Demo 1



[pic]

|Materials |Coefficients of Static Friction[pic] |Coefficients of Kinetic Friction [pic] |

|Steel on Steel |0.74 |0.57 |

|Aluminum on Steel |0.61 |0.47 |

|Copper on Steel |0.53 |0.36 |

|Rubber on Concrete |1.0 |0.8 |

|Wood on Wood |0.25-0.5 |0.2 |

|Glass on Glass |0.94 |0.4 |

|Waxed wood on Wet snow |0.14 |0.1 |

|Waxed wood on Dry snow |- |0.04 |

|Metal on Metal (lubricated) |0.15 |0.06 |

|Ice on Ice |0.1 |0.03 |

|Teflon on Teflon |0.04 |0.04 |

|Synovial joints in humans |0.01 |0.003 |

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Why do you feel thrown backwards when a car starts up quickly from rest? Are you really thrown backward? What is actually going on?

Why do you feel thrown forward when a speeding car stops abruptly? Are you really thrown forward? What is actually going on?

B

A

1) Which object has more of a tendency to remain at rest?

2) Which object has more of a resistance to changing its state?

3) Which object has more inertia?

4) Which object has more mass?

Definitions:

1. Inertia:

2. Equilibrium:

3. Net External Force:

Observations:

1. A force is not necessary for

Two natural states of motion:

2. A force is necessary for

Forces and Motion

Newton’s Third Law of Motion (Action-Reaction Law)

If object 1 exerts a force on object 2, then object 2 simultaneously exerts a force equal in magnitude but opposite in direction on object 1.

Sir Isaac Newton (1643-1727), English mathematician and physicist, considered one of the greatest scientists in history, 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.

1. What is the title in English?

2. By what title is it commonly known?

3. When was it published?

4. What did it contain?

a)

b)

Newton’s Second Law of Motion (Force Law)

[pic].

The net external force on an object is equal to the mass of that object multiplied by the acceleration of the object. With a constant force, the mass of an object and its acceleration are inversely related.

Newton’s First Law of Motion (Law of Inertia)

An object at rest remains at rest and an object in motion with a constant velocity remains in motion with a constant velocity until acted on by a net external force

Newton’s Three Laws of Motion (original version translated from Latin)

“Every object persists in its state of rest or uniform motion in a straight line unless it is compelled to change that state by forces impressed on it.”

“The alteration of motion is ever proportional to the motive force impressed; and is made in the direction of the right line in which that force is impressed.”

“To every action there is always opposed an equal reaction: or the mutual actions of two bodies upon each other are always equal, and directed to contrary parts.”

Below is the title page of Isaac Newton’s greatest work and one of the most influential books in all of science.

Isaac Newton

Fg:

FA:

FT:

1. What is the cause of the normal force?

2. What is the cause of the frictional force?

Note: Objects can be considered as point particles or point sources from which the forces acting on them originate. They are idealized objects that have mass but no extent in space.

[pic]

4.

2.

1.

What is a “free-body diagram” or a “force diagram”?

Rank the fundamental forces from strongest to weakest:

FN or R:

Ff:

3.

Identifying Forces: Free-Body Diagrams

Range

Property

Name

Forces

Types of Forces

Holds nucleus together

9.

8 N

8 N

5 N

16 N

8.

10 N

6 N

7.

In each case below, determine the net force acting on the cart (magnitude and direction).

Net force (Fnet or ΣF):

4. What will happen to the cart if weight #1 is heavier than weight #2?

5. What will happen to the cart if weight #2 is heavier than weight #1?

6. What will happen to the cart if both weights are the same?

1. What will happen to the cart? Why?

2. What will change if a heavier weight is used?

3. What will change if a heavier cart is used?

A second weight is now hung over a pulley by a string pulling on the cart in the opposite direction.

A weight is hung over a pulley by a string that is attached to a cart. The weight is dropped.

Balanced and Unbalanced Forces

Write the unit for force in terms of fundamental units:

Balanced Forces

If all the forces acting on an object are balanced,

Unbalanced Forces

If all the forces acting on an object are not balanced,

Newton’s Second Law of Motion:

Conclusions:

1)

2)

3)

|Variable: |Fnet |m |a |

|Quantity: | | | |

|Units: | | | |

|Type: | | | |

1) A net force of 100. N acts west on a 5.0 kg mass. Determine the magnitude and direction of the acceleration of the mass.

2) A 1.6 kg box is accelerated at 2.0 m/s2. Determine the magnitude of the net force.

3) An 80. kg student is pulled on roller blades by a friend who exerts a force of 20.0 N. Friction between the wheels and the ground exert a force of 5.0 N. What is the student’s acceleration?

4) A 1000. kg car accelerates from rest to 20. m/s in 5.0 seconds. What net force acts on the car?

5) A 15.0 kg crate is dragged across the floor with an acceleration of 0.80 m/s2 by an applied force of 22 N. How much friction is acting on the crate?

Mass and Weight

1. What is the weight of a 1.0 kilogram mass: a) here on Earth? b) In deep space?

2. What is the mass of a 1.0 N apple: a) here on Earth? b) In deep space?

Estimation Skills - some common masses and weights:

Penny = 3 grams (0.003 kg) 1 kilogram mass = 2.2 pounds 1 apple = 1 newton

|Variable: |Fg |g |m |

|Quantity: | | | |

|Units: | | | |

|Type: | | | |

Mass:

Property:

Weight:

Property:

Relationship between mass and weight:

| |Earth |Moon (g = 1.6 m/s2) |Deep Space |

|Mass | | | |

|Weight | | | |

4. What is your mass in the metric system of measurement?

5. Complete the chart below for your mass and weight in various places using the metric system.

6. The weight of a plumber’s wrench on Planet Goozh is 22 N and its mass here on Earth is 8.2 kg. For Planet Goozh, calculate the:

a) acceleration due to gravity b) gravitational field strength

Comparing “a” and “g”

b) the tennis ball thrown upwards on its way up

c) the tennis ball thrown upwards at its peak

3. For each object below, draw and label a free-body diagram, then determine the magnitude of all the forces acting on it, the net force acting on it, and its acceleration.

a) a 0.25 kg tennis ball at rest on a table

Applied force (N)

Friction force (N)

Kinetic Friction (Ffk) (dynamic friction, sliding friction)

1)

2)

3)

Static Friction (Ffs)

1)

2)

3)

c) movement begins

d) while moving

a) no movement

b) no movement

Cause of solid friction:

Solid Friction

In each case below, the car is screeching to a halt across a different surface. A value known as the “coefficient of friction” is listed below each case.

2. Why are there two types of coefficients of friction?

3. Compare the coefficients of kinetic friction and static friction.

4. What materials on top of one another are the:

a) easiest to start moving?

b) hardest to start moving?

5. What materials are the:

a) easiest to slide over one another?

b) hardest to keep moving?

6. What is the coefficient of friction that applies while you are dragging a wooden crate across a hardwood floor?

7. What is the coefficient of friction that applies when a car skids across a dry asphalt roadway?

1. What does the coefficient of friction measure?

Coefficient of friction

Force of Friction

Normal force

Weight

a) Draw and label a free-body diagram from this situation.

b) Calculate each force acting on the block.

c) Calculate the coefficient of kinetic friction ((k) for the block and the floor?

d) If that brick is replaced with one that has a mass of 8.0 kg, which of the following will change? If so, how much?

8. A 4.0 kg block is being dragged across the floor by a student who finds that he must exert a force of 14.0 N to keep the block moving at a constant velocity.

|Variable: |Ff |μ |FN |

|Quantity: | | | |

|Units: | | | |

|Type: | | | |

Relationship:

10. A driver slams on the brakes and her 2.0 x 103 kg car skids to a stop on a dry asphalt highway.

a) What is the force of friction stopping the car?

9. A 45 N sled rests on a frozen pond. A child pulls the sled with a horizontal force of 15.0 N and causes it to accelerate at a rate of 1.50 m/s2.

b) Determine the coefficient of friction.

b) What is the acceleration of the car while it is stopping?

a) Determine the force of friction.

12. A 20. N wooden box is dragged across a wooden floor with a force as shown. Determine if the box is accelerating or not and justify your answer. If the box is accelerating, determine its rate of acceleration.

b) normal force acting on the box.

a) force of friction acting on the box.

11. A 5.7 kg block is dragged at a constant velocity of 3.2 meters per second by a 100. N force as shown. Determine the:

3) The force exerted by ____________________ on ____________________ is equal and opposite to the force exerted by _____________________ on ____________________.

2) The force exerted by ____________________ on ____________________ is equal and opposite to the force exerted by _____________________ on ____________________.

1) The force exerted by ____________________ on ____________________ is equal and opposite to the force exerted by _____________________ on ____________________.

Give some examples of “action-reaction pairs” of forces:

Action-Reaction pairs:

If a heavier student pushes a lighter student, who exerts more force? Explain.

|Activity 4 |Activity 5 |Activity 6 |

| | | |

Newton's Third Law of Motion:

|Activity 1 |Activity 2 |Activity 3 |

| | | |

Newton’s Third Law

30.0

30.0

3. Calculate the tension in the rope holding this 50. N box held at rest on a hill if there is a 10. N force of friction.

2. Calculate the force of tension in the rope holding this 50. N box held at rest on a frictionless hill.

1. Draw and calculate the magnitude of all the forces acting on this box sliding down a hill at a constant speed of 12 m/s.

If the box is in equilibrium, then . . .

Fg┴ =

Fg║ =

Resolving Fg into components that are parallel and perpendicular to the surface of the inclined plane:

Draw a free-body diagram for this box at rest on an inclined plane.

θ

The Inclined Plane

5. Calculate how fast this box will accelerate down the hill if the hill is frictionless.

6. Calculate the force of friction acting on the box if it now accelerates down the incline at a rate of 0.67 m/s2.

7. As the angle θ increases, what happens to the . . .

weight?

normal force?

force of friction?

coefficient of friction?

4. Calculate the tension in the rope when this 50. N box is being dragged up the hill at a constant speed of 5.0 m/s if there is a 10. N frictional force acting on it.

30.0

20.0

4.5 kg

20.0

4.5 kg

What are some properties of an object

in translational equilibrium?

Translational Equilibrium

Draw the resultant.

Sketch a free-body diagram for this box at rest on a hill.

For each system below, sketch a resultant vector and an equilibrant vector.

Based on your drawings, what is another property of an object in translational equilibrium?

1. Which of the following sets of forces could be in equilibrium?

a) 2 N, 3 N, 4 N b) 10 N, 20 N, 30 N c) 20 N, 20 N, 50 N d) 30 N, 30 N, 30 N

Rule for equilibrium of 3 force vectors:

2. Which of the following forces could put a 5 N and a 10 N force into equilibrium?

a) 2 N b) 5 N c) 7 N d) 12 N e) 15 N f) 17 N

Equilibrant:

Now, draw a single vector that will put the system into equilibrium.

Is the system below in equilibrium?

Then, find the resultant of the vectors you drew.

What is the relationship between the resultant and the equilibrant?

5. A 20.0-kg floodlight in a park is supported at the end of a horizontal beam of negligible mass that is hinged to a pole, as shown. A cable at an angle of 30.0° with the beam helps to support the light. Find the tension in the cable.

4. Find the resistive force F caused by the drag of the water on this boat moving at a constant velocity.

b) bird feeder

a) picture frame

3. Find the tension in each wire in each diagram shown below.

2. Compare the force of gravity to the force of air resistance for this 100. kg skydiver as well as his acceleration.

1. What are some factors that affect the amount of air resistance a falling object experiences?

Fluid Resistance and Skydiving

6. Which experiences a greater force of air resistance: an elephant or a feather? Explain.

4. As the skydiver falls,

5. What is the force of air resistance acting on a 60 kg skydiver at terminal velocity?

3. What happens when the force of air resistance equals the force of gravity?

Terminal velocity:

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