Entry #5: Force

Watch : target accident http://www.youtube.com/watch?v=x_4TqtHATWI Answer: what happen? And why, share with your group and give the best answer to write on the board

Inertia

• Watch the video http://www.youtube.com/watch?v=A6snX4M2_8U

• and list at least 3 ideas about inertia

Check out yourself

Washer and cup tryout1. Put a piece of paper to cover the cup2. Put one washer on the paper3. Quickly remove the paper4. What will happen to the washer?5. Why

What factor(s) affect inertia?

• http://www.youtube.com/watch?v=SIsl1TQEcCA&NR=1&feature=endscreen

Newton’s first law of motion: law of inertia

• An object at rest will remain at rest unless acted on by an unbalanced force. An object in motion continues in motion with the same speed and in the same direction unless acted upon by an unbalanced force.

• Objects tend to resist change

http://teachertech.rice.edu/Participants/louviere/Newton/law1.html

Share with your group about your experience with inertia and choose one to share with the class

What do you need to do in the following situations (use arrow as force):

Bowling ball with a broom

Dry ice with a straw

Stay still

Speeding up

Maintaining motion

Stop/slowing down

Make a turn

Feel the forces: types of forcesWhat is a force? How do you know when you

exert a force?

Make a list of three forces you exerted in the last five minutes.

What are some things that can happen to an object when a force acts on it?

Forces• A force is a push or pull upon an object resulting

from the object's interaction with another object. • all forces (interactions) between objects can be

placed into two broad categories:– contact forces– forces resulting from action-at-a-distance

Forces• Push/Pull: Fp

- the force that one object pushes or pulls another object

1. Use arrow to show force acting on the block

2. What are the interaction objects for push and pull? which one give Fp? Which one receive Fp?

Forces• Friction: f– The force when 2 objects rub together– Pushes in the opposite direction of motion

1. Use arrow to show friction force acting on the bear

2. What are the interaction objects for friction? which one give f? Which one receive f?

Forces• Gravitational: Fg – Force of attraction between two objects– The force that pulls things toward Center of Earth

1. Use arrow to show gravitational force acting on the ball

2. What are the interaction objects for gravitational force? which one give Fg? Which one receive Fg?

Forces• Normal Force: Fn– Force pushing up on an object that is not moving

and on a surface– Force is always perpendicular ( | ) to the line

where the object and surface touch

What are the interaction objects for normal force? which one give Fn? Which one receive Fn?

Forces• Tension: Ft– A pulling force of a string, wire, rubber band or

something similar– Usually pulls up or away from the object

1. Use arrow to show tension force acting on the block

2. What are the interaction objects for tension force? which one give Ft? Which one receive Ft?

Forces• Electrostatic– Force between two charged objects– This force can pull two objects together or push

them apart, depending on the charge of each object

1. Use arrow to show electrostatic force acting on a hair

2. What are the interaction objects for Fe? which one give Fe? Which one receive Fe?

Forces• Magnetic Force : Fm– A force that either pushes or pulls – Acts between two magnets, between a magnet

and a magnetic object, or between an object with a magnetic field and a magnet or magnetic object

1. Use arrow to show magnetic force acting on a clip

2. What are the interaction objects for magnetic force? which one give Fm? Which one receive Fm?

Forces

Contact ForcesFrictional Force Tension Force Normal Force Air Resistance Force Applied Force (push and pull)elastic Force

Action-at-a-Distance ForcesGravitational Force Electrical Force Magnetic Force

Describing Forces in Free Body Diagrams

FNPE

FGEP FGPE

FNPE

How to draw a free body diagram:

1. Direction 2. Amount/strength 3. Label: types of force: Fpush or pull , ffriction , FGravity

FNormal, FTension , FElectrostatic , FMagnetic

FP , f , FG , FN , FT , FE , FM 4. Label: object acts On and By

F OB 5. Object is represented by a dot

Force DiagramWarm-Up• identify the forces that the bowling ball has and by

what object ( hint: force are interaction by two object, one give force and the other one receive force)

• draw a picture to show all the forces the bowling ball received.

Force Label

FgAB

Type of Force On A objectBy B object

Practice Force labels

1. Normal force on a ping-pong ball2. Magnetic force on a paper clip3. Electric force on balloon4. Friction on block5. Gravitational force on paper ball6. Tension force on weights7. Applied force on scales

Force Diagram– A dot represents an object– An arrow from the dot to show the force the object (dot)

received– The length of the arrow represent the strength of the force– The arrow head represent the direction of the force– Label of Force: FgAB: gravitational force on object A by object B– Forces of different direction are originate from the object

(the dot)– All the forces (arrows) will be balanced when the object does

not move or move in constant velocity

Force DiagramDraw force diagram of the block

Draw force diagram of the bear

Practice Force Diagrams1. a ping-pong ball on water

2. a paper clip pulled by a magnet

3. Ball attracted to a wool

4. Block on Ramp

5. Weight hung by the rubber band

Exit ticket of force diagram

• A book rests on a table

• A book hangs from a spring