Download An intro to forces

Lesson 5 ~
INTRO. TO FORCES NOTES
Defined as any push or pull on an object, we normally will use a spring scale to measure Force. (You need
to read the Force in Newtons.)
A small list of Forces we will look at
1. For small masses, the ELASTIC force a rubber band exerts is related to how much to rubber
band stretches and then returns to the original shape. Elastic forces are related to objects like rubber bands
and other materials that stretch easily when acted on by a force. The tension force is the force that is
transmitted through a string, rope, cable or wire when it is pulled tight by forces acting from opposite
ends. OUR rubber bands will break if stretched beyond 20 cm, and once at that length, will never return
to original shape.
2. GRAVITATIONAL force is the force of attraction between two different masses. The larger
the mass, the more gravitational force is present. LAW OF UNIVERSAL GRAVITATION: Every object
in the universe attracts every other object in the universe with a force that is directly related to the product
of their mass and opposite to the square of the distance between them.
An object’s mass stays the same as it moves from place to place, but weight does not. For
example, a rock from the moon has the same mass at both places, but weighs 6 times more on Earth
because Earth’s gravity is 6 x more than the moon’s.
* Weight and Mass are different characteristics of an object and are measured in different units –
Weight, a measurement of gravitational force on a body is measured as a force: measured in Newtons (N),
while Mass, the amount of matter in a body, is measured in grams (g). The gravitational force on a body
is directly related to the mass of the body. We use the term load force when we are talking about the
gravitational pull on an object we are experimenting on.
3. FRICTIONAL force: The spring scale measures the effort force needed to overcome the
frictional force caused by the sliding surface. Since for every action there is an equal and opposite
reaction, we can use this Newton’s Law to know that the frictional force is also shown by the spring scale.
There are multiple types of friction: fluid, sliding, rolling, static.
*The force of friction on the block depends on the type of surface the block moves over. The
surface that had the most frictional force was the fine sandpaper due to more particle surfaces to cross
over. The greater the load (or weight) the more force needed to overcome friction. The force of friction
does NOT depend on the amount of surface area touching the sliding surface.
Air resistance can be called buoyant force if the object is floating in the gas, but mainly refers to
the fluid frictional force of the air pushing back on the object travelling through the air.
4. CENTRIPETAL force: A force caused by movement within a circular motion
5. MAGNETIC force: When magnets push away, it is called repulsion because the two like forces,
either positive or negative, do not connect. Magnets that have unlike forces near each other will move
together due to attraction. + or – sides on the magnets.
Repelling= like forces (-- or ++) Attracting=unlike forces (+-)
In most materials, the forces are in balance. Half of the electrons are spinning in one direction; half
are spinning in the other, and they are randomly scattered throughout the materials. Magnets are different.
In magnets, most of the electrons at one end are spinning in one direction. Most of the electrons at the
other end are spinning in the opposite direction. This creates an imbalance in the forces between the ends
(poles) of a magnet. This creates a magnetic field around a magnet.
Because of the magnetic field, magnets are special materials that are attracted to each other and other
materials such as some metals.
Like poles of magnets repel each other, opposite poles of magnets attract each other.
Metals that can be magnetized include iron, nickel, and cobalt. Steel is made of iron and other metals.
The Earth is a magnet with North and South poles, because its core is iron. Scientists believe that the
Earth’s poles have reversed several times in the past, but they don’t know why.
There is also an electromagnetic force that follows the same principles only it is caused by the electric
current flowing through the wire.
6. Buoyant Force – the upward force caused by the mass of liquid or air on an object with less
density than what it is floating in.
7. Nuclear force – this is the force that exists in the atomic structure of an atom.
8. NORMAL force is responsible for the structural integrity of tables and floors as well as being
the force that responds whenever an external force pushes on a solid object. An example of the normal
force in action is the impact force on an object crashing into an immobile surface and the object being
pushed back.
9. CENTRIPETAL force acts on an object accelerating in circular motion, the unbalanced force
acting on the object causes a circular motion while the force is pushing the object to the outer edge of the
circular path.
10. APPLIED force or better known to us as EFFORT force. An applied force is
a force that is applied to an object by a person or another object. If a person is pushing a desk across the
room, then there is an applied force acting upon the object. The applied force is the force exerted on the
desk by the person.
11. The SPRING force is the force exerted by a compressed or stretched spring upon any object
that is attached to it. An object that compresses or stretches a spring is always acted upon by a force that
restores the object to its rest or equilibrium position. For most springs (specifically, for those that are said
to obey "Hooke's Law"), the magnitude (strength) of the force is directly proportional to the amount of
stretch or compression of the spring
To move anything, you must use a force. The more energy used to create the force, the stronger the force
will be. To stop any movement, we must have a counterforce opposing the directional movement. Forces
are mentioned as either balanced or unbalanced. Inertia is when forces are equal so no movement
change will occur and so the forces are balanced.
NET FORCES- The combined Forces acting on the mass.
We use diagrams to show net forces as being balanced or unbalanced (remember balanced means no
movement or continuing a steady straight movement; unbalanced means to start moving, stop moving,
change direction, slow down, or speed up
10 N
5N
15 N
5N
80 N
20N
What will happen? (the first mass will not move due to the balanced forces on either side not being
greater to overcome the force of gravity) (the second mass will move right due to the force pushing in that
direction being greater than gravitational force and the opposing force)