Last Class…
Work, Kinetic Energy, WorkWork-Energy Theorem, Power
B1, Ch5 Sec.1Sec.1-2, B2, Ch7 Sec.3
Today..
Examples
1
Example: Projectile Motion and Kinetic energy
2
1
iClicker Quiz 1
Work done by a gravity on a falling elephant
W = F ∆x
is _________.
a) Positive
b) Negative
c) zero
If force and displacement point
the same direction, the work is _______.
3
iClicker Quiz2: Work done by a gravity on a ball that is going up
Gravity
Displacement
W = F ∆x
Is work positive, negative, zero?
a) Positive
b) Negative
c) zero
Gravity
If force and displacement point
the opposite directions, the work is ______.
4
2
Work-Energy Theorem
As shown for 1D motion,
K f − K i = ∆K = Wnet
Æ Work-Energy Theorem
Positive net work: KE increases (iClicker Quiz 1)
Negative net work: KE decreases (iClicker Quiz 2)
5
Example: Falling elephant
Example: Ball thrown vertically up
6
3
Examples for work in 2D
Example: Work by multiple forces
Example: Work, scalar product, angle
7
Work done by a constant force
W = F d cos θ F ,d ≡ F id
magnitude
Scalar (dot) product
(B2, Ch7, Sec.3)
Force
θ
Displacement
8
4
Math Review: Scalar (dot) product
A B cos θ A, B
θ = 180o
Æ
Ai B = − A B
+A B
Ai B = Bi A,
(A + B)iC = AiC + BiC
9
5