STARTER

1. STARTER A rock is dropped from rest from a tower. Two seconds later its moving at 19.6 m/s. What is the acceleration of the rock?

2. When an object is dropped from rest, and there is no air resistance,the velocity is as follows:

3. Free Fall An object in free fall is only subject to the acceleration due to gravity ( -9.8 m/s2). Air resistance is ignored or there is none. In free fall, all objects fall at the same rate.

4. Kinematic Equations for Free Fall Because the acceleration is constant, the kinematic equations still apply. We just replace the x’s by y’s.

5. Free Fall Equations 1. vf = vi + at 3. vf2 = vi 2+2a(yf –yi) 2. yf = yi + (t/2)(vi +vf) 4. yf = yi + vit + (1/2)at2 Free Fall a = -9.8 m/s2

6. Coordinate system

7. How To Use Them 1st List the possible unknowns yf = vf = a = -9.8m/s2 yi = vi = t = 2nd Read the problem and fill in all you can. Don’t forget, a = -9.8m/s2 always. 3rd Choose a kinematic equation with just one unknown in it.

8. Example A rock is dropped from rest from a building 20m tall. yf = 0 vf = ? a = -9.8m/s2 yi = 20m vi = 0 t = ? How long does it take to hit the ground? How fast is it going when it hits?

9. To get t, you need an equation with t in it, but without vf. Which one is it? yf = yi + vit + (1/2)at2 0 = 20 + 0t + (1/2)(-9.8)t2 = 2.02 seconds

10. To get Vf you have a choice. vf = vi + at vf = 0 + (-9.8)(2.02) = -19.8m/s

11. Free Fall Summary 1. vf = vi + at 3. vf2 = vi 2+2a(yf –yi) 2. yf = yi + (t/2)(vi +vf) 4. yf = yi + vit + (1/2)at2 Free Fall a = -9.8 m/s2

12. Exit How can you determine the height of a building you’re on top of with a rock and a stopwatch? Explain carefully.