Chapter 5 and 6 Topics: Forces and Friction

Chapter 5 and 6Topics: Forces and Friction

5.1 The cause of forces

• A force is a push or pull, or an action that has the ability to change motion.

• Forces can increase or decrease the speed of a moving object.

• Forces can also change the direction in which an object is moving.

5.1 How are forces created? • Forces are created in many ways. • For example, your muscles create

force when you swing a baseball bat.

Four Elemental Forces

• All forces in the universe come from only four basic forces.

• Electromagnetic forces are important to technology.

• Gravity is a universal force.

Force And Net Force 1•Force–Produce changes in motion–Produce a change in velocity–Cause an acceleration

Force And Net Force 2

• Force–A quantity that is capable of producing motion or a change in motion–Change in velocity–Change in acceleration

Force And Net Force 3• A force may be canceled by one or more other forces•Net effect is zero• If a force acts alone–Change in velocity or acceleration

Force And Net Force 4•Balanced forces–Equal in magnitude–Opposite in direction•Unbalanced forces–Motion occurs

5.1 Units of force

• The pound is a unit of force commonly used in the United States.• For smaller amounts, pounds are

divided into ounces (oz.). • There are 16 ounces in 1 pound.

5.1 Newtons

• Although we use pounds all the time in our everyday life, scientists prefer to measure forces in newtons. • The newton (N) is a metric unit of

force.

5.1 Unit conversions• The newton (N) is a smaller unit

of force than the pound (lb).• If one pound of force equals

4.448 newtons, then a 100 lb person weighs 444.8 newtons.

5.1 The force vector• The direction of a force makes a

big difference in what the force does. • That means force is a vector,

like velocity or position.• Arrows are often used to show

the direction of forces in diagrams.

Forces and vectors• Forces and vectors

5.1 Drawing a force vector

• The arrow points in the direction of the force.

5.1 How forces act 1• One way forces act is

the result of direct contact.• A contact force is

transmitted by matter directly touching other matter such as wind acting to slow a parachute.

5.1 How forces act 2• The force of gravity

between Earth and Moon appears to be what people once called “action-at-a-distance”.

• Today we know that the gravitational force is carried from the Earth to the Moon by a force field.Click pic

4 video

Classify these forces as contact forces or the result of force fields.

5.1 Contact forces from ropes and springs

• Ropes and springs are often used to make and apply forces.• Ropes are used to transfer

forces or change their direction.• The pulling force carried by a

rope is called tension. • Tension always acts along

the direction of the rope.

5.1 Spring forces• The force created

by a spring is proportional to the ratio of the extended or compressed length divided by the original (resting) length.

• If you stretch a spring twice as much, it makes a force that is twice as strong.

5.1 Gravity• The force of gravity on an

object is called weight. • At Earth’s surface, gravity

exerts a force of 9.8 N on every kilogram of mass.

5.1 Weight vs. mass• Weight and mass are not the

same.• Mass is a fundamental property

of matter measured in kilograms (kg). • Weight is a force measured in

newtons (N).• Weight depends on mass and

gravity.

Weight depends on mass and gravity

A 10-kilogram rock has the same mass no matter where it is in the universe. On Earth, the10 kg. rock weighs 98 N.. On the moon, the same rock only weighs 16 N.

5.1 Calculating weight

• Calculate the weight of a 60-kilogram person (in newtons) on Earth and on Mars.

1. Looking for: – …weight of person in newtons on both

planets– Given: – …mass = 60 kg; g = 3.7 N/kg on Mars; – …implied g = 9.8 N/kg on Earth

2. Relationships: – W = m x g

3. Solution: – 60 kg x 9.8 N/kg = 588 N– 60 kg x 3.7 N/kg = 222 N

Solving Problems

5.2 Learning Goals

• Define what friction is.• Know what causes friction• Be able to list different types of

friction.• From a picture identify the type of

friction.• Describe how to reduce friction and

uses of friction.

5.2 Friction 1• Friction is a

force that resists the motion of objects or surfaces.

• Many kinds of friction exist.

• Video Bill Friction 8 min

5.2 Friction 2 Fill in boxes

5.2 Friction and two surfaces• Friction depends on both of the

surfaces in contact.• When the hockey puck slides on ice,

a thin layer of water between the rubber and the ice allows the puck to slide easily.

5.2 Identifying friction forces 1• Friction is a

force, measured in newtons just like any other force.

• Static friction keeps an object at rest from moving.

• Sliding friction is a force that resists the motion of an object moving across a surface.

• Friction clip play @ 3:34

5.2 Identifying friction forces 2

5.2 A model for friction• Friction depends

on a material’s properties such as roughness, how clean the surfaces are, and other factors.

• The greater the force squeezing two surfaces together, the greater the friction force. v

5.2 Reducing the force of friction

• Unless a force is constantly applied, friction will slow all motion to a stop eventually.

• It is impossible to completely get rid of friction, but it can be reduced.

5.2 Reducing the force of friction• The friction

between a shaft (the long pole in the picture) and an outer part of a machine produces a lot of heat.

• Friction can be reduced by placing ball bearings between the shaft and the outer part.

5.2 Using friction 1• Friction is also

important to anyone driving a car.

• Grooved tire treads allow space for water to be channeled away from the road-tire contact point, allowing for more friction in wet conditions.

5.2 Using friction 2

• Shoes are designed to increase the friction between their soles and the ground.

How do you think these shoes increase friction?

5.2 Friction and energy 1Friction changes

energy of motion into heat

energy.

5.2 Friction and energy 2• Friction is always

present in any machine with moving parts.

• If the machine is small, or the forces are low, the amount of heat produced by friction may also be small.

5.2 Friction and energy 3

• Each time two moving surfaces touch each other, tiny bits of material are broken off by friction.

• Breaking off bits of material uses energy.

Chapter 5.3 Learning Goals

• Determine the net force acting on an object.

• Define equilibrium.• Draw free-body diagrams to

represent all forces acting on a body.

Newton’s First Law of Motion 1

•What is the natural state of motion?• Aristotle–Objects require a force to stay in motion–Natural state of an object is at rest


•Galileo–Objects could naturally remain in motion–Do not come to rest on their own


•Newton–First Law of Motion–An object will remain at rest or in uniform motion in a straight line unless acted on by an external, unbalanced force


• Uniform motion in a straight line–Constant velocity

• Object at rest–Zero velocity–External forces–Internal forces


•Constant velocity–No friction–No gravitational attraction


• Inertia–Galileo–Objects tend to remain in •A state of rest•Uniform motion in a straight line


•Newton–Mass is a measure of inertia–Greater the mass, the greater the inertia


•Newton’s First Law–Law of inertia–Can apply to everyday life

Newton’s Second Law of Motion 1

• Acceleration–The greater the unbalanced force, the greater the acceleration–The greater the mass of an object, the smaller the acceleration

acceleration1

Newton’s Second Law of Motion 2

• Newton – Unit of force in the metric system (SI)• Pound – Unit of force in the

British system–Gravitational system

• Weight is a force!

Mass and Weight•Mass – The amount of matter an object contains•Weight – The gravitational force acting on an object

w vs. m

Newton’s Third Law Of Motion 1

•Newton–Impossible to have a single force–Forces always occur in pairs!


•Newton’s Third Law–For every action there is an equal and opposite reaction–Forces always come in pairs!


• Newton’s Third Law–If an object exerts a force on a second object, the second object exerts an equal and opposite force on the first object

Newton’s Law Of Gravitation 1

•Gravity–We do not know what causes it–We can only describe it


• Gravity–Associated with mass–Describes the mutual attraction of mass particles–video


• Law of universal gravitation–Every particle in the universe attracts every other particle with a force that is directly proportional to the product of their masses and inversely proportional to the square of the distance between them


• Newton’s law of universal gravitation–Universal gravitational constant (G)–6.67 10-11 N-m2/kg2

221

rmGm

F

Newton’s Law Of Gravitation 5• Satellites–Gravity supplies centripetal force

for orbit• Zero g and weightlessness

incorrect terms–Astronauts and spacecraft would

not maintain orbit–Both have weight since gravity is

acting on them


•Weightless–Astronaut and spacecraft falling toward the earth–Scale would read zero as you are falling•Not weightless and g is not zero

CYU 8.2

Momentum•Linear momentum–Product of mass and velocity–Larger mass more difficult to stop

Momentum•Linear momentum of a system–No external forces–Conserved–Does not change with time

Momentum• Linear momentum of a system–With no unbalanced forces•No acceleration•No change in velocity•No change in momentum

momentum initial Total momentum final Total

Momentum• Jet Propulsion–Burning rocket fuel–Internal work–Exhaust gas goes out back–Rocket goes in opposite direction•Conserve linear momentum

The words 8.3• Inertia- the tendency of an

object to remain at rest or in motion with constant velocity.

• Free Fall-the motion of a body when only the force of gravity is acting on it.

• Terminal Velocity- max. velocity reached by falling object when the resistance is equal to gravity.

The Quiz 8.3• Inertia

• Free Fall

• Terminal Velocity

CYU 8.3Part 1

Angular Momentum• Angular momentum–Arises when objects go in paths around a fixed point–Conserved–Elliptical orbit of planets around sun

mvr L

Angular Momentum•Torque–Twisting effect caused by one or more forces–Changes angular momentum–Produces a rotational motion

CYU 8.3 Part 2

Angular Momentum• Law of conservation of angular momentum–The angular momentum of an object remains constant if there is no external, unbalanced torque acting on it

Angular Momentum•Conservation of angular momentum–As planets get closer to the sun•Distance decreases•Velocity increases•Planets move faster closer to the sun

Angular Momentum• Conservation of angular momentum–Ice skating•Slow rotation –Extend both arms and one leg

•Gain velocity–Drawing arms and leg inwards–Decreases radial distance of the mass

CYU1-9

Momentum

• Review of the 3 laws video

5.3 Forces and Equilibrium

• The sum of all the forces on an object is called the net force.

• The word net means total but also means the direction of the forces has been taken into account.

In what direction will this plane go?

5.3 Adding forces

• To figure out if or how an object will move, we look at ALL of the forces acting on it.

• Four forces act on a plane: 1. weight2. drag (air friction)3. the thrust of the engines, and 4. the lift force caused by the flow of air

over the wings.

5.3 Equilibrium

When several forces act on the same object:

1. The net force is zero, or

2. The net force is NOT zero.

5.3 Normal forces

• When the forces are balanced, the net force is zero.

• When the net force on an object is zero, we say the object is in equilibrium.

5.3 Equilibrium and normal forces

• A normal force is created whenever an object is in contact with a surface.

• The normal force has equal strength to the force pressing the object into the surface, which is often the object’s weight.

The normal force is sometimes called the support force.

5.3 The free body diagram• How do you keep

track of many forces with different directions?

• Draw a free-body diagram that contains the objects, like a book on a table.

5.3 Solving equilibrium problems• For an object to be in equilibrium,

all the forces acting on the object must add up to zero.

Is this object in equilibrium?

Solving Problems

Two chains are used to support a small boat weighing 1,500 newtons.

One chain has a tension of 600 newtons.

What is the force exerted by the other chain?

1. Looking for: – …tension on chain 2

2. Given– …weightboat = 1,200N; tension1 = 600

N– Implied: weight and tension are forces

3. Relationships: – Net force on boat = zero

Solving Problems

4. Solution: – Draw free body

diagram

Solving Problems

Upward force of chains = weight of boat 600 N + tension2 = 1,200 N tension2 = 900 N

Chapter 5 and 6 Topics: Forces and Friction

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Transcript of Chapter 5 and 6 Topics: Forces and Friction