Motion and Energy, Forces, Forces and Fluids

Motion and Speed

 An object is in motion if it changes position relative to a reference point  Stationary objects make good reference points

 Whether or not an object is in motion depends on the reference point you choose.

 Distance is the total length of the actual path between two points. Displacement is the length and direction of a straight line between starting and ending points. What is the total distance this person traveled (in blocks)? 7 Blocks What is the total displacement of this person? 5 Blocks Northeast

 Quantities that have both a magnitude and a direction  Example: Displacement

 If you know the distance an object travels in a certain amount of time, you can calculate the speed of the object.

 The speed of most moving objects is not constant

 Rate at which object is moving at a given instant in time

 Speed in a given direction  Velocity is a vector because it has both magnitude and direction  Changes in velocity may be due to changes is speed, changes in direction, or both

 You can use distance-versus-time graphs to interpret motion.

1.Is a moving bus a good reference point from which to measure your position? a. No, because it is often late. b. No, because it is not a stationary object. c. Yes, because it is very large. d. Yes, because it can travel very far.

1.Is a moving bus a good reference point from which to measure your position? a. No, because it is often late. b. No, because it is not a stationary object. c. Yes, because it is very large. d. Yes, because it can travel very far.

2.To describe a friend’s position with respect to you, you need to know a. Your friend’s distance from you. b. The direction your friend is facing. c. Your friend’s distance and direction from you. d. Your friend’s distance from a nearby object.

2.To describe a friend’s position with respect to you, you need to know a. Your friend’s distance from you. b. The direction your friend is facing. c. Your friend’s distance and direction from you. d. Your friend’s distance from a nearby object.

3. Two cars traveling in the same direction pass you at exactly the same time. The car that is going faster a. moves farther in the same amount of time. b. has more mass. c. has the louder engine. d. has less momentum.

3. Two cars traveling in the same direction pass you at exactly the same time. The car that is going faster a. moves farther in the same amount of time. b. has more mass. c. has the louder engine. d. has less momentum.

4. To describe an object’s motion, you need to know its a. position. b. change in position. c. distance. d. change in position over time.

4. To describe an object’s motion, you need to know its a. position. b. change in position. c. distance. d. change in position over time.

Acceleration

 Rate velocity changes with time  Vector quantity  In science, acceleration refers to increasing speed, decreasing speed, or changing direction  Decreasing speed = deceleration

 To determine the acceleration of an object, you must calculate its change in velocity per unit of time.  Acceleration = Final Velocity – Initial Velocity Time Chapter 9 Motion and Energy

 Calculate the plane’s acceleration in the first 5 seconds of motion. A= V f – V i time A = 40 m/s – 0 m/s 5 s A = 8 m/s 2

 As a roller-coaster car starts down a slope, its velocity is 4 m/s. But 3 seconds later, its velocity is 22 m/s in the same direction. What is its acceleration?  Read and Understand  What information have you been given?  Initial velocity = 4 m/s  Final velocity = 22 m/s  Time = 3 s Chapter 9 Motion and Energy

 As a roller-coaster car starts down a slope, its velocity is 4 m/s. But 3 seconds later, its velocity is 22 m/s in the same direction. What is its acceleration?  Plan and Solve  What quantity are you trying to calculate?  The acceleration of the roller-coaster car = __  What formula contains the given quantities and the unknown quantity?  Acceleration = (Final velocity - Initial velocity)/Time  Perform the calculation.  Acceleration = (22 m/s - 4 m/s)/3 s = 18 m/s/3 s  Acceleration = 6 m/s 2  The acceleration is 6 m/s 2 down the slope. Chapter 9 Motion and Energy

 As a roller-coaster car starts down a slope, its velocity is 4 m/s. But 3 seconds later, its velocity is 22 m/s in the same direction. What is its acceleration?  Look Back and Check  Does your answer make sense?  The answer is reasonable. If the car’s velocity increases by 6 m/s each second, its velocity will be 10 m/s after 1 second, 16 m/s after 2 seconds, and 22 m/s after 3 seconds. Chapter 9 Motion and Energy

 Practice Problem  A falling raindrop accelerates from 10 m/s to 30 m/s in 2 seconds. What is the raindrop’s acceleration?  (30 m/s - 10 m/s) ÷ 2 seconds = 10 m/s 2 Chapter 9 Motion and Energy

 Practice Problem  A certain car can accelerate from rest to 27 m/s in 9 seconds. Find the car’s acceleration.  (27 m/s - 0 m/s) ÷ 9 s = 27 m/s ÷ 9 s = 3 m/s 2 Chapter 9 Motion and Energy

 You can use both a speed-versus-time graph and a distance-versus-time graph to analyze the motion of an accelerating object. Chapter 9 Motion and Energy

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 1 st Law: An object will remain at rest or moving at a constant velocity unless it is acted upon by an unbalanced force.  2 nd Law: Acceleration depends on the net force acting on the object and on the object’s mass.  3 rd Law: If one object exerts a force on another object, then the second object exerts a force of equal strength in the opposite direction on the first object.

 A push or pull  Vector quantity  Described by its magnitude and by the direction in which it acts  Arrow represents direction  Magnitude unit = Newton (N)

 Often there is more than one force acting on an object at the same time  The result is net force, a combination of all the forces  Net force determines whether an object moves and in which direction

 The strength and direction of the individual forces determine the net force.

 Unbalanced forces acting on an object result in a net force and cause a change in the object’s velocity.

 Balanced forces acting on an object do not change the object’s velocity.

 An object will remain at rest or moving at a constant velocity unless it is acted upon by an unbalanced force  Inertia is the tendency of an object to resist a change in motion  Newton’s First Law of Motion = The Law of Inertia

 A force 2 surfaces exert on each other when they rub against each other  Acts as an unbalanced force to slow motion down  The strength of the force of friction depends on the types of surfaces involved and how hard the surfaces push together  4 types of friction

 Acts on objects that are not moving  You must exert a force greater than the force of static friction to make the object move

 Occurs when two solid surfaces slide over each other

 Occurs when an object rolls across a surface  Rolling friction is less than sliding friction for similar surfaces

 Occurs when a solid object moves through a fluid, such as air, water, oil, etc.  Fluid friction is usually less than sliding friction

 Force that pulls towards the center of the earth  Newton realized that gravity acts everywhere, not just on earth  Called the Law of Universal Gravitation  Any 2 objects in the universe attract each other

 The force of gravity between objects increases with greater mass and decreases with greater distance.

 Mass is how much matter is in an object  The gravitational force exerted on a person or object at the surface of a planet is known as weight.  Weight = Mass x Acceleration due to gravity Acceleration due to gravity at Earth’s surface = 9.8 m/s 2

 If the only force acting on the object is gravity, it is said to be in free fall  An object in free fall is accelerating because of the force of gravity at a rate of 9.8 m/s/s  This means that every second an object is free falling, it increases its velocity 9.8 m/s  Is this affected by mass? If dropped from the same height at the same time, will a heavier object fall faster?

 No! If there are no other forces to consider, then the objects will fall at the same rate, regardless of mass

 A type of fluid friction that acts on objects falling through the air  An upward force acting on a falling object

 Falling objects with a greater surface area experience more air resistance.

 Occurs when an object is thrown horizantally  Gravity will act on the object in the same way as it does when an object is dropped vertically

 An object will remain at rest or moving at a constant velocity unless it is acted upon by an unbalanced force.

 Acceleration depends on the net force acting on the object and on the objects mass  Acceleration = Net Force Mass Or Net Force = Mass x Acceleration

 A speedboat pulls a 55-kg water-skier. The skier to accelerates at 2.0 m/s 2. Calculate the net force that causes this acceleration.  Read and Understand  What information have you been given?  Mass of the water-skier ( m ) = 55 kg  Acceleration of the water-skier ( a ) = 2.0 m/s 2

 A speedboat pulls a 55-kg water-skier. The skier accelerates at 2.0 m/s 2. Calculate the net force that causes this acceleration.  Plan and Solve  What quantity are you trying to calculate?  The net force (F net ) = __  What formula contains the given quantities and the unknown quantity?  a = F net / m or F net = m x a  Perform the calculation.  F net = m x a = 55 kg x 2.0 m/s 2  F = 110 kg m/s 2  F = 110 N

 A speedboat pulls a 55-kg water-skier. The skier accelerates at 2.0 m/s 2. Calculate the net force that causes this acceleration.  Look Back and Check  Does your answer make sense?  A net force of 110 N is required. This does not include the force that overcomes friction.

 Practice Problem  What is the net force on a 1,000-kg object accelerating at 3 m/s 2 ?  3,000 N (1,000 kg x 3 m/s 2 )

 Practice Problem  What net force is needed to accelerate a 25-kg cart  at 14 m/s 2 ?  350 N (25 kg x 14 m/s 2 )

Elastic Forces

 Matter is considered elastic if it returns to its original shape after being squeezed or stretched.

 Compression  tension

 Elastic force that squeezes or pushes matter together  Example: sitting on a coach  Balanced force

 An elastic force that stretches or pulls matter  Example: Swinging on a tire swing  Balanced forces

 If one object exerts a force on another object, then the 2 nd object exerts a force of equal strength in the opposite direction on the first object.  WHAT????  = for every action there is an equal but opposite reaction.

Pressure

 The amount of pressure you exert depends on the area over which you exert a force.

Pressure = ForceArea = Length x Width Area Units: Force- Newton (N) Area-square meters (m 2 ) Pressure- Pascal (Pa)

 The area of a surface is the number of square units that it covers. To find the area of a rectangle, multiply its length by its width. The area of the rectangle below is 2 cm X 3 cm, or 6 cm 2.

 Practice Problem  Which has a greater area: a rectangle that is 4 cm X 20 cm or a square that is 10 cm X 10 cm?  The square has the greater area.  4 cm X 20 cm = 80 cm 2  10 cm X 10 cm = 100 cm 2

 A material that can easily flow.  Examples?  Liquids  Gases  Tiny particles are constantly moving and colliding with surfaces, which exerts forces on the surfaces.

 All of the forces exerted by the individual particles in a fluid combine to make up the pressure exerted by the fluid.

 Right now, there is approximately 100 km of fluid on top of you…  AIR!  The weight of the air exerts a force which causes air pressure or atmospheric pressure.  Why are you not crushed by these fluids?  The forces are exerted from all directions so they are balanced.

 As your elevation increases, atmospheric pressure decreases.

 Water pressure increases as depth increases.

 You can measure atmospheric pressure with a barometer  Meteorologists use barometers to measure pressure to help forecast the weather  Decrease in pressure = storm

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Floating and Sinking

 Density is a measure of how closely packed the atoms in a substance are  Density is a physical property  All matter has measurable density  Density = Mass Volume

 Measured in grams using a balance scale

 Measured in mL using a graduated cylinder

 Measured in cm 3 using math  Volume = L x W x H

 Measured in mL by using the displacement method

 g/mL Or  g/cm 3

 The density of a substance is its mass per unit of volume.  For example, a sample of liquid has a mass of 24 g and  a volume of 16 mL. What is its density?

2.9 g/cm 3 A piece of metal has a mass of 43.5 g and a volume of 15 cm 3. What is its density?  Practice Problem

 By comparing densities, you can predict whether an object will sink or float in a fluid.  An object that is more dense than the fluid it is in sinks.  An object that is less dense than the fluid it is in floats on the surface  An object with a density equal to that of the fluid floats at a constant depth.

 Changes in density cause a submarine to dive, rise, or float.

 Ability to float  Ships are designed to have buoyancy

 The pressure on the bottom of a submerged object is greater than the pressure on the top. The result is a net force in the upward direction.

 The buoyant force works opposite the weight of an object.

 Archimedes’ principle states that the buoyant force acting on a submerged object is equal to the weight of the fluid the object displaces.

 A solid block of steel sinks in water. A steel ship with the same weight floats on the surface.

 What is the volume of a box with the dimensions 8cm by 5cm by 5cm?  8cm x 5cm x 5cm = 200 cm 3

 What is the volume of the dinosaur?  5.6mL-4.8mL = 0.8mL

 If an object has a mass of 20 g and a volume of 40 cm 3, what is its density?  0.5 g/cm 3  Water has a density of 1.0 g/cm 3. If the object described above were placed in water, will it sink or float?  Float!

Pascal’s and Bernoulli’s Principles

 Pressure increases by the same amount throughout an enclosed or confined fluid  When force is applied to a confined fluid, the change in pressure is transmitted equally to all parts of the fluid.

 In a hydraulic device, a force applied to one piston increases the fluid pressure equally throughout the fluid.

 By changing the size of the pistons, the force can be multiplied.

 Uses liquids to transmit pressure and multiply force in a confined fluid  Multiplies force by applying the force to a small surface area. The increase in pressure is then transmitted to another part of the confined fluid, which pushes on a larger surface area.

 The hydraulic brake system of a car multiplies the force exerted on the brake pedal. bDyJhBb4c bDyJhBb4c

 Bernoulli’s principle states that as the speed of a moving fluid increases, the pressure exerted by the fluid decreases. 

 Bernoulli’s principle helps explain how planes fly.

 An atomizer is an application of Bernoulli’s principle.

 Thanks in part to Bernoulli's principle, you can enjoy an evening by a warm fireplace without the room filling up with smoke.

 Like an airplane wing, a flying disk uses a curved upper surface to create lift.

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