Ex 1) An object slides across a horizontal table
which changes its kinetic energy from 20 J
to 18 J in 2
sec. What work does the object
do against friction?

W_{f } = 2 J

**Work against friction increases
an object's Internal Energy** (makes it hotter)

Friction is a nonconservative force
(doesn't store energy)

(can't get back)

Joule Paddle

Conservation of Energy

Harper's New Monthly Magazine, No. 231, August, 1869.

Work done by the ball on
the floor changes the ball's mechanical energy

When the WORK equation
doesn't WORK,
use an ENERGY equation.
When an ENERGY
equation
doesn't
WORK,
use the WORK equation

Ex 2) A 10 kg
frictionless cart is resting on a horizontal table. A force of 10 N is
applied to the cart for a distance of 8 m. What is the carts new KE?

Ex 2) A 10 kg
frictionless cart is resting on a horizontal table. A
force of 10 N is applied to the cart for a distance of
8 m. What is the carts new
kinetic energy?

m = 10 kg

F = 10 N

d = 8 m

KE = (˝)mv^{2 }??

ΔKE
=
W

= Fd = 10N(8m)

= 80 Nm

Ex 3) 100
Joules of work is needed to lift a 100 kg object from the ground.

What is the objects new P.E.?

Ex 3)100
Joules of work is needed to
lift a 100 kg
object from the ground.

What is the objects new
P.E. ?

W = ΔPE
= 100J

Ex 4) Crate
is pulled 6.00 m up an incline with a force of 50.0 N. P.E. of cart
increases by 250. J.

Ex 4) Crate is pulled 6.00 m up an incline
with a force of 50.0 N.
If the PE of
the cart increases by 250. J.
, what is the total work done against
friction?