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Transcript
1-D Projectile Motion First Some Definitions • Projectile – An object that only has the force of gravity acting on it. • Trajectory- path a projectile travels through space • Apex-highest point reached along a trajectory Free Body Diagram Hint: Remember the definition of a projectile Free Body Diagram Hint: Remember the definition of a projectile Does the ball travel the same distance every second? Does the ball travel the same distance every second? No. The ball travels more distance per second as it falls. Is the ball’s velocity constant as it falls? Is the ball’s velocity constant as it falls? No, It decreases by 10 m/s every second So…... is the acceleration constant? Yeah … a = -10 m/s2 the whole time We can find the velocity at any time Velocity of Free Falling Projectile a is the acceleration due to gravity (g) Now to find the distance… The distance increases as the square of the time Problems • A 3-kg watermelon is dropped off the Sundial restaurant in downtown Atlanta. It takes 3 seconds to hit the ground. 1. What is the velocity of the watermelon on impact? 2. How far had the watermelon fallen by the 2nd second? 3. How tall is the Sundial restaurant? 4. What is the weight of the watermelon at the top of the restaurant? 1. v= gt; v = (10)(3) = 30 m/s 2. d = 1/2gt2 ; d= ½ (10)(2)2 = ½(10)(4) = 20 m 3. d = 1/2gt2 ; use 3s since that’s how long it took to fall from the top; d= ½ (10)(3)2 = ½(10)(9) = 45 m 4. Fw = mg = (3kg)(10) = 30 N Conditions/assumptions for freefall projectile • Gravity is the force that causes the acceleration (the change in velocity) • Assume no air resistance during freefall, so acceleration due to gravity is a constant 10 m/s2 • Assume object start from rest (initial velocity will be zero) • Velocity can be calculated at any instant (v=gt) • Distance traveled can be calculated at any instant (d = ½ gt2) Questions • When an object is in freefall: • Does its velocity change by regular amounts each second that it’s falling? If yes, why; if no, why not? • Yes, changes by ten for every second the object falls • Is the distance it falls per second the same amount each second? Explain why. • No, as it falls it gets faster so for every second it falls it covers more distance per second