to air molecules acts upon objects as they travel through the air opposes the motion of an object most noticeable for objects traveling at fast

speeds Examples

Space shuttle re-entry Meteorite in Freehold Meteor over Russia

Minimizing Air Resistance (Drafting)

used in variety of competitive events (bicycle and car racing, swimming, etc.) to reduce air resistance

Although it does not workexactly the same way, where

is these seen in nature?

Image taken from: http://www.bbc.com/news/magazine-19166035

Notice how the second biker experiences less air resistance because he is

shielded by the first biker.

Tension force that is transmitted through a string, rope,

cable or wire when it is PULLED tight by forces acting from opposite ends

directed along the length of the wire and PULLS equally on the objects on the opposite ends of the wire

Spring force exerted by a compressed or stretched

spring upon any object that is attached to it for most springs, the magnitude of the force is

directly proportional to the amount of stretch or compression of the spring

If both springs are the same size when not compressed,

which spring will apply more force to the ball when

released? Explain your reasoning.

Images taken from: http://www.lesjoforsab.com/standard-springs/compression-springs.asp

Gravity

natural force of attraction between any two objects

factors: distance – increased

distance less gravitational pull or vice versa

mass – increased mass more gravitational pull or vice versa

Why does the force of gravity have more of an impact on holding our solar system

together compared to holding the parts of an atom

together?

Electromagnetic Force force that moving charges exert on one

another results from the repulsion of like

charges and the attraction of opposites

-

+

+

+

--

Notice how the particles with the

same charge move apart and

the particles with different charges move together.

Compare and contrast gravitational force and electromagnetic force.

Free Body Diagrams

visuals that help show net force use a square and draw all forces acting

on the object. remember size and direction of vector

arrows are important!

Practice

Fgrav = 5 N

Ffric = 3 N

Fnorm = 5 N

Fapp = 3 N

What do you think thesymbols w/subscripts

represent?

What is the netforce on this object?

What’s the Net Force

Fgrav = 10 N

Fapp = 20 N Ffric = 5 N

You throw a baseball to your friend who is to your left.

Your dog pulls you down the street on a skateboard in an

eastward direction.

Fgrav = 10 N

Ffric = 5 N

Fnorm = 10 N

Fapp = 15

N

What’s the Net Force (An Interesting Case)

Fgrav

Ffric

Fgrav

Ffric

A skydiver is descending with a constant velocity. Consider air resistance.

The same skydiver is descending after 30 seconds.

Consider air resistance.

What has the skydiver reached in this

scenario?

Laws

Newton’s 1st Law

Newton’s 2nd Law

Newton’s 3rd Law

Newton’s 1st Law

objects at rest remain at rest, and objects in motion remain in motion with the same velocity, unless acted upon by an unbalanced force

also considered the Law of Inertia

How is this illustrated when riding in a car? Can you think of other

experiences where this is illustrated?

Inertia

the resistance of an object to a change in the speed or the direction of its motion

directly related to mass

Newton’s 2nd Law

the acceleration of an object increases with increased force and decreases with increased mass

the direction in which an object accelerates is the same as the direction of the force

Formula: F = ma (or a = F/m) Shopping Cart Simulation

Centripetal Force

any force that keeps an object moving in a circle

directed toward the center of the circle

In this case, the force of theball as it accelerates around the circle is pointing inward,

toward the center.

Practice Problems - Force1. What net force is needed to accelerate a 24 kg

dogsled to a rate of 3 m/s2?

2. A 1.5 kg object accelerates across a smooth table at a rate of 0.5 m/s2? What is the unbalanced force applied to it?

F = ma

F = ma

F = =(24 kg)(3 m/s2) 72 kg·m/s2

or 72 N

F =

(1.5 kg)(0.5 m/s2)=0.75 kg·m/s2

or 0.75 N

Newton’s 3rd Law

states that every time one object exerts a force on another object, the second object exerts a force that is equal in size and opposite in direction back on the first object.

How was this illustrated during the Scooter Games competition? Can

you think of other experiences where this is illustrated?

Vector a quantity that has both direction and

magnitude (size) drawn as an arrow which shows direction

and magnitude (length of arrow) consists of two parts: tail and head

Tail

Head

Consider the vectors above. Describe the direction and relative magnitude (force) of each ball based on the

vector.

can be combined/added to help determine net force the overall force acting on an object when all of

the forces acting on it are combined

Combining Vectors

What is the hockey puck’s net force?

Applied Force = 25 N

Gravity = 14 N

Friction = 2 N

Normal Force = 14 N

Normal Force = 14 N

Gravity = 14 N

Applied Force = 25 N Friction = 2 N

23 N