WED (21 Oct):

 

 

 

Key Concept's) Today: Conservation of Energy

 

 

L #9: 1- 8,  11-16    Due Today Wed 20 Oct (Torque)

 

L #12: 1- 14,  16, 17    Due Fri 22 Oct (Work & Power)

 

L #13: 1- 13,  18, 19    Due Wed 27 Oct (Instantaneous Vel & Acc)

 

 

L #15: 1- 9,  11-15, 17    Due Thurs 28 Oct (Static Beam)

 

L #16: 1- 9,  11-15, 17    Due Monday 1Nov (Simple Machines)

 

L #17: 1- 16    Due Wed 3Nov (Sliding Blocks)

 

 

 

Test 9, 12, 13, 15, 16, 17  Monday 8 Nov 

 

 

 

                                       

                                                                                           

 

 

 

1. a. What is the speed of a "bob" on a pendulum if it falls .4m?      

 

 

 

                              h1 = .4m

 

 

 

 

 

 

 

 

 

                                                                   h2 = 1.2m

 

                                                                                                      X

 

 

 

b. If the bob is released at the bottom of the swing and it falls 1.2 m to the ground, then how long will it take for the bob to hit the floor?

 

 

c. How far will it move horizontally ?

 

 

 

 

 

 

 

a.   mgh = 1/2 mv2

        (10)(.4) = 1/2 v2

       V = 2.8m/s

 

b. h2 = Vot + 1/2at2

 à -1.2 m = 0 + 1/2(-10m/s2)t2

 à -1.2 m/(-5 m/s2)  = t2

      t = .49s 

 

 

 

c.  x =  Vot + 1/2at2   

 à x = (2.8 m/s)(.49s) + 0

           x = (2.8 m/s)(.49s)   = 1.4 m

 

 

 

 

 

 

 

 

 

 

L#12:

Work = (Force)(Distance)

(in direction of motion)  (Label-Joules)

 

Power = Work / time            

(Label-Watts)

 

Energy is the ability to do Work   

(Label-Joules)

 

 

Work also = DKE + DPE

 

 

 

2) a. How much work is required to lift a 20kg object  5 m in the air?

 

b. Power used if done in 20 s?

 

c. What is the velocity just before it hits the ground if the object falls the 5m?

 

 

 

 

 

 

 

 

 

 

 

Answer: a. W =  DPE = mgh

              = (20kg)(10N/kg)(5m)

                                  W  =  1,000J

 

b. P =  W/T  = 1,000J / 20s

   P = 50 Watts

 

c. PE = KE

     mgh = 1/2 mv2

       gh = 1/2(v2)

(10m/s2)(5m) = 1/2(v2)

 

        v2   = √100 = 10m/s

 

3. A pump lifted water up to a house 12m above the stream.

If it was a 400W pump how much water did it move in 3 minutes.

 

 

 

 

 

Ans #2:

PE (mgh) per 1.0 kg

PE = (1kg)(10N/kg)(12m) = 120 J per kg

 

P=W/t  --> W = Pt      also    W = DPE

W =  (P)(t) = (400 J/s)(180s) =  72,000 J   (of PE)

72,000 J / 120 J per kg =  600 kg of water