Forces

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

Force

Forces come in pairs A force is a vector Forces have a magnitude and a direction

5N, north (up) Magnitude: 5NDirection: north (up)

Types of Forces

There are two types of forces Contact forces Forces at a distance

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

Applied Force

Identified as Fa

Applied to an object by a person or another object

Acts in the horizontal direction, parallel to the surface

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

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

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

Air Resistance Force

Identified as Fair

Type of frictional force that acts upon objects as they travel through the air

Acts opposite motion

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

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

Balanced and Unbalanced Forces

Forces occur in pairs and they can be either balanced or unbalanced

Balanced Forces

Balanced forces do not cause change in motion, they are in equilibrium

They are equal in size and opposite in direction

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

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

Unbalanced Forces

3 N, right – 6 N, left = 3N, left

Unbalanced Forces

4 N, left – 10 N, right = 6N, right

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

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.”

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

How to Draw a Force Body Diagram

Step 6: Draw and label the vertical forces Step 7: Calculate the sum of all the forces

Example

A book is at rest on a tabletop. Diagram the forces acting on the book.

Newton’s 2nd Law

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

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?

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

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

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

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

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

Bellwork

Pick up today’s notes Work on Tuesday’s bellwork Get out a calculator and complete the

remaining practice problems from yesterday

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?

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?

Second Law Simulation

Forces Simulation Independent variable: Dependent variable: Control Variable:

Acceleration Vs. Force Graphs

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

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.

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

Example 1A net force of 24 N is applied to an object whose mass is 14 kg. What is the object’s acceleration?

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?

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

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

Practice problems

Diagram: Balanced Vs. Unbalanced Forces

Bellwork

Pick up today’s notes and bellwork Work on today’s bellwork Do not play with the materials

Friction

Question of the day . . . Once a skateboard has been pushed, what are the forces acting on it?

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

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

Harmful friction

Causes

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

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

Coeffieicent of Friction

Is symbolized with greek letter mu. Mu is calculated using the ratio between

the frictional force and the normal force Equation

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?

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?

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

Terminal velocity Drag Air friction Aerodynamics

Terms to Know

Demo- Drag

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?

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

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

Newton’s 3rd law

Forces act in force pairs For every action, there is an equal

and opposite reaction.

Baseball pushes glove to the left

Man pushes car

Foot pushes floor

Bowling ball pushes pin leftward

Bus hits bug

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?