Newtonian Physics

Laws of Motion

Force, Mass and InertiaO FORCE

O Force is needed to change motion. O There can be no change in an object’s motion

without the presence of a Force.O Forces can increase or decrease the motion of

an objectO Forces can cause a change in direction

O MassO Mass creates InertiaO Objects with greater mass have greater inertia,

or resistance to a change in their motion.

Units of ForceO The NEWTON is the Metric unit

used to express force

O 1 Newton is needed to cause 1 Kg to speed up by 1 m/sec (Units for math problems need to be meters and Kg)

O The Newton is smaller than a pound of forceO 4.448 N to 1 Pound

Net ForceOForces act together in nature

OAdding the forces that are acting on an object is very different than just adding the numbers

OForces act in certain directions, and the Net Force is the result

Balanced and Unbalanced Forces

Newton’s First LawO Considered the Law of Inertia

O An object in motion will remain in motion, an object at rest will remain at rest, unless acted upon by an unbalanced force.

O Balanced forces result in no movement/motionO Inertia: An object’s resistance

to a change in its motion

1. What is the force that will stop an object that is rolling

across the floor?2. Describe how to find the Net

Force acting on an object.3. What are the units for force?

Friction:OFriction is a force between

objects that are sliding past one another.

OFriction creates the following:O HeatO Wear and tear on an objectO Decrease in speed

Newton’s Second LawO Newton’s first law states force is needed to

change an object’s motion.O Newton’s Second Law addresses and

explains that change in motion.

O The change in motion is called Acceleration

O Acceleration: The rate of change in an object’s speed.

O Units for Acceleration are m/sec2

Solve these problems

O If a car increases its speed from rest to 40 miles per hour, and it takes 5 seconds to do so, what is its acceleration?

O If a car changes its speed from 60 miles per hour to 20 miles per hour in 4 seconds, what is its rate of acceleration?

Try another!!

OA car accelerates at a rate of 5 m/sec/sec for 6 seconds. It started at a velocity of 10 m/sec, what is its final velocity?

Force, Mass and Acceleration

ONewton’s Second law also describes the relationship between Acceleration and the Net Force on that object or Acceleration and the Mass of that object.

Relationships

Acceleration is directly/positively related

to Net Force

Acceleration is inversely related to Mass

OWhat is the difference between Acceleration

and Velocity?

Solve the followingOIf a 2000kg car has an acceleration

of 15m/sec/sec, what was the force required to do so?

OA 20 N force was applied to a 100 kg box, what acceleration resulted?

To use Newton’s second law properly, keep the following important ideas in mind. They are a good guideline for how to apply the second law to physics problems.

1. The net force is what causes acceleration.2. If there is no acceleration, the net force must be zero.3. If there is acceleration, there must also be a net force.4. The force unit of Newtons is based on kilograms, meters, and seconds.

Applying Newton’s Second Law

OWhat are the 2 different mathematical formulas that are going to be used when working with Newton’s Second Law?

OWhat does Newton’s Second Law address/explain.

Acceleration and GravityAcceleration Due to

Gravity

O The acceleration of objects that are dropped, is a constant rate.

O That rate is the acceleration due to gravity, (g)

O The value of (g) is 9.8 m/sec/sec

Acceleration Due to Gravity

OLargest Vacuum Chamber

OIf the acceleration of objects in free fall is the same no matter what, then why does a feather take longer to fall than a baseball?

How Do We Calculate Free Fall

OThe speed is equal to the acceleration of gravity multiplied by the time that it took for the object to fall

Solve the Following:

OWhat is the speed of a rock, dropped from a bridge, that falls for 5 seconds?

Upward LaunchesOWhen you drop an object it will

accelerate at 9.8 m/sec/sec due to the downward force of gravity.

What do you think happens if you throw an object up into the air?

The object will decelerate on its way up, stop for an instant and then accelerate as it falls to the ground

It will decelerate at a rate of 9.8 m/sec/sec until it reaches the top

+ Sign means moving upward

- Sign means moving downward

OAn object is thrown up into the air and when it lands its speed was 29.4 m/sec.

1. How long was the object in the air?2. Set up a chart of the object’s speed from start to finish.

Draw a picture to help if necessary

Distance With Acceleration

When an object has constant speed, the distance is easy to calculate: d = vt

The problem is when an object is accelerating, the speed is changing and the distance becomes difficult to predict.The formula to use is:

d

Free Fall Distance

When an object is in free fall, you use (g) as your

accelerationd = g

O \

OBefore we go on, solve the following problem!

A rock falls of a cliff and splashes into a river 5 seconds later, how far did it fall?

Air ResistanceOEarlier in this unit we said

that forces do not act aloneOGravity is NOT the only

force acting on an object that is in “Free Fall”

OAir exerts an an additional force on the object falling

Air Resistance and Terminal VelocityO The size and shape of

an object affect the force of air resistance

O A feather has its weight spread out over a larger surface area

O Shape of the object is also a factor in the ease of falling objects

O Objects will stop accelerating at a certain point and proceed to fall at a constant speed.

O Terminal velocity is a ratio of an object’s weight to its air resistance.

O When the net force becomes zero, the object has reached terminal velocity

OA falling object reaches terminal velocity when the force of gravity is balanced by the force of air resistance.

OExplain this fact as it relates to Newton’s first and second laws.