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Motion and Force

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Frame of Reference Motion of an object in relation to a fixed body or place. To describe motion accurately and completely THE MOST COMMON FRAME OF REFERENCE: The Earth

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Relative Motion Is the movement in relation to a frame of reference Example: A train going past a platform (see movement) People riding the train feel no movement

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Measuring Displacement Displacement—the direction from the starting point and the length of a straight line from the starting point to the ending point Involves distance and direction

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Combining Displacement Vector—is a quantity that has magnitude and direction Add displacements using vector addition If vectors are in the same directions, add them Ex: 10meters 5 meters = 15 meters If vectors are in the opposite direction, subtract them Ex: 20 meters 15 meters = 5 meters Resultant vector—the vector sum of two or more vectors

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Speed Rate of Motion: Change of position in a given amount of time. Formula: Speed = Distance/time Units: km/hours, m/s

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Speed Average Speed Is computed for the entire duration of a trip Instantaneous Speed Is the rate at which an object is moving at a given moment of time Speed right now

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Speed Practice Problems 1. How long does it take a bird to fly 300km if it travels at a speed of 50 km/hr? 2. Alex rode his bicycle 60 kilometers in 4 hours. How fast was he going? 3. Joe can pitch a baseball 32 m/s and the ball crosses the plate in 1.5 seconds. How far away is the plate? 4. Kayla runs 90 meters with a speed of 15 m/s. How long did it take?

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Velocity Velocity= Speed in a given direction. Velocity is a vector You combine velocities the same way you combine displacement Same direction, add them Opposite direction, subtract them

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Velocity Formula V=Δd/Δt V=velocity d=distance t=time Units: m/s

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Graphs of Speed and Velocity Constant Speed Not Moving Constant speed and then not moving

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Acceleration Rate of change in velocity. Acceleration is a vector Formula: A = v f – v i or Δv t Unit: m/s 2 Variables need to solve acceleration problems 2 Velocities Time

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Deceleration Negative Acceleration Solve it just like the acceleration problem but the answer is always NEGATIVE

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Acceleration/ Deceleration Problems 1. A car has starts from rest and goes to 20 m/s in 4 seconds. What is the car’s acceleration? 2. A runner starts to run and reaches a velocity of 100 m/s in 25 seconds? What is the car’s acceleration? 3. A train slows down from 200 m/s to a stop in 50 seconds. What is the car’s deceleration? 4. A train has an acceleration of 40 m/s 2 during a period of 15 seconds. What is the train’s velocity?

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Graphs of Acceleration (Speed verse Time) Acceleration Deceleration Constant Acceleration

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Graphs of Acceleration (distance verse time) Acceleration Deceleration Constant Speed

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Centripetal Forces Centripetal Force is a force that pulls an object towards the center. Center Seeking

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Constant Speed and Circular Motion An object moving in a circular motion may have constant Speed even though it is accelerating. Why?

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Answer: That object needs to change direction.

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3 ways objects accelerate Change in Speed only Change in Direction only Change in Speed and Direction

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Momentum Momentum is the product of the mass times the velocity. The heavier an object, the harder it is to stop its movement. Formula: Momentum = m x v

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Forces Force is a push or pull on an object. Can cause a resting object to move or it can accelerate a moving object by changing the object’s speed or directions Unit for force: N or Newton.

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Balanced forces: Where there are two forces that counteract each other and result in no movement (EQUAL) Unbalanced forces: One force is greater than the other force. (UNEQUAL)

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Friction A force that resist motion and can cause heat Lubricant help reduce friction Types of friction Static Sliding Rolling Fluid

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Types of Friction Static—is the friction force that acts on objects that are not moving Sliding—is the force that opposes the direction of motion of an object as it slides over a surface Rolling—the friction force that acts on rolling objects Fluid—opposed the motion of an object through a fluid Fluids= water and mixture of gases Fluid friction action on an object moving through the air is known as air resistance

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Newton’s Laws of Motion 1 st Law: Objects at rest remain at rest, or objects in motion remain in motion unless acted upon by a force. 2 nd Law: The acceleration of a body depends on the ratio of the acting force to the mass of the body. 3 rd Law: for every action there is an equal and opposite reaction.

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1 st Law of Motion (Law of Inertia) Inertia: force that is resistant to the direction of the motion Unbalanced forces Examples: Inertia belts (seat belts)

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2 nd Law of Motion Concept: Acceleration Unbalanced forces Examples: hitting a golf ball gently vs. hard. Answer = ___ newtons

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2 nd Law of Motion Formula: Force=mass x acceleration F=ma 1. How much force is needed to accelerate a 500.0 kg car at a rate of 4.000 m/s/s? 2. A 100 N causes an object to accelerate at 2 m/s/s. What is the mass of the object? 3. A 1.5 kg ball is kicked with a force of 450 N. What acceleration did the ball receive?

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3 rd Law of Motion Action/ Reaction Concept: Action/Reaction of objects Balanced forces Examples: Stationary objects, rockets being launched

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Gravity and Free Falling Objects Gravity: is a force that acts between two masses Acts downward towards the center of the Earth All objects fall at a rate of 9.8 m/s/s Galileo did an experiment at the leaning tower of Pisa in Italy with bowling balls. Both balls fell at the same rate. Galileo's experiment at the Tower of Pisa Galileo's experiment at the Tower of Pisa

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Projectile Motion The motion of a falling object after it is given an initial forward velocity

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Air Resistance Force that slows down falling objects due to the atmosphere and surface area of the object.

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Newton’s Law of Universal Gravitation Shows that objects are attracted to one another by their mass and their distance away from the object.

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Feather vs. Rock Earth: feather would float down (air resistance) while the rock would drop at 9.8 m/s/s Moon: feather and rock would drop at same rate due to no atmosphere and air resistance.

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What about a vacuum? Both fall at the same rate due to no air being in the jar.

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Gravity and Weight Weight—is the force of gravity acting on an object Expressed as F g Formula Weight=mass x acceleration due to gravity (g) F g =mg g=9.8 m/s/s

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