lecture4

lecture4 - Example: You are driving along in the country at...

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Example : You are driving along in the country at 90 km/hr, when you suddenly notice a cow in the road 30 m ahead. You attempt to stop the car by pressing on the brakes, but the distance is too short. With what velocity does the car hit the cow? Assume your reaction time is 0.75 s and that the deceleration of your car is 7.84 m/s 2 when the brakes are applied. a x m/s 25 = o v 30 m m/s 25 = o v 18.75 m 11.25 m First, let’s convert 90 km/hr to m/s: m/s 25 = o v Once you see the cow 30 m away, does the car start decelerating? No! m) 25 . 11 )( m/s 84 . 7 ( 2 m/s) 25 ( 2 2 2 + = f v m/s 2 . 21 = f v mph) 5 . 47 ( uch! You have a reaction time of 0.75 s, so the car moves closer to the cow by an amount: m. 18.75 ) m/s)(0.75s 25 ( = = = Δ t v x o This only leaves: 30 m – 18.75 m = 11.25 m of distance left to the cow. Negative sign – very important! Ouch! Now you apply the brakes. Given the deceleration of the car, what will its final velocity be after it travels a distance of 11.25 m? e can use: 2 2 e know we know We can use: a v v x o f 2 = Δ x a v v o f Δ + = 2 2 2 We know a , we know Δ x , and we know v o . We can solve for v f :
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Problem solving strategy 1. Relax, breathe – It’s just a word problem! . Draw a figure for the problem. 2. Draw a figure for the problem. 3. Assign a set of convenient and appropriate axes, and set their zero locations. 4. Note what they are asking you to solve. Identify which equations are appropriate for what they are asking 5. Identify which equations are appropriate for what they are asking.
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This note was uploaded on 08/04/2009 for the course PHYS 2001 taught by Professor Sprunger during the Fall '08 term at LSU.

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lecture4 - Example: You are driving along in the country at...

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