forces. a force is a push or a pull exerted on an object forces cause objects to speed up, slow...
TRANSCRIPT
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Forces
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Forces
A FORCE is a push or a pull exerted on an object
Forces cause objects to speed up, slow down, or change direction.
Force is measured in Newtons
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Force
Forces come in pairs A force is a vector Forces have a magnitude and a direction
5N, north (up) Magnitude: 5NDirection: north (up)
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Types of Forces
There are two types of forces Contact forces Forces at a distance
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Contact Forces
Contact forces- types of forces that result when the two interacting objects are perceived to be physically contacting each other
Examples of contact forces: 1. Applied force
2. Friction force
3. Tension force
4. Normal force 5. Air resistance
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Applied Force
Identified as Fa
Applied to an object by a person or another object
Acts in the horizontal direction, parallel to the surface
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Friction Force
Identified as Ff
Acts opposite sliding motion between surfaces
Acts in the horizontal direction, parallel to the surface and opposite the direction of sliding
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Tension Force
Identified as FT
Pull exerted by a rope, string, or cable attached to a body and pulled tight
Acts away from the object and parallel to the string, rope or cable
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Normal Force
Identified as FN
Exerted by a surface on an object (presses two surfaces together)
Acts in an upward, vertical direction, perpendicular to the surface
Sometimes referred to as the support force
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Air Resistance Force
Identified as Fair
Type of frictional force that acts upon objects as they travel through the air
Acts opposite motion
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Long Range Forces
Long Range Forces- types of forces that result even when the two interacting objects are not in physical contact with each other, yet are able to exert a push or pull despite their physical separation
Examples of long range forces: 1. Gravitational force
2. Electrical force
3. Magnetic force
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Gravitational Force
All objects on Earth experience a force of gravity that is directed downward towards the center of the earth.
Identified as FW
Referred to as the force of weight Acts in the vertical direction
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Balanced and Unbalanced Forces
Forces occur in pairs and they can be either balanced or unbalanced
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Balanced Forces
Balanced forces do not cause change in motion, they are in equilibrium
They are equal in size and opposite in direction
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Unbalanced Forces
An unbalanced force always causes a change in motion
Forces are not in equilibrium When unbalanced forces act in opposite
directions you can find the net force The net force is the sum of all the forces
acting on an object
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Net Force
A net force shows magnitude and direction The magnitude is the difference between
forces acting on an object The resulting motion of an object is in the
direction of the largest force
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Unbalanced Forces
3 N, right – 6 N, left = 3N, left
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Unbalanced Forces
4 N, left – 10 N, right = 6N, right
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Force Body Diagrams
Used to show all the forces acting on an object (can be 1,2,3, or 4 forces)
Can help you understand the motion of an object
Force vectors are represented by arrows The length of the arrow represents the
magnitude (size) of the force
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Bellwork
Write two multiple choice questions. You must give 4 answer choices. Follow these rules: 1. One wrong-answer choice must be funny.
2. Only one answer choice may be right.
3. You may not use “all or the above” or “none of the above.”
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How to Draw a Force Body Diagram
Step 1: Draw a sketch of the situation Step 2: Identify the horizontal forces Step 3: Identify the vertical forces Step 4: Draw a box, place a dot in the
middle Step 5: Draw and label the horizontal
forces
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How to Draw a Force Body Diagram
Step 6: Draw and label the vertical forces Step 7: Calculate the sum of all the forces
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Example
A book is at rest on a tabletop. Diagram the forces acting on the book.
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Newton’s 2nd Law
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Review: Net Force The sum of all forces acting on an object When net force is greater than zero, the object
accelerates in the direction of the largest force If the net force is zero the object is either
1. At rest OR
2. Moving with constant velocity
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Physics of Footballhttp://www.nsf.gov/news/special_reports/football/lawofmotion.jsp What is Inertia? What is an unbalanced force? What does Newton’s First Law of
Motion state?
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Newton’s First Law• An object at rest stays at rest and
an object in motion stays in motion with the same speed and in the same direction unless acted upon by an unbalanced force.
• Newton’s first law is sometimes called the Law of Inertia
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Inertia• Inertia is a property of matter that
tends to resist a change of motion• If an object is at rest, it wants to
stay at rest• If an object is in motion, the object
wants to stay in motion
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Mass Mass is the amount of matter that makes
up an object Mass is a measure of an object’s inertia The more mass an object has, the
greater its inertia The unit for measuring mass is the
kilogram
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Weight Weight is the force that results from the
pull of gravity on an object’s mass The SI unit for weight is Newtons Can be determined by using the
equation
mgFw
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Example On Earth, a scale shows that you weigh
585 N. What is your mass? What would the scale read on the moon, where gravity is 1.67 m/s2?
Mass = 59.7 kg
Force on moon: 95.5 N
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Bellwork
Pick up today’s notes Work on Tuesday’s bellwork Get out a calculator and complete the
remaining practice problems from yesterday
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Example 2: On Planet X a 50 kg barbell can be lifted by exerting a force of 180 N. What is the acceleration of gravity on Planet X?
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Example 3: A 71.1-kg man travels from an area where the gravity is 9.78 m/s2 to where it is 9.80 m/s2. A. What is his change in mass?
B. What is his change in weight?
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Second Law Simulation
Forces Simulation Independent variable: Dependent variable: Control Variable:
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Acceleration Vs. Force Graphs
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Newton’s 2nd Law Newton’s 2nd Law helps is identify how an
object’s motion changes when a net force is applied to the object
When forces are unbalanced, the object accelerates
Acceleration is dependent on two variables
1. Net force
2. The mass of an object
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Newton’s 2nd Law http://www.nbclearn.com/nfl Causes a change in motion A change in motion means a change in
velocity A change in velocity is called an
acceleration Newton’s 2nd law describes motion when
unbalanced forces act on an object If forces are unbalanced, the net force is
greater than zero.
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Newton’s 2nd Law Summary
The acceleration of an object depends directly on the net force acting on the object, and inversely on the mass of the object.
As the force acting on an object is increased, the acceleration of the object is increased.
As the mass of an object is increased, the acceleration of the object is decreased
The acceleration is always in the direction of the net force
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Example 1A net force of 24 N is applied to an object whose mass is 14 kg. What is the object’s acceleration?
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Example 2 Thomas, the 72 kg star forward on
Buford High School’s basketball team, collides with Killer, a guard for the other team, and is brought to a stop with an acceleration of –20 m/s2. What force does Killer exert on Thomas?
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Bellwork
Work on Wednesday’s bellwork Get out calculator and practice from
yesterday Complete the practice problems from
yesterday Complete today’s assignment Read October Sky CH. 8-9 OR work on
research for Science Fair project
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Bellwork
Complete Thursday’s bellwork Work on practice from yesterday Tomorrow:
Quiz over Newton’s Laws/Forces and October Sky
Be sure to bring your logbook and October Sky
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Practice problems
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Diagram: Balanced Vs. Unbalanced Forces
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Bellwork
Pick up today’s notes and bellwork Work on today’s bellwork Do not play with the materials
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Friction
Question of the day . . . Once a skateboard has been pushed, what are the forces acting on it?
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Predict
On which material will the toy walk fastest, your lab table or the sandpaper?
Design a mini lab to test your prediction? Was your hypothesis correct? Why or why not? http://www.nbclearn.com/olympics
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Amount of friction
depends on:
Types of Friction
STATIC FRICTION
Friction Notes
KINETIC FRICTION
A force that opposes motion
ROLLING
Increasing force/weight, increases the friction
2)Roughness of the surface
1) Force (weight)
pushing surfaces together
Rougher the surface, greater the friction
Applying a force & NO movement
SLIDING
Friction b/w MOVING surfaces
Examples
Examples
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Harmful friction
Causes
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Coefficient of Friction
The friction between surfaces is a direct result of the surface and the force pressing surfaces together, the normal force
On a flat surface, when vertical forces are balanced, the normal force is equal to the force of weight
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Coefficient of Friction
The coefficient of friction refers to the “stickiness” of two surfaces
The closer to 1, the greater the friction The closer to 0, the less the friction
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Coeffieicent of Friction
Is symbolized with greek letter mu. Mu is calculated using the ratio between
the frictional force and the normal force Equation
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Example
A hockey puck has a coefficient of kinetic friction of μk = .10. If the puck feels a normal force (FN) of 5 N, what is the frictional force that acts on the puck?
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A block has a weight of 5 N. An applied horizontal force of 1.5N is required to cause the block to slide with constant speed on a smooth concrete floor. What does “constant speed” tell you about the shoe?
Draw a force diagram for the block.
What is the coefficient of sliding friction?
What is the mass of the block?
If the coefficient of friction were 0.10, what would be the acceleration of the block?
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Bellwork
Answer Tuesday’s bellwork questions Define each of the terms (not already
defined) on the side board on your vocab organizer for this unit
Get out a calculator
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Terminal velocity Drag Air friction Aerodynamics
Terms to Know
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Demo- Drag
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Drag is the force exerted by a fluid on the object moving through the fluid
Air is a fluid Drag is dependent on the motion of the object,
the properties of the object, and the properties of the fluid.
Drag is a resistance to motion, so it acts opposite of motion (air friction)
What is Drag?
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Aerodynamics is the study of forces and the resulting motion of objects through the air.
Drag also effects objects not in free fall Air exerts force in all directions http://www.nsf.gov/news/special_reports/s
os/drag.jsp
Drag and Aerodynamics
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Terminal Velocity is the constant velocity that is reached when the drag force equals the force of gravity
http://www.youtube.com/watch?v=kNED5RzqxOo
Terminal Velocity
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Newton’s 3rd law
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Forces act in force pairs For every action, there is an equal
and opposite reaction.
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Baseball pushes glove to the left
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Man pushes car
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Foot pushes floor
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Bowling ball pushes pin leftward
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Bus hits bug
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Newton’s Third Law Problem:
How can a horse pull a cart if the cart is pulling back on the horse with an equal but opposite force?
NO!!!
Aren’t these “balanced forces” resulting in no acceleration?
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