1. Forces 8th Grade Science
Chapter 10
Forces
8th Grade Science

2. Force _______- a push or a pull One object exerts a _______ on another
Force is described by its ______ and by the ________ in which it acts
Example – if you ______ on a door, you exert a _____ in a different _______ than if you ____ on the door.
Force
Force
strength
direction
push
Force
direction
pull

3. Static Friction
_____________ - the friction that acts on an object that is not moving.
You must use________ to start motion of a stationary object.
If you use a force that is less than force of ___________, then the object that is stationary will not move.
Example: pushing a desk on the floor
Static Friction
Extra Force
Static Friction

4. Sliding / Rolling Friction
____________ - occurs when two surfaces slide over each other.
Example: a ballet dancer puts sticky powder on the bottom of ballet shoes so that they do not slip on the floor.
_____________ - when an object rolls across a surface
Example: skateboard – rolling friction acts in the _________ direction of the skateboards motion.
Sliding Friction
Rolling Friction
opposite

5. Fluid Friction
___________ - occurs when a solid object moves through a liquid.
Example: parts of a machine are bathed in oil so that they move smoothly over each other.
Example 2: a bike rider experiences fluid friction between the individual and the air.
When an object ____________, fluid friction occurs
Fluid Friction
Pushed fluid aside

6. Gravity _______ - a force that pulls objects towards each other
__________ - concluded that force acts to pull objects straight downward towards the center of the Earth.
Newton realized that ________ acts everywhere in the _______, not just on Earth = ___________________
Gravity
Isaac Newton
gravity
universe
Law of Universal Gravitation

7. Law of Universal Gravitation
Law of Universal Gravitation states that _____________________________________
Any two objects in the universe, _______________, attract each other.
We are attracted to the _______ and all the other objects around us.
Objects all around us are _________ to us as well – we don’t know it because the forces are small compared to the force of Earth’s attraction.
The force of gravity acts between all objects in the universe
Without Exception
Earth
attracted

8. Factors Affecting Gravity
Two factors that affect gravitational attraction between two objects are ____ and ________
The more _____ an object has, the greater the ______________.
Because the ______ mass is so great, it exerts a large gravitational force on the ________
________ - the farther the two objects are, the _________ the gravitational force between the objects
mass
distance
mass
Gravitational force
Sun’s
planets
distance
lesser

9. Weight and Mass
________ - the measure of the amount of matter in an object
_________ - the measure of the gravitational force exerted on an object
When you step on a scale, you are determining the amount of gravitational force the Earth is exerting on you!
_________ varies but _______ does not
mass
weight
Weight
Mass

10. Gravity and Motion
On Earth, gravity is a ________ force that affects __ objects.
__________ - when the force of gravity is an ____________ force, which causes the object to _________.
Near the surface of the Earth, the acceleration due to gravity is _____ - regardless of the object’s ______.
downward
all
Free Fall
Unbalanced
accelerate
9.8 m/s
mass

11. Math / Analyzing the Data
Page 346 (1-4)

12. Air Resistance
____________ - fluid friction that objects encounter while falling through the air.
Air resistance is an _______ force that is exerted on an object falling _________.
Falling objects with a greater ____________ experience ______air resistance than objects with less surface area.
Air resistance increases with ________
Air Resistance
upward
downward
Surface area
more
velocity

13. Air Resistance Continued
As a falling object gains speed, ______________ increases also – eventually the velocity of the falling object will be fast enough that the ________ force of air resistance is equal to the ________ force of gravity = _______________
When forces are ________ there is no _________ - velocity remains constant
Air resistance
upward
downward
Balanced force
balanced
acceleration

14. Projectile Motion
__________________ - the greatest velocity of a falling object
________ - an object that is thrown
When a projectile is thrown at an ________ angle, the force of gravity ________ its vertical velocity.
Eventually the ______ motion of the projectile will stop and ________ will pull it ________
Terminal Velocity
projectile
upward
reduces
upward
gravity
downward

15. Section Assessment
Page 348 (1-3)

16. Newton’s Laws Newton proposed three basic laws of motion in the 1600’s
Newton’s first law restated _________ idea about force and motion.
Newton’s first law of motion states: An object at rest will remain at rest, and an object moving at a constant velocity will continue moving at a constant velocity, unless it is acted upon by an unbalanced force.
Galileo’s

17. 1st Law
An object that is not moving, will not move until a ___________.
If an object is moving, it will continue to move at a constant velocity until ________________
_________________________
On Earth, ________ and _________ are _______________ that often change an objects motion.
Force acts on it
force acts to either
Change either speed or direction
gravity
friction
Unbalanced forces

18. Inertia
_______ - is the tendency of an object to resist a change in motion - ______________
Newton’s first law of motion is also called the _________.
Inertia explains why a person moves forward in their seat when a car stops suddenly. ________ moves you forward in your seat while a seat belt ________________
inertia
Galileo’s concept
Law of inertia
inertia
Changes your direction

19. Inertia depends on Mass
The _________ the _______ of an object, the __________ its ______ and the _________ the ________ required to ______________.
greater
mass
greater
inertia
greater
force
Change its motion

20. Second Law of Motion
_________ depends on an objects ________ and the ____________ acting on the object.
Acceleration = Net Force
Mass
acceleration
mass
Net force

21. Acceleration Acceleration is measured in _______________
Mass is measured in __________
According to Newton’s Second Law – force is measured in __________ = _____________
________ = SI unit of force
Math Sample Problem on page 351
Math Practice page 351 (1 and 2)
Meters per second²
Kilograms (kg)
kg times m/s²
Newton (N)
Newton

22. Newton’s Third Law
To increase __________ you must either increase _____ or decrease _____
Newton’s Third Law states: if one object exerts a ________ on another object, then the second object exerts a ________ of _____________ in the ____________ on the first object.
FOR EVERY ACTION THERE IS AN EQUAL BUT OPPOSITE REACTION
acceleration
mass
force
force
force
Equal strength
Opposite direction

23. Why don’t action and reaction forces cancel each other?
Read Examples on pages

24. Momentum
____________ is a characteristic of a moving object that is related to the mass and velocity of the object
Momentum of a moving object is calculated by __________________________
The more momentum a moving object has, the more difficult its is to stop
Objects with greater ______ and ________ have greater ____________
momentum
Multiplying mass times velocity (m x v)
mass
velocity
momentum

25. Conservation of Momentum
____________ - conditions before and after an event
An amount that is conserved is the same amount ______ the event as it was ________
The total amount of momentum objects have is _________ when they ________
Momentum can be __________ but not ____
conservation
after
before
conserved
collide
transferred
lost

26. Law of Conservation of Momentum
_____________ of any group of objects remains the same, or is ___________, unless ___________ act on the objects.
Examples on page three different types of collisions
Total Momentum
conserved
Outside forces

27. Section Assessment
Page 359 (1-5)