Chapter 5 and 6 Topics: 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

ForceForce 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

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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 newtons, then a 100 lb person weighs 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

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 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 3:34 Friction clip 3: 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 1 Friction 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? What is the natural state of motion Aristotle – Objects require a force to stay in motion – Natural state of an object is at rest

Newton’s First Law of Motion 2 Galileo – Objects could naturally remain in motion – Do not come to rest on their own

Newton’s First Law of Motion 3 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

Newton’s First Law of Motion 4 Uniform motion in a straight line – Constant velocity Object at rest – Zero velocity – External forces – Internal forces

Newton’s First Law of Motion 5 Constant velocity – No friction – No gravitational attraction

Newton’s First Law of Motion 6 Inertia – Galileo – Objects tend to remain in A state of rest Uniform motion in a straight line

Newton’s First Law of Motion 7 Newton – Mass is a measure of inertia – Greater the mass, the greater the inertia

Newton’s First Law of Motion 8 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

acceleration 1

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 Of Motion 2 Newton’s Third Law – For every action there is an equal and opposite reaction – Forces always come in pairs!

Newton’s Third Law Of Motion 3 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

Newton’s Law Of Gravitation 2 Gravity – Associated with mass – Describes the mutual attraction of mass particles – video video

Newton’s Law Of Gravitation 3 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 Gravitation 4 Newton’s law of universal gravitation – Universal gravitational constant (G) – 6.67  N-m 2 /kg 2

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

Newton’s Law Of Gravitation 6 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 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 8.3 Part 1

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

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

CYU 1-9 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.weight 2.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 – …weight boat = 1,200N; tension 1 = 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 + tension 2 = 1,200 N  tension 2 = 900 N