Transcript Force and PRessure

Journal P-5: Jumping Spider
A small spider, less than 2 cm in diameter, spots an insect.
The spider crouches and crawls slowly forward. Then and
leaps, landing right on the victim! A jumping spider has the
amazing ability to jump 10-40 times its body length. To
capture prey from that far away, it has to accurately
estimate its beginning velocity. Once the spider jumps, the
force of gravity controls its motion. , causing it to follow a
curved path. Its velocity changes at every point along its
path until it lands on its prey.
1. Once the spider jumps and is in the air, can it change its
course? Explain.
2. Does the direction of motion change throughout its
flight? Explain.
3. Think of an activity or sport in which the goal is to hit a
specific target from far away. What are some of the
challenges?
A. Force
1. Force – push or pull
• It has the ability to change an
object’s motion
•
•
•
•
•
Starting
Stopping
Speeding up
Slowing down
Changing direction
2. Forces have both size and
direction
a. Size
•Size is measured in SI units called
newtons (N)
• with a Spring Scale
b. Direction:
•Same direction: Add (+)
•Diff. direction: Subtract (-)
3. Net force = combination of all
forces acting on object
4. Force is shown with vector arrows
(remember these?).
• they point in the direction of force
• arrow length or thickness shows the
strength of a force
For example:
5. Forces can be..
• Balanced
• No movement
• Net force=0
• Unbalanced
• Movement
6. Calculating Force
requires multiplying the
mass and acceleration of
an object:
•F=ma=N
•Variations
•m=N/a
•a=N/m
Practice-in your journals!
1. How much force is required to
accelerate a 50 kg mass at 2 m/s2?
2. What is the acceleration of an 18 kg
mass pushed by a 9 N force?
3. A truck is moving with a force of 100
N and an acceleration of 5 m/s2, what
its mass?
Lab: The Nail Challenge
You will receive one block and 12
nails. Your challenge is to balance
11 nails on 1 nail head. You may
work in pairs using only the
materials given and –of course-the
normal force of gravity.
GOOD LUCK!
B. FRICTION
1. Definition-it is a resistance to motion
because of the force that two surfaces
exert on each other
2. Can make moving easier or harder
3. Depends on 2 things
•Type of surface
•How hard the surfaces push against each
other
4. 4 main types
a. sliding
friction-when
solid surfaces
slide over
each other.
b. rolling frictionwhen an object rolls
over a surface.
Until sliding
friction takes
him out!
c. fluid friction-when
objects move through air
or water. Adding
lubricants can change
sliding friction into fluid
friction. Air resistance is
fluid friction.
d. static friction-acts on
objects that are not
moving. It holds them in
place until a force cause
motion.
C. Gravity- an attractive
force between two masses
1. Mass and distance are 2 factors that affect
gravity
•
A big distance between things needs a –
big force
•
A small distance between things needs
smaller force
• The larger the mass, the greater the
gravitational force on an object
2. Also depends on Weight
So what is the difference
between mass and weight?
Mass : How much matter is in
something. Never changes unless we
cut off parts.
Weight: a measure of the force of
gravity in newtons
3. Free Fall
• Objects with gravity as the only
force
• Objects accelerate in free fall at
the same rate of
9.8 m/s
2
4. air resistance –a force that
opposes the movement of an object
in air
• It is a type of fluid friction
• Depends on size , shape and
speed
Remember your paper lab?
Remember most
objects will hit the
ground at the same
time-if there is no
air resistance.
5. Terminal Velocity-the constant acceleration
of a falling object
•increased speed=increased air resistance
until …..
air resistance=force of gravity
6. Projectile Motion- a curved path that is
caused by the downward pull of gravity and the
initial force of velocity
7. Vacuum – empty space
• All objects fall at the same speed
because there are no forces
present.
• In space, the spacecraft and
astronauts are falling “around the
Earth” at the same speed. This is
why they experience weightlessness
D. PRESSURE
1. pressure is the amount of force
acting on a surface
2. pressure equals force divided by
area
•
p = f/a
3. Fluid Pressure-in air and liquids.
4. Air Pressure
a. air pressure is caused by the motion
of particles in the air
b. you do not feel the weight of the air
because the pressure inside your
body is equal to the air pressure
around you
c. Barometer-instrument used to
measure air pressure
d. Changes with altitude-gets lower as
you go higher
5. Water Pressure
a. Manometer-instrument used to
measure pressure in liquids
b. the force of water pressure on an
object is the same in all directions
*This is why we don’t explode
c. water pressure changes with depththe deeper you are, the greater the
water pressure
d. Buoyant force
• The ability of a fluid to have an upward force.
This force is usually equal to the amount of
fluid displaced by an object in it (like a boat)
• This determines whether something sinks or
floats (Archimedes's Principle)
• The density of an object is a factor too.
Density> 1will sink. A density < 1 will float.
6. Bernoulli’s principle
a. Bernoulli’s Principle-principle that
states that as the speed of a fluid
increases, its pressure decreases
b. airplane wings are designed to use
Bernoulli’s principle
• air traveling over the wings moves faster
than air underneath
• this reduces the pressure on top of the
wing, allowing it to be pushed upward
c. Gravity (acting on mass=weight) pulls down
on a plane but three forces combine to help
an airplane fly
• the upward force on a plane’s wing is
called lift
• a forward force, or thrust, helps the
plane take off and maintain air speed
• the air resistance on a plane is called
drag