Newton’s Second Lawof

Motion

F=ma

Why can a rabbit run faster than a turtle?

It’s really quite simple. He can

apply more force to his mass than I can.

Truck A Truck B

Which of these trucks can accelerate the easiest?Why?

GET OUTSIDE THE BOXProve your answer with realistic numbers.

Remember Force (F) and mass (m)

F = ma

a =Fm

a =10,000 N 8,000 kg

a = 1.26 m/sec2

A

Ba =

Fm

a =10,000 N 2,000 kg

a = 5 m/sec2

With the same amount of force applied to each, B willaccelerate easier because it has less mass.

How does an ice skater use force?1. The skater uses force to

get started and to speedup.

2. The skater uses force to change direction.

3. C.

ALL OF THESE ARE

m = 0.5 kgF = 2 N

A car rolls down a rampand you measure a forceof 2 newtons pulling thecar down. The mass of thecar is 0.5 kg. What is itsacceleration?

Solve for acceleration.

a = Fm

a = 2 N 0.5 kg

a = 4 m/sec2

What formula would youuse to solve for Force (F)?

F = m x afor mass (m)?

M = _F_ a

BALANCED FORCES

Balanced forces are always equal in size andopposite in direction.

Balanced forces produce NO MOTIONThis man is pushing against awall, the wall is pushing backagainst the man. Nothing moves so the forces are inequilibrium, (balanced)…

NOTHING MOVES !

Notice that the arrows arethe same size and pointingin opposite directions.

UNBALANCED FORCES

With unbalanced forces the forces will be in opposite directions but the size of one force willbe larger than the size of the other.

UNBALANCED FORCES ALWAYS PROCDUCE MOTION !

As this man applies anunbalanced force to thedolly with the drinks, itmoves because his force ismore than the force of the object. (notice the arrows)

Analyze this force diagram

Force applied by Dozer Rat.

Opposite forceapplied by dirt

Direction of motion

What is the actual“force” applied bythe dirt?(not a number)

The weight of thedirt

Analyze these force diagrams.

The sum of all forcespulling in this direction

The sum of allforces pulling inthat direction

Direction of motion

Are you pulling ?

We Need

HELLLP!

3.2 WEIGHT, GRAVITY, & FRICTION

In order to understand what weight is you must under-stand what gravity is. So…..

Explain gravity

Gravity is a force that pulls every mass towardsevery other mass.

So… how does that relate to the weight of an object?

Gravity depends on mass, the greater the mass anobject has, the greater its gravitational pull.

For example, the earth has more mass than our moon, so the gravitational pull of earth is more than the moon.

Let’s look at Mrs. Shelly weighing. How do these scales“weigh” us?

The floor pushes upon the scales

Gravity pulls downon the mass of the person

The top platform of the scale is able tomove and is attached by springs to thedial which are calibrated to show poundsof weight.

Calculating weight

Fw = mg

Weight force (N)

Mass (kg)

Acceleration ofgravity (9.8 m/sec2)

The correct unit for weight force is the newton (N).

What is your metric weight? (reminder 1 kg=~2.2 lb.)

Divide your weight by 2.2 lbs to convert it to mass,then multiply by 9.8m/sec2. This will give your wt.in newtons.

FRICTIONFriction is a term used to describe forces that resultfrom the relative motion between objects. Simply defined, friction is a force that opposes motion.

Friction is caused by two surfaces rubbing againstone another. The amount of friction depends on thesmoothness or roughness of the surfaces. This iscalled texture.

REVIEW

Newton’s 2nd Law of Motion is a mathematicalequation which explains the relationship betweenforce, mass, and acceleration.

F =Force (Newtons, N)

Acceleration (m/sec2)

Mass (kilogams, kg)

ma