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P2:
Forces, Springs, & Pressure
Forces Notes
• A force is
any kind of push or pull on an object
– SI unit is in Newton (N)
• A force has both DIRECTION and
MAGNITUDE (vector!)
• Forces can:
– change the speed of an object
– change the direction of movement of an obje
ct
– change the size or shape of an object
Types of Forces
• BALANCED
– Equal forces acting in opposite directions
– Cancel exactly so zero NET (total) force is
produced
Image obtained from: http://sites.wff.nasa.gov/code810/edu_newton.html
Types of Forces
• UNBALANCED
– Unequal forces acting in opposite directions
– Object accelerates in the direction of the
unbalanced force
– A larger unbalanced force = larger
acceleration
Image obtained from: http://www.bbc.co.uk/schools/ks3bitesize/science/images/unbalancedforces.gif
Simulation
• Go to: http://phet.colorado.edu/
• In the search box: forces and motion
– Questions to answer with a partner
• How much force must be applied to the following
objects before it moves?
–
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Crate
Dog
File cabinet
Textbook
Mystery object
refrigerator
Forces and Motion
• Formula for linking together Force, Mass,
and Acceleration of Objects
F = ma
• F is the force in N
• m is the mass of the object in kg
• a is the acceleration in m/s2
• MAKE A FLASHCARD!!
Hooke’s Law
• The formula to calculate force on a spring:
F = kx
• F is the force on the spring
• k is the spring constant (springs with different
tension have different spring constants)
• x is the extension from equilibrium
• Make a FLASHCARD!
• How can you determine the spring
constant using your data?
Forces and Springs
• Today we are going to examine what
happens to the extension of a spring when
a force is exerted on it.
• Create a data table with space to collect
data for 3 trials of at least 5 different
masses. Include a space for your initial
measurements too!
Data Collection
• When you collect data it is important that you
start by marking the load position on your
meter stick when your spring is empty.
– This will serve as a fixed point to use in your
measurements during the lab.
• Be sure to choose 5 masses that are quite
different from one another and do NOT
overload the spring (i.e. if it appears that it
will break or fail to stretch back)
Calculating Pressure
• Each time you take a step or sit down in a
chair you exert pressure on the surface
below you!
• You can calculate the pressure exerted on
something by using this equation.
Pressure = Force/Area
• Units for pressure are usually Pascals (Pa) which
is another way to say N/m2
Pressure in Real Life
• Why is a small woman wearing high heels
capable of damaging wood floors but a
huge elephant could walk across the same
floor without making a dent?
– Let’s find out!
Sensible shoes?
• The force of our body
pushing down will exert
a pressure
• The surface area of our
shoes in contact with
the ground will affect
the pressure
• These stiletto shoes will
concentrate the force
over a small area
• These shoes will give a
larger pressure and
would not be suitable
for walking on soft
ground
More sensible shoes
• These boots have a large
surface area
• The weight of a person is
spread out over the larger
surface
• The pressure exerted by
the person is smaller than if
they were wearing stiletto
shoes
• A small pressure means
you will not sink into soft
ground
Examples of where a large area is
needed to reduce pressure.
Caterpillar tracks on diggers and on
tanks
Tires on a tractor
Skis and snowboards
Examples of where a small area is
needed to increase the pressure
• Nails
•
•
•
•
•
Needles
Kitchen knives
Scissors
Push pins or tacks
Spade
Practice Calculations
1. A box weighs 500N. It’s base
has an area of 5 m2. What
pressure does it exert?
Answers:
1. 100 Pa
2. Another identical box is
placed on top of the first
box. What is the pressure
now?
2. 200 Pa
3. A snowboarder weighs 600N.
The area of the snowboard is
0.5m2. What is the pressure
3. 1200 Pa
Examples
• Two people are pushing on a refrigerator
from the same side, one with a force of
150N and the other with a force of 85N.
How much force is being exerted on the
fridge?
Example
• The teachers and students are involved in
a tug-of-war competition on field day.
There are 15 teachers, each pulling with a
force of 110N and 25 students, each
pulling with a force of 75N. Who will win?
Practice Time
• Complete the sheet in your packet called
“Addition of Force and Pressure”