net
KE = 1/2mv
2
W = Fdcosθ
F=ma
CONSERVATION OF ENERGY:
- energy can be converted from one form to another, but can not be created or destroyed (in regular chemical and physical changes)
- mechanical energy involves the energy of both position and motion ME = KE + ∑ PE
- mechanical energy is conserved as long as there is no friction present; if friction is present, mechanical energy is "lost“, generally as heat energy (beyond this class)
0 0.5 1 1.5 2 2.5 3 3.5 4 4.5 5 0 50 100 150 200
PEg (J)
Height from ground
Jo
u
le
e.g. 18) A 5 kg ball is dropped from a height of 5.0 m above the ground. How fast is it moving when it reaches the ground? When it reaches a height of 1.0 m?
0 0.5 1 1.5 2 2.5 3 3.5 4 4.5 5 0 50 100 150 200 250 ME (J)
PEg (J)
Height from ground
Jo
u
le
0 0.5 1 1.5 2 2.5 3 3.5 4 4.5 5 0 50 100 150 200 ME (J)
PEg (J)
KE (J)
Height from ground
Jo
u
le
e.g. 18) A 5 kg ball is dropped from a height of 5.0 m above the ground. How fast is it moving when it reaches the ground? When it reaches a height of 1.0 m?
0 0.5 1 1.5 2 2.5 3 3.5 4 4.5 5 0 50 100 150 200 250 0 2 4 6 8 10 12 ME (J) PEg (J) KE (J) Velocity (m/s)
Height from ground
e.g.19) The same ball is thrown upward at a speed of 1.5 m/s and lands on the ground traveling 7.0 m/s. From what height was it thrown? What is its maximum height?
* note: since KE depends on speed, the direction does not matter!
e.g.20) A 200 g pendulum bob is released when the 2.0 m long string it is attached to forms a 30˚ angle with the vertical.
a) From what height was it released?
SPRING LAB
Spring lab: we will do the lab today, then go to the computer lab to analyze data FRIDAY 1. Objective: to demonstrate Mechanical Energy and gain more experience in Excel 2. Goal: determine k for 5 Spring Monsters.
3. Plan: With your partner, determine your approach What equations do you need?
What data do you need to gather? What equipment do you need?
4. Test: Make the Spring Monsters Bounce and record the data
5. Analyze: Use excel to calculate values for k, and make a graph that shows the comparison of ME, KE, and PEg for one complete Spring Monster bounce.
Homework:
Answers: page 171 practice E #1,2,3,(4),5
1. 20.7 m/s (VECTOR ADDITION!!! Vx+Vy) 2. 9.9 m/s; 14.0 m/s
3. 14.1 m/s
(4.) 0.25 m hint: mghtop=1/2mv2 bottom
- power is the rate at which work is done P = W/t
e.g.22) A small motor lifts a 5.0 kg bucket of water 2.0 m in 1.5s. a) What is the power of the motor?
b) At what speed can the same motor lift a 2.5 kg bucket?
Homework:
page 175 practice F #1, 3, 4, (5)
3. 2.61 x 108 s 4. 3600 s
Chapter Review:
2. A 50.0 kg parachutist is falling at 20 m/s when her parachute opens. Her speed upon landing is 5.00 m/s. What was the work done on the parachuter? By what factor would the work change is she decelerates from 40 m/s to 10 m/s?
3. A 0.060 g piece of straw is embedded in a tree trunk during a tornado. The tree trunk exerts an average 65 N force on the straw. If the straw is embedded a distance of 2.0 cm, what was its initial speed? How would the initial speed change if the force was double? 4. A 50 g block slides along a surface, acted upon by 2 forces. The first force is 1.0 N at 45 ˚ and the second force is 1.5 N at 325˚. If the block moves 2.0 m at 15˚, what is the net work done on the block? To what speed can it be accelerated over the given distance?
