Chapter 2Forces and Motion

Dr. Gluck

Forces and MotionLaws of Motion

2.1 Newton's First Law

2.2 Acceleration and Newton's Second Law

2.3 Gravity and Free Fall

2.4 Graphs of Motion

ForcesPS3 (9-11) – 9b, 8a

How do Newton’s laws predict the motion of an object when forces act on it?

What is the difference between forces applied to an object and forces applied by an object?

According to Newton’s third law, why is motion possible?

How does the motion of a mass change due to forces acting on it?

How can we explain the change in motion of a mass as caused by different forces?

2A Investigation: Law of Inertia

Key Question:

Why are heavier objects harder to start or stop moving?

*Students read text section 2.1

AFTER Investigation 2A

2A Investigation Topics

Launch cars of different mass.

Measure time intervals.

Calculate speed based on measured time intervals.

Graph speed vs. mass.

Research and define terms: inertia, weight and mass.

Chapter 2.1 Objectives and Vocabulary

force Newton's first law inertia newton net force

Recognize that force is needed to change an object's motion.

Explain Newton's first law.

Describe how inertia and mass are related.

What is the difference between mass and weight?

Mass – measure of the amount of matter in an object.

Mass – is constant

Weight – measure of gravitational force between two objects.

Weight – changes depending on the amount of gravity

Sir Isaac Newton (1642-1727)

English Physicist

Invented calculus

His three laws explain the relationship of forces, an objects mass, and it's motion.

How do we change the motion of an object?

Force

Push or pullAny action that has the

ability to change motion.

Club hits ball. What are the possibilities?

1. Ball stays in motion at constant

speed.2. Another force acts on the ball.

Force is opposite

of the motion causing ball to stop.

Force applied at an

angle and it

changes direction.

Bottom Line: There has to be a force applied to the ball OR the ball moves unchanged forever.

What is going on here?

Describe what is going to happen.

An object in motion stays in motion…..

How could this have been prevented??

Inertia

Property of an object to RESIST a change in motion.

But what causes inertia?

MASS

If mass determines how much inertia an object has, which of these has

more inertia?

The heavier object has more inertia.

1 pound = 4.448 NIs a Newton (N) LARGER

or SMALLER than a pound?

A Newton is SMALLER than an pound.

NET FORCE is the total of ALL forces acting on an object.

Can we just add all the forces acting on the

object mathematically in Newton’s?

NO Direction matters

NET FORCE IS A VECTOR QUANTITY.

Needs magnitude and direction.

Forces in the SAME direction

Add them up: 500 200 = 700 ----

Forces in OPPOSITE directions

Add them up using direction:

500N -200N

Total : 300N

2B Investigation: Newton's Second Law

Key Questions:

What is force?

What is the relationship between force and motion?

*Students read text section 2.2

AFTER Investigation 2B

2B Investigation Topics

Measure time intervals of car moving along track. Calculate and compare speeds of car at different

points on track. Evaluate forces acting on car. Calculate acceleration of car. Use Newton's second law to calculate the force. Graph force vs. time for the car on the track. Explain how speed and acceleration are different

using graphs.

Chapter 2.2 Objectives and Vocabulary acceleration

deceleration Newton's

second law

Define and calculate acceleration.

Explain the relationship between force, mass, and acceleration.

Determine mass, acceleration and force given two of the quantities.

Newton’s First Law says that a force is needed to change an object’s motion.

What kind of change happens?

What is acceleration?Rate at which speed changes.

f i

What does the acceleration of an

object depend on?FORCE

MASS

This is Newton’s Second Law

Force causes acceleration, and mass resists

acceleration.

Some things to consider when applying Newton’s second law:

1.The NET force is what causes the acceleration.

2.If there is no acceleration, the net force must be zero.

3.If there is acceleration, there must be a net force.

4.The force unit of newtons is based on kg, m, and s.

Chapter 2.3 Objectives and Vocabulary free fall

acceleration due to gravity

velocity weight air resistance terminal speed

Describe the motion of an object in free fall.

Calculate the speed and distance for an object in free fall.

Distinguish between mass and weight.

Explain how air resistance affects the motion of objects.

Free Fall

Objects in free fall accelerate downward due to gravity and no other forces.

Abbreviated by “g”

Has a value of 9.8 m/s/s

ms s

s m =s

So, is the speed (velocity) of a falling object constant

or is there an acceleration?

If you know the acceleration of an object in free fall, you

can predict its speed.

A skydiver falls for 6 seconds before opening her parachute. Calculate her

actual speed at the 6-sec mark and the distance she has fallen in this time.

Given: initial speed = 0 m/s

Time = 6 sec

Looking for: final speed and distance

Formulas: vf = gt vavg = v1 + v2 d=vavgt

2

vf = gt

Vf = 9.8 m/s/s (6 sec)

Vf = 58.8 m/s

vavg = v1 + v2 d=vavgt 2

Vavg = (0+58.8m/s/s) = 29.4 m/s2

d=(29.4 m/s)(6 sec) = 176.4 m

Formulas: vf = gt vavg = v1 + v2 d=vavgt

2

Calculate the final speed and distance for a skydiver who waits only 4 seconds to open his

parachute.

Given: initial speed = 0 m/s

Time = 4 sec

Looking for: final speed and distance

t = 4 sec

Vf = (9.8 m/s/s)(4 sec)

Vf = 39.2 m/s

Vavg = 0 + 39.2 m/s = 19.6 m/s

d = (19.6 m/s)(4 sec) = 78.4 m

An apple falls from the top branch of a tree and lands 1 second later. How tall

is the tree?

Given: initial speed = 0 m/s

Time = 1 sec

Looking for: distance

Formulas: vf = gt vavg = v1 + v2 d=vavgt

2

t = 1 sec

Vf = (9.8 m/s/s) (1 sec) = 9.8 m/s

Vavg = 0 + 9.8m/s = 4.9 m/s2

d = (4.9 m/s)(1 sec) = 4.9 m

Is there a difference between the mass of these astronauts?

Is there a difference between the weight of these astronauts?

Weight depends on gravity.

The more gravity, the more weight.

Chapter 2.4 Objectives and Vocabulary slope Describe motion with position

versus time and speed versus time graphs.

Use a position versus time graph to calculate speed from the slope.

Use a speed versus time graph to calculate acceleration and distance traveled.