In this lesson, students will learn how gravity & friction can take affect on the simple things we do everyday of our lives. Learn the 3 main types of friction, & air resistance, plus a few examples. Finally, learn about terminal velocity & get a basic introduction to magnetic & electric forces with a slight distinction for buoyancy & density.

1. Gravity, Friction, & More
Forces in Everyday Life
By Emmanuel Dikolelay

2. Introduction
Everywhere we look in our day-to-day lives, we see forces in
action.

3. Gravity
Gravity is not just the force we see when objects fall to the ground, gravity affects
all masses. Gravity is the force of attraction between all things that have mass.
The strength of gravity depends on both the mass & the distance between masses.
Larger masses have more gravity. Also, objects that are closer will always pull on
you with more gravity. If gravity affects all objects, why aren't we pulled to a
building when we walk past it? The force of gravity between objects on Earth is so
tiny that we don't even feel it, especially when compared to the force of gravity
from Earth itself.
gravity: the attractive
force between
objects with mass

4. Gravity (con.)
Gravity is also what keeps the Earth in orbit around the sun. The sun has so much
mass that it exerts a gravitational force on our entire solar system, keeping all of the
planets, including Earth, in orbit.

5. Planets in Motion
Why aren't planets in orbit around the sun pulled toward it? Because, despite the sun´s
gravity, the planets are also moving sideways. If you swing a yo-yo around your body,
you are pulling it toward yourself by holding on to its string, that´s like the sun´s
gravity, but the motion sideways keeps the yo-yo swinging around in the same circle.

6. Weight
Weight is actually a measure of gravitational force. Weight depends on both the
force of gravity & the mass of the object. If you weigh two objects on a scale, the
object with more mass will weigh more.

7. Weight (con.)
Mass doesn´t depend on location, but weight does because gravitational force can
change depending on your location. For example, the force of gravity is less on the
moon than it is on Earth. (The force of gravity depends on mass, & because the moon
has less mass than the earth, it has a smaller gravitational force.) Therefore, the same
object will weigh less on the moon than it will on Earth (about ⅙ as much).

8. Negative Acceleration
Because gravity pulls you down towards the Earth, there is a constant acceleration
toward the ground. The force of gravity on Earth is approximately 9.8 m/s(squ.).
So, when you throw something up into the air, it gets slower until it stops in mid
air, & then comes back down towards the Earth, accelerating down until it crashes
into the ground.
Also known as Negative
Acceleration!

9. Friction
Newton's first law states that an object in motion will remain in motion unless
acted on by a net force. Try sliding your computer across the table. It slows down
& eventually stops. So what is the net force that is affecting the book? Friction!
It´s the force that opposes movement between touching surfaces & always acts
against the direction of motion. When you ride a skateboard, the wheels slow
down due to friction with the sidewalk & with the bearing in the wheels.

10. Friction (con.)
In general, rougher surfaces have more friction. Sandpaper is harder to slide than
normal paper because it has a rougher surface & therefore more friction. On the
other hand, you can minimize the force of friction by greasing surfaces. Even our
bodies have ways to reduce friction: We have fluid in our knees to reduce joint
friction.

11. Types of Friction: Air Resistance
There is also friction with air & water. The more technical term for friction with the
air is air resistance. When you drop a feather, it floats from side to side because
air resistance opposes the downward movement. Because friction is the force that
opposes movement between surfaces in contact, objects with more surface area
will have more air resistance.

12. Types of Friction
Some other types of friction include:

13. Static Friction
The force of friction between surfaces not in motion. Static Friction is the result of
molecules on one surface adhering to the other surface.

14. Sliding Friction
Also known as kinetic friction, the force of friction that affects surfaces in motion.
When you are pushing a box, the force of friction that resists the motion is sliding
friction. Because the surfaces aren't always bonded like they are in static friction,
sliding friction is less powerful than static friction.

15. Rolling Friction
The friction between surfaces, when an object, such as a wheel or a ball, rolls
freely over a surface.
The friction between the wheels of a skateboard & the sidewalk is rolling friction.
Rolling friction is weaker than sliding friction, which is why it is much easier to
move something on wheels!

16. Terminal Velocity
When an object is falling to the ground, there are 2 forces acting on it: the force of
gravity & the air resistance that is opposing the motion. When the air resistance
equals the force of gravity on the object, there is no net force on the object, the
forces are balanced. Without a net force, the object stops accelerating & it
continues falling at a constant speed.
Terminal Velocity is the speed at which the force of gravity equals air resistance.
This speed depends on many factors, including the object's surface area, mass,
how the object is oriented, & even the thickness of the air!

17. Magnetic Forces
When you play with magnets, you can feel either an attractive or a repelling force.
A magnet is a material that is attracted to iron, steek, or other magnets. Magnets
have a positive & negative end. Opposite charges attract & like charges repel. So,
when you place a positive with a negative end, the magnetic forces pull together. If
you try to touch a negative end with another negative end, or a positive with a
positive, you will feel a repulsive force (they push each other away).

18. Electrical Forces
Electrical Forces are like magnetic forces in that they are caused by positive &
negative charges in the matter. While magnetic forces are created by unmoving
changes, electric forces are created by moving charges. The strength of both
electric & magnetic forces is related to the amount of charge & the distance
between charges. Electric & magnetic forces increase when the charge is stronger
& when the charges are closer together.

19. Electromagnetism
Electromagnetism is the interaction of electric forces & magnetic fields, any
electric charge in motion has a magnetic region around it. A wire that carries an
electric current is surrounded by a magnetic field. You can make an electromagnet
by wrapping a current-carrying wire around an iron core. Just like other magnets,
one end is a north pole, & the other is a south pole.

20. Electromagnet Diagram
The north & south ends of the bar
are determined by the direction of
the flow of electricity. If you
reverse the flow, the charges
reverse!

21. Centripetal Force
Because velocity includes both speed & direction, an object in circular motion is
constantly changing its velocity, & therefore accelerating. Because an object in
circular motion is accelerating, there must be an outside force acting on it
(according to Newton's second law, force equals mass multiplied by acceleration).
The force affecting an object in circular motion is called a centripetal force. A
centripetal force always points towards the center of the circular path of motion.

22. Centripetal Force
centripetal force: the force that
causes an object to follow a curved
or circular path & is directed inward,
toward the center of rotation

23. Centripetal Force Examples
A centripetal force can be any number of things. The moon moves in a circular
orbit around the Earth & is affected by the centripetal force of gravity. If you swing
a yo-yo around in a circle, tension from the string is the centripetal force that
keeps the yo-yo in circular motion.

24. Buoyancy & Density
The force that keeps a rubber ducky floating is called the buoyant force. The
buoyant force is an upward force exerted by a fluid onto an object that is
immersed in the fluid.
The buoyant force depends on the density of the fluid & the amount of fluid that
the object displaces. The denser the fluid & the less fluid displaced, the stronger
the buoyant force. The buoyant force is actually equal to the weight of the fluid
that has been displaced, which is a principle called Archimedes´ Principle.

25. Buoyancy Example
Don´t Forget: An object placed in a
fluid will float if it is less dense than
the fluid, & it will sink if it is denser
than the fluid.
https://jr.brainpop.com/science/forces/sinkorfloat/

26. Lesson Review: Quizizz
https://quizizz.com/admin/quiz/5c351d721a1651001b2c8138/gravity
https://quizizz.com/admin/quiz/5b985d1eea47280019db2f44/mass-vs-
weight

27. Lesson Review: Kahoot
https://create.kahoot.it/details/acceleration/234b0138-e3d1-4c4b-9554-
ccd54be4cf50
https://create.kahoot.it/details/friction-vocabulary-quiz/720dce1f-6fc8-46f5-
badf-14f6f253f971
https://create.kahoot.it/details/93da38b3-1a8d-4594-a08c-51eeafcb2def

28. Lesson Review: Quizlet
https://quizlet.com/307601607/terminal-velocity-flash-cards/
https://quizlet.com/316902050/density-and-buoyancy-flash-cards/