force and motion two families of forces type a type b man...

Students will be able to discriminate between field and contact forces and be able to list and describe the four main field forces.

Force and Motion What is a force? Units for Force:

Two families of Forces

Type A Type B Man pushing a shopping cart An object in freefall

A lady lifting up a barbell Electic charges repelling each other Friction between a tire and the road Attraction between the Sun and the Earth

Punching a punching bag A north and south pole of a magnet attracting Common Properties: Common Properties: Type: Type:

1. 3.

2. 4.

TELJR Publications © 2014 1

Students will be able to construct force vectors to create a free-body diagram

Representing Forces To show all of the forces (contact or field) acting on an object we use a …… Use a free-body diagram to show all of the forces acting on the following:

A B On a standing person

On the person

C D On the key

On the chess piece

Conclusions:


Students will be able to summarize the contributions of Aristotle, Galileo and Newton to the current theory of forces

Physics from the past – Aristotle, Galileo and Newton Aristotle’s Views: Four Elements: Natural Motions: Violent Motions

Demo 1 Demo 2 Demo 3

Q.1. Based on the preceding demonstrations, what would Aristotle conclude? Q.2. Give two other examples that would fit with Aristotle’s conclusions.

Aristotle (384-322 BC), Greek philosopher and scientist, who shares with Plato and Socrates the distinction of being among the most famous of ancient philosophers.



Galileo developed the following thought experiment: Where would the ball stop, given the following three surfaces?

Based upon this thought experiment, what is another natural state of objects?

Surface A - Surface B - Surface C -

Hockey puck demo 1 Hockey puck demo 2

Galileo Galilee (1564-1642), Italian physicist and astronomer, who, with the German astronomer Johannes Kepler, initiated the Scientific Revolution that flowered in the work of the English physicist Sir Isaac Newton.



Why is this second state of motion so difficult to observe on Earth? Pen and index card: Stand the pen on one end on top of the index card. Without touching the pen, remove the index card without causing the pen to fall.

Toy car on paper: Place the toy car on top of the paper. Move the paper back and forth quickly. Observe the motion of the car.

Observations: Observations:

Coin and cup: Place a coin on top of an index card and then place the index card on top of a cup. Without touching the coin, remove the index card, having the coin fall into the cup.

Physics hat: Watch!

Observations: Observations:

What are some common properties of the preceding demos? Inertia: Give some other examples of inertia:

Activities



Newton – Putting it All Together

Newton’s 1st Law Newton’s 2nd Law Newton’s 3rd Law

Example:

Example:

Example:

Forces are….

Forces are….

Forces are….

Sir Isaac Newton (1643-1727), English mathematician and physicist, considered one of the greatest scientists in history, who made important contributions to many fields of science. His discoveries and theories laid the foundation for much of the progress in science since his time.


Students will be able to define mass and weight, describe how they are related, calculate weight and its connection to gravity

Newton’s Second Law of Motion

Determine the net force in each of the following diagrams (all forces are 30 Newtons):

A B C

Common Properties: If the net force acting on an object is we say that the forces are___________________ . Therefore, the acceleration of the object is _____________________ and the velocity of the object is _____________________________________________. Formula for Newton's 2nd Law Factors: Relationship: Formula: Units: Vector/Scalar:

Q.1. A force of 100 N acts on a 10 kg mass, moving the mass to the west. Determine the magnitude and direction of the acceleration of the mass. Given Need Relation Solution



Q.2. What effect, if any, would friction have on your answer from question 1? Q.3. A 1.6 -kg box is accelerated at 2 m/s2. Assuming a frictionless surface, determine the magnitude of the net force. Given Need Relation Solution

Q.4. An 80 kg student is pulled on roller blades by a friend who exerts a force of 20 N. Friction between the wheels and the ground exert a force of 5 N. What is the student’s acceleration? Given Need Relation Solution

Q.5. An object has the following forces acting on it. Determine the net force and the acceleration of the object:

Item Value

F1

F2

Fnet

m

a

30 N 20 N

F1 F2

Item Value

F1

F2

Fnet

m

a



Q.2. Two forces act on an object that is at rest. One is unknown the other is measured to be 45 N. What is the value of the net force and the value of the unknown force? (Include a sketch)

Q.3. A force on an object is determined to be 68 N. The object has a mass of 17 kg and is resting on a surface that has friction. The acceleration of the object is found to be 2 m/s2.

a) Create a freebody diagram.

b) Determine the net force acting on the object and the force of friction. (Include a sketch)

c) Assuming the object started from rest, how far will it travel after 7 seconds?

Conclusions:

Item Value

F1

F2

Fnet

m

a

Item Value

F1

F2

Fnet

m

a



Comparing Balanced and Unbalanced Forces

Conclusions:

Characteristics Balanced Unbalanced Similar Different


SWBAT compare and contrast action and reaction force pairs

Newton’s Third Law of Motion Activities: Perform the activities with the force meters as directed by your teacher. Make your observations below.

Construct a theory to explain your observations. Newton's Third Law of Motion:

Q.1. Who is exerting more force in each of the diagrams below:

A B C Fly on Windshield

Bat on Ball

Fist on Face

Q.2. Who ‘feels’ this force more? Circle that object.

Force Meter 1 Force Meter 2 Force Meter 3


SWBAT compare and contrast action and reaction force pairs

Q.3. A carpenter hits a nail with a hammer. Compared to the magnitude of the force the hammer exerts on the nail, the magnitude of the force the nail exerts on the hammer during contact is (1) less (2) greater (3) the same Q.4. If a 65-kilogram astronaut exerts a force with a magnitude of 50. N on a satellite that she is repairing, the magnitude of the force that the satellite exerts on her is (1) 0 N (2) 50. N less than her weight (3) 50. N more than her weight (4) 50. N

Conclusions: TELJR Publications © 2014 12

http://uzar.files.wordpress.com/2007/08/daemon_hammer.jpg

SWBAT compare and contrast mass and weight, how they change with environment and calculations of each

Mass and Weight

Earth Deep Space The Moon The Sun

mass = 100 kg weight = 1000 N




From the above drawings, clearly describe the difference between mass and weight:

Calculating Weight (w) – The Force of Gravity (Fg) Factors Relation Formula Direction Metric Units English Units: Q.1. Using your weight in the English system of measurements, figure out your mass in the metric system of measurements. (1 kg = 2.2 lbs.) Then complete the following chart.

Quantity Earth Moon (g = 1.62 m/s2) Mars (g = 3.7 m/s2)

Mass (kg)

Weight (N)



Q.2. How could you measure mass directly? How could you measure weight directly?

Mass Weight Q.3. As you ‘fly’ away from earth what will happen to your…

Mass Weight Q.4. Would it be easier to lift a sack of potatoes off the floor here on Earth or on the Moon? Explain. Q.5. Imagine you are in deep space where you are weightless. I drive my car at 60 mph (also weightless) directly at you. Should you get out of the way? Explain. Q.6. Which quantity, mass or weight, do you feel is also a good measurement for the inertia of an object? Conclusions:



The Normal Force Why don’t you fall through your chair? Show the direction of weight and normal force on the box below. Normal Force: Symbol: In many cases: Q.1. When we step on the bathroom scale we say we are measuring our weight (weighing ourselves). What is the scale really measuring? Q.2. Determine the magnitude and direction of the weight and normal force for each of the following situations.

A B m = 25 kg

m=300 kg

Q.3. What is true about the net force in the above problems?

Item Value

FN

Fg

Fnet

a

Item Value

FN

Fg

Fnet

a



Q.4. Sometimes I like to fool myself by making myself lighter or heavier on the bathroom scale. How could I do this?

Heavier Lighter What am I really changing…… Mass Weight Normal Force Q.5. Determine the normal force in the following situations:

A B A 1 kg pizza falling through the air.

A 30,000 kg submarine in the ocean.

Conclusions: TELJR Publications © 2014 16


Drawing the Normal Force on Inclined Surfaces The normal force is drawn in each of the diagrams below and the magnitude its magnitude given. Based upon these drawings, develop a general rule for drawing the vector that represents the Normal Force.

A B C D

15 degree incline 30 degree incline 45 degree incline

E F Rule for Direction Observations of Magnitude 60 degree incline

90 degree incline Normal Force: Normal Force:

Force of Gravity (weight): Force of Gravity (weight):

Q.1. How is the angle used in determining the normal force on an incline? Q.2. If you were standing on a bathroom scale on the Titanic as the floor begins to tilt, what would happen to the reading on the scale? Conclusions:

Fg = 200 N

FN = 200 N

Fg = 200 N

FN = 193 N

Fg = 200 N

FN = 173 N

Fg = 200 N

FN = 141 N

Fg = 200 N

FN = 100 N

Fg = 200 N

FN = 000 N


Students will be able to: determine conditions where weight changes, calculate the weight in an accelerated and non accelerated reference frame.

Riding in the Elevator: Weight and Normal Force Weight of the man when elevator is at rest: Going Up: Coming Down: Start: Start: Middle: Middle: End: End: Under what conditions do the scale readings change? Under what conditions do the scale readings read the “true” weight of the person? A man (m = 60 kg) stands in an elevator and undergoes the different motions below. Determine the normal force (i.e. the reading on the scale) in each case.

A B C D

FN

w=mg

FNet=ma

a

Elevator moving at constant velocity

FN

w=mg

FNet=ma

a

Elevator accelerating upwards at 2.0 m/s2

FN

w=mg

FNet=ma

a


FN

w=mg

FNet=ma

a




Q.1. What is true about the normal force (the reading on the scale) when the elevator is accelerating upwards?

E F G H

Q.2. What is true about the normal force (the reading on the scale) when the elevator is accelerating downwards? Q.3. We would say the person in elevator H is in free-fall and the reading on the scale would be zero. In this case our elevator rider would be considered weightless. Astronauts on the space shuttle also appear to be weightless, yet they are under the influence of Earth's gravitational field. What kind of motion are the astronauts in if they appear weightless. Q.3. Some pilots, in aerobatic maneuvers, can withstand an acceleration of 8 g's (eight times that of normal gravity). How much would the pilot weigh? Conclusions

FN

w=mg

FNet=ma

a

Elevator at rest

FN

w=mg

FNet=ma

a

Elevator accelerating downwards at 3.0 m/s2

FN

w=mg

FNet=ma

a


FN

w=mg

FNet=ma

a




Being Weightless Objects in orbit around the Earth are actually in a constant state of free-fall, hence (usually) no normal force acts on them.

Objects in orbit around the Earth are actually in a constant state of free-fall, hence (usually) no normal force acts on them.


Students will be able to: use trigonometric functions, algebra and apply Newton’s 1st Law to determine tensions in ropes at various angles or masses.

Real-World Forces A force is applied to a cart. The cart moves with a constant velocity. Are the forces balanced or unbalanced? Where/what is the other force?

Friction Definition: Units: Direction: What causes friction?

Friction at Rest and Friction in Motion

At Rest In Motion

Observations:

F

F = F =



Coefficients of Friction: Friction is determined by the ‘roughness’ between two surfaces. To the right is information about this ‘roughness’ for various surfaces: Q.1. Using your prior knowledge, which surface do you think is the ‘smoothest’? What is the value of the coefficient? Coefficient of Friction - µ The coefficient is independent of ….

CAF (Consider All Factors) that determine the Force of Friction (static or kinetic):

Factors: Relationship: Formula: Units:

Q.2. A 2.00 kg rubber ducky is at rest on dry concrete. Determine the maximum value for the force of static friction. b) The rubber ducky is then pulled with enough force to cause it to move and is moving with a constant velocity. What is the value of the force of kinetic friction? What would be the value of the pulling force.

Approximate Coefficients of Friction

Kinetic Static Rubber on Concrete (Dry) 0.68 0.90 Rubber on Concrete (Wet) 0.58 - Rubber on Asphalt (Dry) 0.67 0.85 Rubber on Asphalt (Wet) 0.53 - Rubber on Ice 0.15 - Waxed Ski on Snow 0.05 0.14 Wood on Wood 0.30 0.42 Steel on Steel 0.57 0.74 Copper on Steel 0.36 0.53 Teflon on Teflon 0.04 -



Q.3. Interpret the following graph: Q.4. If the coefficient of kinetic friction for dragging a brick across the tabletop is found from experiment to be 0.23, how much force would you need to drag a 2 kg brick across the tabletop at a constant speed? Given Need Relation Solution Q.5. If that brick is replaced with one that has a mass of 4 kg, which of the following will change? If so, how much? Normal force: Force you apply: Coefficient of friction:

Item x-dir y-dir

Fp

Ffr

Fg

FN

Fnet

a

Force of Friction

Pulling Force

Slope = 1

Slope = 0



Q.6. A student applies a force of 18 N to push a 60 N cart down the hallway at a constant speed. What is the coefficient of friction between the cart and the floor? Given Need Relation Solution Q.7. Describe the motion of the cart if the student applied more than 18 N of force. Explain. Q.3. A 20-kg sled rests on a bed of ice. A pulling force of 30 N is applied to the sled and it is found to accelerate at a rate of 0.5 m/s2. Determine the following: a) Construct a free-body diagram b) Determine the weight and the normal force. c) Determine the force of friction.

Item x-dir y-dir

Fp

Ffr

Fg

FN

Fnet

a

Item x-dir y-dir

Fp

Ffr

Fg

FN

Fnet

a



The Inclined Plane Q.1. A box is at rest on an incline. Complete the following: A. Draw Free-Body Diagram B. Sketch in components of gravity that are

and to the inclined plane.

m = 10 kg

20o

C. Determine the magnitude of each of the forces acting on the incline. The box is at rest. Fg = Fgll = Fg⊥ = FN = Ffr =

Fll

F⊥

Fgll

Ffr

Fg⊥

FN

Fnet

a

Q.2. As the incline gets steeper what happens to the component of gravity that acts parallel to the incline? Compare to coasting down a shallow hill to one that is steeper. Q.3. As the incline gets steeper what happens to the component of gravity that acts perpendicular to the incline? Compare doing a push-up normally, to doing one on an incline plane.



Q.4. Determine the magnitude and direction for all forces acting on a box that is moving with a constant speed down an inclined plane.

Q.5. A 100-kg box accelerates at 2.0 m/s2 down an incline of 15 degrees. Determine the magnitude and direction of all forces acting on the box.

m = 25 kg

Fll

F⊥

Fgll

Ffr

Fg⊥

FN

Fnet

a

Fg = Fgll = Fg⊥ = FN = Ffr = 30o

15o

Conclusions:

Fll

F⊥

Fgll

Ffr

Fg⊥

FN

Fnet

a



Falling Through the Air A skydiver jumps from a plane. The forces acting on him are and . The relative magnitudes of these forces for each second of fall are shown below. The acceleration of the sky diver is given as well.

1 second 2 seconds 3 seconds 4 seconds a = -9.81 m/s2

a = -9.00 m/s2

a = -7.00 m/s2

a = -4.5 m/s2

5 seconds 6 seconds Observations: a = -1.2 m/s2

a =0

At six seconds the sky diver has reached velocity. Factors that effect this velocity are: TELJR Publications © 2014 27


Terminal Velocity of Various Objects:


force and motion two families of forces type a type b man...

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