/PP_Revised

Forces and Motion Prentice Hall Physical Science, Concepts in Action, pages 354 – 387; http://www.physicsclassroom.com/ Class/newtlaws/newtltoc.html

Objective 1 

Define force and describe how forces affect the motion of an object; state the SI unit for force.

Forces  A force is a push or pull that acts on

an object.  A force can set an object at rest into motion, or it can accelerate a moving object by changing the object’s speed or direction.

The Newton  Force is measured in Newtons,

abbreviated as N.  One Newton is the force that causes a 1­kilogram mass to accelerate at a rate of 1 meter per second each second (1 m/s2).  One Newton is equal to one kilogrammeter per second squared (1 N = 1 kgm/s2).

Objective 2 

List the four universal forces and describe each of these forces.

Universal Forces  Observations of planets, stars and

galaxies strongly suggest four different forces exist throughout the universe.  These forces are known as universal forces.  The four universal forces are the electromagnetic, strong nuclear, weak nuclear, and gravitational.

1. Electromagnetic Forces  Electric and magnetic forces are two

different aspects of the electromagnetic force.  Electromagnetic force is associated with charged particles.  Electric force and magnetic force are the only forces that can both attract and repel.

Electric Forces  Electric forces act between charged

objects or particles such as electrons and protons.  Objects with opposite charges – positive and negative – attract one another.  Objects with like charges repel one another.

Magnetic Forces  Magnetic forces act on certain

metals, on the poles of magnets and on moving charges.  Magnets have two poles, north and south.  Two poles that are alike repel each other.  Two opposite poles attract each other.

2. Strong Nuclear Forces  The strong nuclear force is a powerful

force of attraction that holds the nucleus together.  Although this force acts over only extremely short distances, it is 100 times stronger than the electric force of repulsion.

3. Weak Nuclear Forces  The weak nuclear force is weaker in

strength than the strong nuclear force.  The weak nuclear force is an attractive force that acts only over a short range and affects all particles.

Your mass on different planets

 http://www.solarviews.com/eng/edu/weight.htm

4. Gravitational Forces  Gravitational force is an attractive force

that acts between any two masses.  Newton’s law of universal gravitation states that every object in the universe attracts every other object.  Gravitational forces act over large distances.  Gravity is the weakest universal force, but it is the most effective force over long distances.

Objective 3 

Identify forces as vector quantities and define net force.

Forces  Forces are vector quantities.  That is, they have both magnitude and

direction.  Vectors are represented by using arrows.  The direction of the arrow represents the direction of the force.  The length of the arrow represents the strength, or magnitude, of the force.

Combining Force Vectors  Vector addition may be used to combine

force vectors.  Forces acting in the same direction add together; forces acting in opposite directions are subtracted.  The Pythagorean Theorem may be used to combine vectors acting at right angles.  A net force is the overall force acting on an object after all the forces are combined.

Finding Net Force

Objective 4  Distinguish

between balanced and unbalanced forces.

Balanced and Unbalanced Forces  Balanced forces are forces that combine

to produce a net force of zero.  When balanced, there is no change in the object’s motion. It is at rest.  An unbalanced force results when the net force acting on an object is not equal to zero.  When an unbalanced force acts on an object, the object accelerates.

Balanced Forces

Unbalanced Forces

Situation A  What is the net

force vertically? 20 N – 20 N = 0

 What is the net

force horizontally? 0

 Are the forces

balanced? yes

Situation B  What is the net

force vertically? 3 N – 3 N = 0

 What is the net

force horizontally? 5 N – 5 N = 0

 Are the forces

balanced? yes

Situation C  What is the net force

vertically?

 40 N – 25 N = 15 N ↑

 What is the net force

horizontally? 0

 Are the forces

balanced?  no

 What is the net

force?

 15 N upward

Situation D  What is the net

force vertically?  3 N – 3 N = 0

 What is the net

force horizontally?  5 N leftward

 Are the forces

balanced?  no

 What is the net

force?

 5 N leftward

Sample Problem  A rightward force of 24 N is applied to

a 100 N object to move it across a rough surface at constant velocity.

The frictional force is 24 N. Draw a force diagram showing all forces acting on the object. Are the forces balanced?

Answer: net F = 0 Ftable = 100 N constant velocity

Ff = 24 N

Fa = 24 N

Fgrav = 100 N

Sample Problem  Suppose the object in the above

problem has an applied force of 36 N. Draw the new force diagram. Are the forces balanced? What is the net force?

Answer: net F = 12 N right Ftable = 100 N

Ff = 24 N

Fa = 36 N

Fgrav = 100 N

Objective 5  Define friction and describe the

four main types of friction.

Friction  All moving objects are subject to friction,

a force that opposes the motion of objects that touch as they move past each other.  Friction acts at the surface where objects are in contact.  There are four main types of friction:  static friction,  sliding friction,  rolling friction, and  fluid friction.

1. Static Friction  Static friction acts on objects that

are not moving.  Static friction always acts in the direction opposite to that of the applied force.

2. Sliding Friction  Sliding friction opposes the direction

of motion of an object as it slides over a surface.  Less force is needed to keep an object moving than to start it moving.

3. Rolling Friction  When a round object rolls across a

flat floor, both the object and the floor are bent out of shape.  This change in shape at the point of rolling contact is the cause of rolling friction, the frictional force that acts on rolling objects.

Rolling Friction, continued  For a given set of materials, the force

of rolling friction is about 100 to 1000 times less than the force of static or sliding friction.  Because of this, wheeled dollies are used to move heavy objects and ball bearings are used to reduce friction in machines.

4. Fluid Friction  Water and air are both fluids.  The force of fluid friction opposes

the motion of an object through a fluid.  Fluid friction increases as the speed of the object moving through the fluid increases.

Air Resistance  Fluid friction acting on an object

moving through the air is known as air resistance.  At higher speeds, air resistance can become a significant force.

Falling With Air Resistance  As an object falls

through air, it usually encounters some degree of air resistance.  Air resistance is the result of collisions of the object's leading surface with air molecules.

Objective 6  Describe how Earth’s gravity and air

resistance affect falling objects.

Gravity  Gravity is a force that acts between

any two masses.  Gravity is an attractive force, that is, it pulls objects together.  Earth’s gravitational force exerts a force of attraction on every other object that is near Earth.

Gravity, continued  The force of gravity does not require

objects to be in contact for it to act on them.  Gravity can act over large distances.  Earth’s gravity acts downward toward the center of Earth.

Gravity and Air Resistance  Gravity causes objects to accelerate

downward, whereas air resistance acts opposite to the direction of motion.  As objects fall to the ground they continue to accelerate and gain speed.  With increasing speed comes increasing air resistance.

Terminal Velocity  If an object falls for a long time, the upward

force of air resistance becomes equal to the downward force of gravity.  At this point, acceleration is zero and the object continues falling at a constant velocity.  Terminal velocity is the constant velocity of a falling object when the force of air resistance equals the force of gravity.

Gravity and Air Resistance

Gravity and Air Resistance  Suppose that an elephant and

a feather are dropped off a very tall building from the same height at the same time.  We will assume the realistic situation that both feather and elephant encounter air resistance.  Which object ­ the elephant or the feather ­ will hit the ground first?

An Explanation

Warm­Up Question Universal Forces A. Electromagnetic B. Strong Nuclear C. Weak Nuclear D. Gravitational

Characteristics 1. Only forces that can both attract and repel.

2. Weakest force, but most effective over long distances.

3. Affects all particles in nucleus.

4. Holds neutrons and protons together in the nucleus.

Warm­Up Question  A rightward force is applied to a 60­N

object to move it across a rough surface at constant velocity. Constant velocity means net force = 0. The object encounters 15 N of frictional force. Draw the force diagram and determine the applied force.

Warm­Up Question Answered  Weight is downward

60 N

 Table pushes upward

15 N

15 N

60 N

with the same force  Frictional force pushes backward  Net force = 0 (constant velocity)  Applied force = 15 N

Prelab: Frictional Forces 1. What is friction?

2. What is the difference between static and sliding friction?

3. What effect does rolling friction have on an object?

4. Compare the strengths of static, sliding and rolling friction.

5. Draw a force diagram for a block that is being pulled across a table.

Force Diagram  FGravity = ­ FTable

Velocity

FTable

 FApplied = ­ FFriction  Starting Force –

just starts to move

FApplied

FFriction

 Three trials  Moving Force –

FGravity

moves at constant speed  Three trials

Lab: Frictional Forces Velocity

 Effect of Type of

FTable

FApplied

FFriction FGravity

Surface  Effect of Force Pressing the Surfaces Together  Effect of Rolling Friction

Lab Frictional Forces      

Start with the blue force measurer.

The four surfaces are: wood, vinyl, sandpaper and cardboard.

Three trials with each surface with a 500 g mass on top of the block.

Record starting force and moving force.

Add an extra 500 g mass, pull on wood side down.

They place wooden dowels under using 1 500 g mass and wood side down.

Objective 7  State Newton’s first law of motion

and apply it to physical situations.

Newton’s First Law of Motion  Newton’s first law of motion states:

the state of motion of an object does not change as long as the net force acting on the object is zero.  Unless an unbalanced force acts, an object at rest remains at rest, and an object in motion remains in motion with the same speed and direction.

Inertia  Newton’s first law of motion is

sometimes called the law of inertia.  Inertia is the tendency of an object to resist a change in motion.

The Motorcyclist

The Car and the Wall

Activity, pages 357 ­ 359 Figure 2A 2B 3 5A 5B

Is Net Force 0?

Effect on Motion

Activity, pages 357 ­ 359 Figure

Is Net Force 0?

2A

Yes

2B 3 5A 5B

Effect on Motion

Activity, pages 357 ­ 359 Figure

Is Net Force 0?

Effect on Motion

2A

Yes

No motion

2B 3 5A 5B

Activity, pages 357 ­ 359 Figure

Is Net Force 0?

Effect on Motion

2A

Yes

No motion

2B

Yes

3 5A 5B

Activity, pages 357 ­ 359 Figure

Is Net Force 0?

Effect on Motion

2A

Yes

No motion

2B

Yes

No motion

3 5A 5B

Activity, pages 357 ­ 359 Figure

Is Net Force 0?

Effect on Motion

2A

Yes

No motion

2B

Yes

No motion

3

Yes

5A 5B

Activity, pages 357 ­ 359 Figure

Is Net Force 0?

Effect on Motion

2A

Yes

No motion

2B

Yes

No motion

3

Yes

No motion

5A 5B

Activity, pages 357 ­ 359 Figure

Is Net Force 0?

Effect on Motion

2A

Yes

No motion

2B

Yes

No motion

3

Yes

No motion

5A

Yes

5B

Activity, pages 357 ­ 359 Figure

Is Net Force 0?

Effect on Motion

2A

Yes

No motion

2B

Yes

No motion

3

Yes

No motion

5A

Yes

No motion

5B

Activity, pages 357 ­ 359 Figure

Is Net Force 0?

Effect on Motion

2A

Yes

No motion

2B

Yes

No motion

3

Yes

No motion

5A

Yes

No motion

5B

No

Activity, pages 357 ­ 359 Figure

Is Net Force 0?

Effect on Motion

2A

Yes

No motion

2B

Yes

No motion

3

Yes

No motion

5A

Yes

No motion

No

Potted tree accelerates

5B

Objective 8  State Newton’s second law of motion

and apply it to physical situations.

Newton’s Second Law of Motion  According to Newton’s second law

of motion, the acceleration of an object is equal to the net force acting on it divided by the object’s mass.

Objective 9  Use Newton’s second law of motion

to calculate acceleration, force and mass

Second Law of Motion net force acceleration = mass

F a= m  The acceleration of an object is always in

the same direction as the net force.

Second Law, continued  Newton’s second law also applies when a

net force produces an acceleration that reduces the speed.  This is the principle used by automobile seat belts.  In a collision, the seat belt applies a force that opposes a passenger’s forward motion.  This force decelerates the passenger in order to prevent serious injury.

Sample Problem  A sailboat and its crew

have a combined mass of 655 kg. If the sailboat experiences a net force of 895 N pushing it forward, what is the sailboat’s acceleration?  1.37 m/s2

Sample Problem  Zookeepers lift a stretcher that holds a

sedated lion. The total mass of the lion and stretcher is 175 kg, and the lion’s upward acceleration is 0.657 m/s2. What is the net force necessary to produce this acceleration of the lion and the stretcher?  115 N

Sample Problem  A child is playing with

some blocks and gets mad. If a block pushed with a force of 13.5 N accelerates at 6.5 m/s2 to the left, what is the mass of the block?  2.1 kg

Objective 10 

Relate the mass of an object to its weight.

Mass and Weight  Mass and weight are not the same.  Mass is a measure of the inertia of an

object.  Weight is a measure of the force of gravity acting on an object.

Weight  An object’s weight is the product of the

object’s mass and the acceleration due to gravity acting on it.

w = m• g

Sample Problem  How much does

a 5.0­kg puppy weigh on Earth?  49 N

Sample Problem  A bag of sugar

weighs 22 N. What is its mass?  2.2 kg

Sample Problem  What is the weight

of the same bag of sugar on the moon, where the acceleration due to gravity is one­sixth that on Earth?  3.6 N

Warm­Up Question  A beach ball is left in the bed of a pickup

truck. Describe what happens to the ball when the truck accelerates forward.

Answer to Warm­Up Question  A beach ball will tend to stay at rest when

the truck accelerates forward.  The ball will move backward relative to the truck.

Warm­Up Question 

A duck is on a scooter accelerating at 2.0 m/s2. If the duck and scooter have a combined mass of 8.0 kg, what net force acts on the duck and scooter?  16 N



Where did this force come from?  The duck’s foot pushes backward on the ground and the ground pushes the duck’s foot forward with an equal and opposite force (3rd law).

Quiz: Forces and Motion  Get out a sheet of

paper.  Place your name at the top of the page.  Number it from 1­15.  You may use a calculator if you have one in class today.

Objective 11 

Explain how action­reaction forces are related according to Newton’s third law of motion.

Newton’s Third Law of Motion  Forces always exist in pairs.  Newton’s third law of motion

states: whenever one object exerts a force on a second object, the second object exerts an equal and opposite force on the first object.  These two forces are called action and reaction forces.

Third Law, continued  When you kick a soccer ball with your foot,

you notice the effect of the force exerted by your foot on the ball.  The ball experiences a change in motion.

But this is not the only force present.  The soccer ball exerts an equal but opposite force on your foot.  The force exerted on the ball by your foot is the action force, and the force exerted on your foot by the ball is the reaction force.

Third Law, continued  Note that the action and reaction

forces are applied to different objects.  These forces are equal and opposite, but this is not a case of balanced forces because two different objects are involved.  The action force acts on the ball and the reaction force acts on the foot.

Action­Reaction Forces  Identify at least six action­reaction

pairs in the picture below.

Objective 12 

Calculate the momentum of an object and describe what happens when momentum is conserved during a collision.

Momentum  A slow moving bicycle is easier to

stop than a fast moving one.  Also, a slow moving bicycle is easier to stop than a car traveling at the same speed.  Increasing either the speed or mass of an object makes it harder to stop.

Momentum, continued  A moving object has a property called

momentum that is related to how much force is needed to change its motion.  Momentum is the product of an object’s mass and its velocity.

Momentum, continued momentum = mass x velocity

p = m •v  The unit for momentum is kgm/s.  Momentum is a vector quantity (it

has direction) because velocity has a direction.

Sample Problem  Calculate the

momentum of a 6.00­kg bowling ball moving at 10.0 m/s down the alley.  60.0 kgm/s down the alley

Sample Problem  Calculate the

momentum of a 48.5­kg passenger on a train stopped on its tracks.  0 kgm/s

Sample Problem  The train starts to

move eastward with a constant velocity of 72 m/s. A baby on the train has a momentum of 360 kgm/s. What is the mass of the baby?  5.0 kg

Sample Problem  A 135­kg ostrich

has a momentum of 2190 kgm/s north. What is its velocity?  16.2 m/s north

Conservation of Momentum  Imagine that two cars of different

masses and traveling with different velocities collide head on.  Momentum can be used to predict the motion of the cars after the collision.  This is because, in the absence of outside influences, the momentum is conserved.

Conservation of Momentum, cont’d.  In physics, the word conservation

means that something has a constant value.  That is, conservation of momentum means that momentum does not increase or decrease.

Closed Systems  Momentum is conserved when the

objects are part of a closed system.  A closed system means other objects and forces cannot enter or leave a system.  Objects within the system can exert forces on one another.

Law of Conservation of Momentum  According to the law of conservation

of momentum, if no net force acts on a system, then the total momentum of the system does not change.  In a closed system, the loss of momentum of one object equals the gain of momentum of another object – momentum is conserved.

Conservation of Momentum, cont’d.  In the example above, the total

momentum of the two cars before the collision is the same as the total momentum after the collision.  This is true if the cars bounce off each other or get tangled together.

Conservation of Momentum, cont’d.  Cars can bounce off each other to

move in opposite directions.  If the cars stick together after a head­ on collision, the cars will continue in the direction of the car that originally has the greater momentum.

Car “Rear Ends” Truck

Truck “Rear Ends” Car

Head­On Collision

Warm­Up Question  The pairs of forces referred to in

Newton’s third law are equal in __________; strength opposite in __________; and direction act on __________ __________. different objects

Warm­Up Question  For every force there

exists an equal and opposite force. If the action force is considered to be that of the earth pulling down on the ball, can you identify the reaction force? Ball pulling up on earth

Warm­Up Question  If a truck and a car have a head­on

collision, which vehicle will experience the greater impact force?  The truck  The car  Both the same  It depends on other factors

Warm­Up Question  Jocko, who has a

mass of 80 kg and stands at rest on ice, catches a 20­kg ball that is thrown to him at 5 m/s. How fast do Jocko and the ball move across the ice?  500 km­m/s

Warm­Up Question The law of conservation of momentum states that a. In a closed system, the total momentum of all objects equals zero. b. In a closed system, the loss of momentum of one object is less than the gain in momentum of another object. c. In a closed system, the loss of momentum of one object is greater than the gain in momentum of another object. d. In a closed system, the loss of momentum of one object equals the gain of momentum of another object. 

Lab: Investigating a Balloon Jet 



 

Blow up the balloon to the same length for all trials.

Observe the motion of a balloon as it moves across the room.

Identify the action and reaction forces.

Use Newton’s second and third laws to explain the motion you observe.