Forces and MotionForces and Motion

Scientists construct explanation of Scientists construct explanation of phenomena (something that happens) phenomena (something that happens) by analyzing a situation, applying ideas by analyzing a situation, applying ideas and evaluating an explanation.and evaluating an explanation.

The analysis is good when the interaction is The analysis is good when the interaction is correctly described and the energy diagram (or correctly described and the energy diagram (or force diagram) is correct.force diagram) is correct.

MechanicalEnergyWater Water and String

Decrease in MotionEnergy

Increase in ThermalEnergy

Source Receiver

What we knowWhat we know

An ______ mechanical interaction An ______ mechanical interaction

occurs when two non-elastic (rigid or occurs when two non-elastic (rigid or

stiff) objects push or pull on each stiff) objects push or pull on each

other. other.

applied

What we knowWhat we know

A ______ mechanical interaction A ______ mechanical interaction

occurs when two surfaces rub occurs when two surfaces rub

against each other. against each other.

friction

What we knowWhat we know

A ______ mechanical interaction A ______ mechanical interaction

occurs when an object moves occurs when an object moves

through fluid like a gas or a liquid,through fluid like a gas or a liquid,

and the fluid resists the object’s and the fluid resists the object’s

motion. motion.

drag

What we knowWhat we know

An _______ mechanical interaction An _______ mechanical interaction

occurs when two objects push or pulloccurs when two objects push or pull

on each other and at least one of on each other and at least one of

them is stretchy. them is stretchy.

elastic

What we knowWhat we know

During a mechanical interaction,During a mechanical interaction,

mechanical energy is transferred frommechanical energy is transferred from

the _____ to the ______.the _____ to the ______.source receiver.

What we knowWhat we know

1. Interactions Ideas1. Interactions Ideas

2. Energy Ideas2. Energy Ideas

3. Now we will layer 3. Now we will layer

FORCE ideas to FORCE ideas to

explain those same explain those same

changes in motion changes in motion

during an interaction .during an interaction .

What we want to knowWhat we want to know

How do forces affect motion?How do forces affect motion? How does a constant forward force How does a constant forward force

affect motion?affect motion? How can an arrow be used to How can an arrow be used to

represent a force?represent a force?

Background InformationBackground Information

Force: A force is a push or pull and Force: A force is a push or pull and has both strength and direction. This has both strength and direction. This idea is represented by a force arrow.idea is represented by a force arrow.

WWhat hat DDo o YYou ou TThink?hink?

An air puck glides on a cushion of air. TheAn air puck glides on a cushion of air. The

base of the puck has a stem with a small base of the puck has a stem with a small

hole drilled down the center. An air-filled hole drilled down the center. An air-filled

balloon fits over the stem. The air from the balloon fits over the stem. The air from the

balloon makes an air cushion for the puck toballoon makes an air cushion for the puck to

float on. The air float on. The air does not does not push the puck – push the puck –

it just creates a frictionless surface for the it just creates a frictionless surface for the

puck to glide on.puck to glide on.

Think about itThink about it

What does the teacher do?What does the teacher do? What does the air puck do?What does the air puck do?

Write about itWrite about it

While gliding on a cushion of air, While gliding on a cushion of air, does the puck does the puck – speed upspeed up– slow downslow down– constant motion?constant motion?

Read about itRead about it

Read the following thinking bubblesRead the following thinking bubbles

among four students who are trying toamong four students who are trying to

account for the puck’s motion as it account for the puck’s motion as it

glides on the cushion of air.glides on the cushion of air.

The puck looks like it moves with constant speed, but is really slowing down a bit. Even if the air in the balloon never ran out, the puck would eventually stop because everything eventually slows down and stops.

The puck keeps moving because nothing is stopping it.

The puck looks like it moves with constant speed, but is really slowing down because the hand is no longer pushing it. To keep it moving, you would have to keep pushing it.

The puck keeps moving because the quick shove the teacher gave it stays with it even after the puck is no longer touching her hand.

1 2 3 4

Write, in Your own Words, Why youThink thePuck hadThe motionThat youObserved.

WWhat hat DDo o YYou ou TThink?hink?

If you used a hair dryer to exert a If you used a hair dryer to exert a constant constant forceforce on a small ball, what kind of motion on a small ball, what kind of motion would the ball have?would the ball have?

Will it speed up?Will it speed up? Or move at a constant speed?Or move at a constant speed? Why?Why?

Predict and please answer these inPredict and please answer these in

your notebookyour notebook

FFor or YYou ou TTo o DDo – Part A o – Part A

Ball and Hair Dryer: (teacher demo)Ball and Hair Dryer: (teacher demo)

1.1. What kind of motion did the ball What kind of motion did the ball have? Did it speed up? Or move at have? Did it speed up? Or move at a constant speed?a constant speed?

2.2. Is this what you predicted in the Is this what you predicted in the WDYT section?WDYT section?

3.3. Please answer in your notebookPlease answer in your notebook

WWhat hat DDo o YYou ou TThink?hink?

Suppose you have a fan connected to a Suppose you have a fan connected to a car. (When the fan blades whirl around, car. (When the fan blades whirl around, they push the air backwards. In return, they push the air backwards. In return, the air pushes steadily forward on the fan the air pushes steadily forward on the fan unit and the car.) When the fan is on, the unit and the car.) When the fan is on, the air provides a constant forward force on air provides a constant forward force on the car and fan unit. Would the car speed the car and fan unit. Would the car speed up or have a constant speed? Why?up or have a constant speed? Why?

Predict and please answer in your Predict and please answer in your notebooknotebook

FFor or YYou ou TTo o DDo – Part Ao – Part A

Fan Car (teacher demo). Fan Car (teacher demo).

3.3. What kind of motion did the car What kind of motion did the car

have? Did it speed up? Or move at have? Did it speed up? Or move at a constant speed?a constant speed?

4.4. Is this what you predicted in the Is this what you predicted in the WDYT section?WDYT section?

Please write this in your notebookPlease write this in your notebook

Low Friction Car (YOU do it)Low Friction Car (YOU do it)

Now you come up with one or two Now you come up with one or two

different ways to exert a different ways to exert a constant forward constant forward

force force on the car. This means that any on the car. This means that any

pushing or pulling on the car pushing or pulling on the car needs to needs to

remain at constant strength.remain at constant strength. You wll use a You wll use a

llow-friction car that has very little friction ow-friction car that has very little friction

Between its moving parts. Between its moving parts.

Think about itThink about it

5.5. Describe how you will exert a Describe how you will exert a constant forward force constant forward force on the low-friction on the low-friction car.car.

Exert a constant forward force on the Exert a constant forward force on the

Car and observe its motion.Car and observe its motion.

6. 6. What kind of motion did the car What kind of motion did the car have? Did it speed up? Or move at have? Did it speed up? Or move at a constant speed?a constant speed?