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An Introduction To Physics . Physics is the study of the relationships between matter and energy. Physics deals with the real world Versus……… Physics deals with the interactions of matter and energy, both PE vs KE!!!! Beginning Physics is called Newtonian Physics. Newtonian Physics Named for Sir Isaac Newton, it deals mainly with the interactions of visible objects around us Physics explains the universe in mathematical terms. Motion MOTION: a change in position, measured by distance (also called DISPLACEMENT) and time. REFERENCE POINT Reference point: thepoint from which movement is determined, and which is assumed to be nonmoving itself. What is motion? • If you are standing in one place, and your friend walks by you, are you moving relative to your friend? – Is your friend moving relative to you? – Is either of you moving relative to the earth? Answer: • You are moving relative to your friend, and your friend is moving relative to you! • You (the Joker) are not moving relative to the earth, but your friend is. You are both moving relative to the sun! Who is moving relative to the computer screen? What is motion? • If you and your friend are walking down the hall together at the same speed, in the same direction, are you moving relative to your friend? – Is your friend moving relative to you? – Are either of you moving relative to the earth? Answer: • You are NOT moving relative to your friend, and your friend is NOT moving relative to you. You both are moving relative to the earth. Frame of REFERENCE • EXAMPLE: Tossing a ball • Describe the motion of the ball." • "If we were going down the road in a school bus, me standing in the isle and you sitting in the seats, would the motion of the ball be different?" • "If someone was standing beside the road as we passed, what would they see the ball doing?" Frames of Reference To measure movement, some point must be considered as nonmoving. • Most common is the earth • Motion is a change in position, relative to a frame of reference “Hey Bob!!! …Did I scare Ya?” • Earth is the most common frame of reference, however: • Earth rotates on its axis at almost 1100 miles/hour. • Earth moves around the sun at over 68,000 miles/hour. • The whole galaxy is rotating at about 490,000 miles/hour. • Is there a universal frame of reference we can use to define the motions of all things? Here’s a problem (or two)….. • If the earth spins on its axis at 1100 miles/hour, what is the speed of the Earth's rotation in feet per second? • The earth travels at 68,000 miles/hour as it moves around the sun. How many miles does the earth travel in one trip around the sun? Speed • Speed = Distance ÷ Time S = D/T Example: A car travels 300km in 6 hours. What is the speed of the car? SPEED!!!! • Instantaneous Speed: the rate of speed at any particular moment in time • Average speed: the speed of moving objects is not always constant: – Average speed = total distance / total time MORE SPEED!!!! • Average Speed Practice Problem: You drive 300 kilometers in 3 hours before stopping for 30 minutes for lunch and gas. After lunch you travel 150 kilometers in an hour and a half. What was your average speed for the trip? Answer: • Speed = distance ÷ time • Speed = 450 Km ÷ 5 hours • Speed = 90km/hr More practice • 1. How far can a plane travel if it flies 800km/hr for 9 hours? • 2. How long does it take a ship to go 500 km if it travels at a speed of 50km/hr? Answer If: S = D/T Then: D = S x T 800km ▪ 9hrs = 7200km hr Answer And then: (since S = D/T) T = D/S 500km ÷ 50km = 10 hrs hr RANDOM FAR SIDE!! Velocity • Speed in a given direction. • What is the velocity of a boat that travels from St. Peter to Mankato (10 miles) in 15 minutes? Answer • Speed = distance ÷ time • Speed = 10 miles ÷ 15 minutes • Speed = 0.67 mi/min • Velocity = 0.67 mi/min South Change your answer to mi/hr! • 0.67mi/min x 60min/hr = • 40 mi/hr Distance-time graphs • On your paper, graph the following: – D (m) T (sec) 0 0 5 7 10 14 15 21 Distance - Time Graph: • Time is the independent variable and always plotted on the horizontal axis. Distance is the dependent variable and always plotted on the vertical axis. The slope of this line indicates the speed. Was your graph a straight line? • A distance-time graph which is a straight line indicates constant speed. • In constant speed, the object does not speed up or slow down. The acceleration is zero. SPEED!!!! (one more thing) • Instantaneous Speed: the rate of speed at any particular moment in time – What device measures instantaneous speed on you car??? VELOCITY -- speed in a given direction. • Speed only gives distance and time. • Velocity gives distance, time, and the direction of travel. Velocity is known as a vector quantity because it has both speed and direction. Consider the flight of an airplane: The first arrow (“A arrow”) shows the speed and heading of the plane. The second arrow (“Another arrow”) shows the speed and direction the wind is blowing. Since the wind is changing the speed and direction of the plane, the two vectors are added, head to tail, to determine the actual speed and direction traveled. If the length of the blue vectors is drawn to scale, the length of the resultant vector will indicate the actual velocity of the plane. When you go around a curve at a Constant speed, are you changing Direction??? More VELOCITY NOTES • Constant Velocity = constant speed and constant direction ( the object is moving in a straight path) • Changing velocity means either a decrease or increase in speed, motion along a curved path, or both • Cars have 3 controls to change velocity: • 1) gas pedal 2) brake 3) steering wheel ACCELERATION • Acceleration: The rate of change in velocity. • Acceleration = (Final Velocity) - (Original Velocity) / Time a = (Vf – Vi) / t ACCELERATION!!! • Problem: Suppose a car moving in a straight line steadily increases its speed each second. First from 35 to 40 km/hr, then from 45 to 50 km/hr. What is its acceleration? ACCELERATION!!! • Problem too: In 5 seconds a car moving in a straight line increases its speed from 50 km/h to 65 km/h while a truck goes from rest to 15 km/h in a straight line Which undergoes greater acceleration? What is the acceleration of each vehicle? ACCELERATION!!! • Acceleration also applies to changes in direction • EX: going around a curve at a constant speed of 50 km/hr, what do you notice or feel? • motion is changing every instant because velocity is changing every instant • YOU ARE ACCELERATING!!!! ACCELERATION • Deceleration ( “negative” acceleration): • A term commonly used to mean a decrease in speed. • Acceleration in a direction opposite to the direction of travel. • Deceleration is negative acceleration and has a negative value to indicate direction. Graph the following on a distance- time graph: • D (m) T (s) 0 0 5 1 20 2 45 3 80 4 125 5 0 1 2 3 4 5 Was your graph a curve? • A graph that curves on a distance-time graph shows that the object is accelerating • A graph of acceleration, always has shape!! Distance-time graphs • Describe the motion of the object as shown in the graph. From 0-8 sec, constant speed: (25 m/sec); From 8-12 sec, no motion; From 12-16 sec, acceleration; From 16-20 sec, constant speed Speed-time graphs • Using the distance-time graph from the last frame, draw a speed time graph. First fill in the table below: Average Speed (m/s) Time (sec) ____ 25 0 to 8 ____ 0 8 to 12 ____ 37.5 12 to 20 What does your graph look like? • Constant speed will be a horizontal line on a speed time graph. • If the speed decreases, the line will slant down. • If the speed increases, the line will slant up. What do the following speed-time graphs depict? Acceleration problem • A roller coaster’s velocity at the top of a hill is 10m/s. Two seconds later it reaches the bottom of the hill with a velocity of 26m/s. What is the acceleration of the roller coaster? Answer • Acceleration = ∆V/ ∆T • a = 26m/s – 10m/s 2s a = 16m/s 2s a = 8m/s/s or 8m/s2 More acceleration problems • 1. A car accelerates at a rate of 20mi/hr/s. How long does it take to reach a speed of 80 mi/hr? • 2. A car travels at 60 miles per hour around a curve. Is the car accelerating? • 3. A car travels in a straight line at 60mi/hr. Is the car accelerating? Answers: 1. 4sec = t 2. yes! Because it’s changing direction! 3. no! It’s not changing speed or direction! Answer: • Acceleration = ∆V/ ∆T • Acceleration = Vf – Vi t • a = 20mi/hr – 60mi/hr 4s a = -40mi/hr 4s a = -10mi/hr/s Review: Distance-time graph of acceleration Distance-time graph of deceleration Review:Speed-time graph of acceleration Review: Speed-time graph of deceleration Review: Distance-time graph of constant speed GO………………. Momentum Momentum = Mass x Velocity Which has more momentum: a 300lb football player moving at 5m/s or a 200lb quarterback moving at 10m/s? Answer: Momentum of the 300lb player is 300lbs x 5m/s = 1500lb-m/s Momentum of the quarterback is 200lbs x 10m/s = 2000lb-m/s The quarterback has a greater momentum! Momentum problems 2 cars are heading east, car A is traveling 30mi/hr, car B is traveling 60mi/hr. Each car weighs 2000lbs. What is the momentum of car A? What is the momentum of car B? If car B crashes into car A, what is the total momentum? Answers: P=mv Car A’s momentum = 30mi/hr x 2000lbs PA = 60,000 mi-lbs/hr east Car B’s momentum = 60mi/hr x 2000lbs PB = 120,000 mi-lbs/hr east Total momentum = PA + PB = 60,000 + 120,000 = 180,000 mi-lbs/hr east Inelastic collision Elastic collision Two dimensional collision Another momentum problem! Car X is traveling 30mi/hr east, car Y is traveling 60mi/hr west. Each car weighs 2000lbs. What is the momentum of car X? What is the momentum of car Y? If car X crashes into car Y, what is the total momentum? Answers: P=mv Car X’s momentum = 30mi/hr x 2000lbs PA = 60,000 mi-lbs/hr east Car Y’s momentum = 60mi/hr x 2000lbs PY = 120,000 mi-lbs/hr west Total momentum = PY - PX = 120,000 - 60,000 = 60,000 mi-lbs/hr west Which has more momentum?

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posted: | 8/9/2012 |

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