Free Fall Acceleration due to gravity Free Fall

Free Fall Acceleration due to gravity

Free Fall and Galileo • All objects at same location above earth fall vertically with same acceleration • Ideal motion where air resistance is ignored= free fall • For small falls acceleration is constant • Constant acceleration means we can use the big 5 kinematics equations

Galileo • Galileo famously rocked the scientific establishment by refuting Aristotle’s claim that bigger objects fall faster • At the end of the Apollo 15 mission the astronauts tried it on the moon- without air resistance • http: //www. youtube. com/watch? v=4 m. Tsr. RZEMw A

Lab • Gravity- measuring g with a stopwatch • Each group will have a different size object • We will drop the object 10 times to average • Solve for acceleration due to gravity known as g

Acceleration due to gravity • g=9. 80 m/s 2 • We will use this value but g actually varies with: – Altitude- why? – Latitude- why?

Problem solving • Since a is constant, same kinematics equations apply • Generally we use y for displacement since it is vertical

Example: finding displacement • A stone is dropped from the top of a tall building. After 3. 00 s of free fall, what is the displacement of the stone? • We know: y a ? -9. 80 m/s 2 v v 0 t 0 m/s 3. 00 s

Example: continued • We don’t know v so we can use: y=v 0 t+1/2 at 2 y=(0 m/s)(3. 00 s) + 1/2 (-9. 80 m/s 2)(3. 00 s)2 y=-44. 1 m

Example- how high? • A football game customarily begins with a coin toss to determine who kicks off. The referee tosses the coin up with an initial speed of 5. 00 m/s. In the absence of air resistance, how high does the coin go above its point of release? • Write what you know • What’s missing? t • So which equation can we use?

Acceleration vs. Velocity • Think about flipping a coin • Where is displacement 0? • Where is instantaneous velocity 0? Draw the velocity vector at different parts of coin’s flight. • Is acceleration ever 0?

Symmetry in Free fall • Remember the saying, “what goes up must come down? ” • Objects in free fall experience symmetrical motion • At any displacement on the way up, object’s speed will be the same on the way down • Object loses 9. 80 m/s 2 on the way up and gains it on the way down • Average speed will occur halfway up/down

Hmm…some questions • If I throw a rock straight up, what will be the acceleration at the moment it reaches the top of its trajectory? • 9. 8 m/s 2 - the instantaneous velocity will be 0 as it reverses direction but acceleration is always g!