Transcript 24 newtons laws of motion 2 - lindsey

Newton’s Laws Take 2
take out your 3s
February 13, 2009
Newton’s 2nd Law proves that different masses
accelerate to the earth at the same rate, but with
different forces.
• We know that objects
with different masses
accelerate to the
ground at the same
rate.
• However, because of
the 2nd Law we know
that they don’t hit the
ground with the same
force.
F = ma
F = ma
98 N = 10 kg x 9.8 m/s/s
9.8 N = 1 kg x 9.8 m/s/s
Terminal Velocity – force of friction = force of gravity
air resistance = weight
– acceleration = 0 m/s2
– Fnet = 0 N
Who has a higher terminal velocity, a person who has
a mass of 200 kg or a person with a mass of 100
kg?
200 kg
Video Clip
• fastest-growing sport in Norway
• http://www.youtube.com/watch?v=Nf2ENQ
RHKA4
Free Body Diagrams - A picture that
shows all of the forces acting on an object
What are the forces acting on this object?
http://lectureonline.cl.msu.edu/~mmp/applist/si/plane.htm
Fn
Ff
Fp
Fg
Forces on an inclined plane…
Fg  mg
Fn  Fg cos θ
Fn
Fg
Fp
Fp  Fg sin θ
Ff   Fn
Billy and his dad are sledding down the hill at an angle
of 15.0°. If their combined mass is 55.2 kg, what is
the force pulling them down the hill?
Fp  Fg sin θ
Fp  mg sin θ
Fp  55.2  9.8 sin 15
Fp  140 N
Example) A car of mass of 2000 kg is on an
icy driveway inclined at an angle of 20.0º,
determine the force of the plane?
What info does the problem give us?
m=2000 kg
Fp  Fg sin θ
Ө= 20.0º
Fp= ?
Fp  mg sin θ
Fp  2000  9.8  sin 20
Fp  6700 N
Tension = stretching force in a rope
Demos with
spring scales
The tension in the rope can be
broken down into its horizontal and
vertical components. The vertical
component of Ft has to equal the Fg.
cos θ = Fg / FT
FT  Fg / cos θ
Fg
FT
Tension Practice Problem
• A stop light with a mass of 25 kg is supported by
2 wires. What is the tension in the wire that is 15˚
from the normal? What is the tension in the wire
that is 50˚ from the normal?
15˚
50˚
Tension Practice Problem
• A stop light with a mass of 25 kg is supported by 2
wires. What is the tension in the wire that is 15˚ from
the normal? What is the tension in the wire that is
50˚ from the normal?
15˚
50˚
Fg = m g
Fg = (25 kg) (9.8 m/s2) = 245 N
F
t
F
t

245 N
cos 15
 253.6 N
F

t
F
t
245 N
cos 50
 381.2 N
Pressure
(N/m2) or Pa
Force (N)
F
P
A
Area (m2)
length x width
• Awl Demo
• Bed of Nails Video
• Bernoulli's Principle states that as the
speed of a moving fluid increases, the
pressure within the fluid decreases
• Pipe Demo
• 2 Hole Water Demo
Creates the ability for planes to fly!
If an airplane is traveling at a
constant velocity, how does the
force of thrust compare to the
force of drag?
Newton’s 3rd Law
For every action, there is an
equal and opposite reaction.
What forces are involved in a tug of war?
Fn
Fp
Ff
Fg
Action / Reaction
Arnold Strongman and Suzie Small are having a tug of
war contest. They pull on opposite ends of a rope. Who
exerts a greater force on the rope?
• Rope tension must be the same throughout the
whole rope. Therefore, Arnold can pull no harder
on the rope than Suzie.
• The most important factor in a tug of war is not
how hard you pull on the rope, but how hard you
push on the ground.
Suppose two carts, one twice as massive as the other, fly
apart when the compressed spring that joins them is
released.
1. How does the force exerted by the spring on the 1m-cart
compare to the force exerted by the spring on the 2m-cart?
The forces are equal
2. How fast does the 2m-cart roll compared to the smaller 1mcart?
It would roll half-as fast since it experienced half the
acceleration when released from the spring.
Which will experience the greater
acceleration, the car or truck if the
forces are equal?
F=F
M
a
=M
a
Action / Reaction
Fgun = Fbullet
M
a
=M
a
What gun and bullet would you
want to be shot with?
– A gun with very little mass and a bullet with a lot of
mass?
Or
– A gun with lots of mass and bullet with very little
mass?
•Fan cart demo
•Fire extinguisher
DVD demo
Reminders
• POTW
• Work on 35s Newton’s Laws worksheet #2
• No school Monday
Hundreds Gather to Protest Global Warming