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Acceleration

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Understanding Speed vs. Velocity Speed is simply how fast Velocity is how fast in a direction Speed = distance/time Velocity = displacement/time

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Self check The speedometer of a car moving east reads 100 km/h. It passes another car moving west at 100 km/h. Do they have same speed? Velocity? Same speed but different directions so their velocities are different. During a certain period of time, the speedometer of a car reads a constant 60 km/h. Does this indicate a constant speed? Constant velocity? Speed is constant but direction may not be constant so velocity may or may not be constant.

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Velocity speed in a given direction. Velocities in the same direction: (+) Add + = 2m/s E + 1m/s E = 3m/s E Velocities in different directions: (-) Subtract + = 2m/s E - 1m/s W = 1m/s E Examples EscalatorEscalator TreadmillTreadmill

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Dont be so negative It is important Velocities have direction Since we can have many different directions, in physics we usually use a + and sign Usually this corresponds to the graphing axis (x and y)

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Acceleration Acceleration - the rate of change in velocity. Any time Velocity (Speed or direction) changes acceleration = v final - v initial ------------- time We can usually feel Accelerations

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Try It! If a rocket blasts off at 200km/s N into space and after 8 sec. reaches a velocity of 550km/s N. What was the rockets acceleration after take off? Step 1 acceleration = v final v initial time Step 2 acceleration = 550km/s 200km/s 8 s Step 3 acceleration = 43.8km/s/s or km/s 2

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Deceleration Deceleration a decrease in velocity over time. Same formula, just different name Deceleration is negative acceleration.

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Try It! If a car traveling at a velocity of 50 m/s and quickly stops in 3s for a red light, find the deceleration of the car? Step 1 deceleration = v final v initial time Step 2 deceleration = 0m/s 50m/s 3 s Step 3 deceleration = -16.7m/s/s or m/s 2

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Accelerating a Car Gas Pedal If acceleration is positive, then velocity is increasing Brake Pedal If acceleration is negative, then velocity is decreasing (deceleration) What other way is there to accelerate your car?

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ACCELERATION 1. The slope of a velocity-time graph is acceleration just as the slope of a displacement-time graph is velocity. No slope. No acceleration. a. If the graph is linear, acceleration is uniform or constant and avg. acceleration = instantaneous acceleration. b. If the graph is a curve. The acceleration is different at every point, but instantaneous acceleration on this graph is still the slope at a point on the curve.

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Remember! Acceleration is a Vector This means that it can have a sign (+ or -). So, a velocity vs time graph with a positive slope is a (+) acceleration graph and a velocity vs time graph with a negative slope is a deceleration graph.

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Distance-Time Graph Acceleration is indicated by a curve on a Distance-Time graph. Changing slope = changing velocity Graphing Acceleration

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A Distance vs. Time Graph Acceleration is represented by a curved line. A Speed vs. Time Graph Acceleration is represented by a straight diagonal line.

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Graphing Accelerated Motion Speed-Time Graph slope = straight line = flat line = acceleration + = speeds up - = slows down constant accel. no accel. (constant velocity)

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Speed-Time Graph Specify the time period when the object was... slowing down 5 to 10 seconds speeding up 0 to 3 seconds moving at a constant speed 3 to 5 seconds not moving 0 & 10 seconds

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Graphing (+) & (-) Acceleration

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Why is it + Acceleration?

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Why is it () Acceleration?

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Why is there no Acceleration?

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Graphing the motion of a Ball thrown Upward

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Breaking Down the Throw: Acceleration of the Upward Leg

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Breaking Down the Throw: Acceleration of the Downward Leg

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Position vs Time Graph of Constant Positive (+) Acceleration

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Position vs Time Graph of Constant Positive Negative (-)Acceleration

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Warm-Up What is acceleration? What is the shape of a distance- time graph for accelerated motion? What is the shape of a Speed-time graph for accelerated motion? Why are they different?

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Momentum

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Momentum is equal to the mass of an object multiplied by its velocity. How hard it is to stop a moving object All moving objects have momentum. http://econtent.bucksiu.org/?a=47768&ch=2

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Momentum Inertia in Motion Momentum of an object is, (Momentum) = (Mass) X (Velocity) P = M V or M V = P Examples of objects with a large momentum are supertanker (large mass) and bullet (large velocity). http://econtent.bucksiu.org/?a=47768&s=00:01:40:17&e=00:02:30:26 http://econtent.bucksiu.org/?a=47768&s=00:04:58:00&e=00:06:07:00

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Check Yourself A 2 ton car, going 60 m.p.h. hits a 5 ton truck, going 20 m.p.h.. Which vehicle, the car or the truck, has greater momentum? The car because (2)x(60) = 120 and (5)x(20) = 100. What would the cars speed have to be for the momentums to match? Fifty m.p.h. since (2)x(50) = (5)X(20). Arent you forgetting something? The direction of velocity. How does that matter? Rear-end crash not same as a head-on crash. More about this when we do collisions.

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Conservation of Momentum Law of Conservation of Momentum states that the total momentum of any group of objects remains the same unless outside forces act on the object. http://econtent.bucksiu.org/?a=47768&s=00:07:10:28&e=00:08:01:00

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Recoil Momentum conservation also explains recoil (MASS) x (velocity) (mass) x (VELOCITY)

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Wrap Up! What is momentum? What is the law of conservation of momentum? Using the formula for calculating momentum, determine the units of measurement?