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3rd Law of Motion

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UC Irvine Physics and Astronomy Videos

School blocks YouTube? Download file below.

Sailboat.flv or Force - Sailboat.m4v

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Click on the button below to start the game.

One of these things is not like the other!

The idea for this game and the graphic above come from a great physics teacher named Kelly O'Shea.

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Explain this using Newton's Laws of Motion

inertia

Explain this using Newton's Laws of Motion

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Video Instruction

showmethephysics.com

slideshow

B) Free Body Diagrams

Diagrams that show the forces acting on an object

Ex 1) Lamp on a Table

[ Flash ]

Weight = mg

Fn - normal force

(1) upward force supporting on an object

(2) Always drawn perpendicular to surface where 2 objects meet

(3) When an object is placed on a horizontal surface

Fn = weight of object

Object Resting on an Incline Plane

Weight force?   Normal Force?   Friction?

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Weight force? B   Normal Force? C   Friction? D

Ex 2) Object at Rest on an Incline Plane

Try It

Drag and Drop the vector arrows to show the forces acting on a box resting on a ramp.

Click on the picture above

On A Ramp,

Fn (normal force) ≠ Weight Force

(Fn is less)

Calculating the Components of Weight Relative to a Ramp

| Onsite |

by Dr. Michael R. Gallis - Penn State Schuylkill

Penn State Schuylkill

Go Nittany Lions!!

[Flash]
Show Me The Physics

Ranking Task

Rank the parallel component on the boxes below, from largest to smallest. Justify your answer.

Video Instruction

showmethephysics.com

Slideshow

Ex 3) Force on a Falling Object

(before terminal velocity)

Downward Force - Weight Force

Upward Force - Friction

Ex 4) Block Pulled to the Right at Constant Velocity

What force is on this block that is also on you?

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Now draw the applied force

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Draw the last force

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Last force is the friction force

Fnet = 0

(Always True at Constant Velocity)

How could this box be moving if all the forces are balanced?

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A force is not required to MAINTAIN motion

Ex 5) Forces on a Projectile Moving Left to Right

What force is on this ball that is also on you?

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If there is no forward force on this ball, how could it move forward?
- Aristotle

Aristotle - (384 BC – 322 BC) was a Greek philosopher
Della Porta Collection then Borghese Collection; purchased in 1807
English: Portrait of Galileo Galilei by Justus Sustermans painted in 1636.

Galileo (and Newton) would say ...

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An object in motion doesn't require a force to stay in motion

I Will Derive!!

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Which plots below represent an object in a state of equilibrium?

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A, C, D

Seatbelt locking mechanism before a sudden stop: a steel ball resting on an inclined ramp below a pivoting locking pawl next to a toothed sprocket NYSSLS Reading Passage icon

NYSSLS Reading Passage · Dynamics

The Physics of Survival

Open Reading Passage → Regents Practice Version →

Check Your Understanding

5 quick questions — pick an answer to see if you’re right.

1. What is a free body diagram?

2. A lamp weighing 20 N rests on a horizontal table. What is the normal force on the lamp?

3. A box rests on a ramp. Compared with the box's weight, the normal force on it is…

4. A block is pulled to the right at constant velocity. What is true about the forces on it?

5. A ball flies through the air from left to right (ignore air resistance). Which force(s) act on it?

NYSSLS Regents Practice · Dynamics · Force Models

Free-Body Diagrams of a Kicked Soccer Ball

A soccer ball is kicked into the air and follows a curved path across a field. Air resistance is negligible. Students drew free-body diagrams to model the forces on the ball at several points after it left the player’s foot.

Which statement correctly describes the force(s) on the ball at the highest point of its path?

▶ Want a challenge? Open 6 more Regents-style questions  (harder · mixed formats)

Challenge score: 0 / 6

Multiple choice

C1. A skydiver falls at a constant terminal velocity. Which statement is correct?

Support or Refute · Justify

C2. A student makes the following claim.

Claim: While the soccer ball is rising, its free-body diagram must include an upward force, because the ball is moving upward.

Place a check mark in either the Support or Refute box. Justify your response.

Draw the vector · Determine the magnitude

C3. The soccer ball has a mass of 0.43 kg. While it is rising, choose the direction of the net force on the ball, and determine its magnitude.

ball
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Fnet = N
Multiple choice

C4. A person stands on a scale in an elevator that is accelerating upward. Which comparison is correct?

Substitute values

C5. An 8.0-kg box is pulled to the right with a 40.-N horizontal force while 16-N of friction acts on it. Substitute numerical values into a mathematical representation that can be used to determine the box’s acceleration. Then calculate the acceleration.

a = Fpull − Ffm = N − N kg = m/s2
Constructed response

C6. Describe the free-body diagram for the soccer ball just before it lands, and explain how it compares with the free-body diagram at the highest point of its path.