1. Learning Objectives
I can explain the concept of field in terms of forces that act at a distance.
I can compare the effect of balanced & unbalanced forces on an object and how to solve for net force.
I can explain Newton’s 1st Law using inertia.
I can describe the relationship among force, mass, and acceleration (Newton’s 2nd Law)
I can explain how forces act in pairs (Newton’s 3rd Law).

2. Ohio New Learning Standards for Science
Ohio 8th Grade, Physical Science #1 (8.PS.1) – Forces between objects act when the objects are in direct contact or when they are not touching.
Ohio 8th Grade, Physical Science #2 (8.PS.2) – Forces have magnitude and direction.
Ohio 8th Grade, Scientific Inquiry & Application #5 (8.SIA.5) – Develop descriptions, models, explanations & predictions.

3. Lesson Opener Upward Force Gravity
Striking the ball with the foot sets the ball in motion.
mesh-like material for catching fish
amount remaining after any subtractions or additions have been made to the starting amount

4. Unit 7, Lesson 1 – Forces
What is a force and how does it act on an object?
- in science, a force is simply a push or a pull
- all forces have both a size and a direction, so forces can cause objects to change their speed or direction
- objects changing position over time relative to a reference point (something stationary) are being acted upon by one or more forces

5. Unit 7, Lesson 1 – Forces
- units used to express forces are called newtons (N)
- forces do not always cause objects to move as some objects’ forces may be balanced with one another
Complete #5 & #6, pg.408

6. Unit 7, Lesson 1 – Forces
- when objects come into contact with one another, the force they exert on each other is contact force
- friction is an example of a contact force between two surfaces
- static friction is the force that resists the start of sliding motion between two surfaces that are touching

7. Unit 7, Lesson 1 – Forces
- kinetic friction is the force that resists movement of two surfaces that are touching & are moving
- a friction-like force that opposes the motion of an object in a fluid (liquid or gas) is called drag
Complete #7, pg.409

8. Unit 7, Lesson 1 – Forces D D F

9. Unit 7, Lesson 1 – Forces
- forces can also act at enormous distances across trillions of miles in the Universe (gravity & magnetism)

10. Unit 7, Lesson 1 – Forces What are some examples of fields?
- when a force acts at a distance, the area in which an object experiences that force is called a field
- fields are measureable, can have motion, and lessen with distance until they become undetectable

11. Unit 7, Lesson 1 – Forces
- an electric field surrounds every charged object
- an electric field around a charged object pulls oppositely charged objects toward it & repels like-charged objects (static electricity)
Complete #9, pg.410

12. Unit 7, Lesson 1 – Forces B A

13. Unit 7, Lesson 1 – Forces
- a magnetic field exists in a region around a magnet (iron, nickel, cobalt, gadolinium) in which magnetic forces can act
- the closer together the magnetic field lines are, the stronger the magnetic field is, especially at the poles!

14. Unit 7, Lesson 1 – Forces

15. Unit 7, Lesson 1 – Forces
- an object with mass creates a gravitational field around it
- field lines can be used to show gravitational fields & the strength of a field is modeled by the density of the field lines
- equally spaced field lines model a uniform field!

16. Unit 7, Lesson 1 – Forces

17. Unit 7, Lesson 1 – Forces
- the Earth rotates on its axis once every 24 hours at 1,080 mph if you’re at the Equator & close to 0 mph if you’re standing on the poles (Newton’s 1st Law)!
- the Earth moves in revolution around the Sun every days at 67,000 mph because of the gravitational pull of the Sun at 93,000,000 miles away!
- the Sun moves in revolution around the black hole at the center of the Milky Way Galaxy at 550,000 mph because of the gravitational pull of the black hole at 26,000 light years away!

18. Unit 7, Lesson 1 – Forces
- our complete solar system makes one revolution around our galaxy every 230 million year!
- the last time Earth was in the place its in right now, dinosaurs roamed the Earth!!!

19. Unit 7, Lesson 1 – Forces
What happens when multiple forces act on an object?
- usually, more than one force is acting on an object
- the combination of all the forces acting on an object is called the net force, which can be determined by looking at the directions of all the forces
- add the forces going in the same direction & subtract the forces going in opposite directions
- the net force acts in the direction of the larger force!
Complete #11, pg.412

20. Unit 7, Lesson 1 – Forces
210N – 190N = 20N; the box WILL move to the right!

21. Unit 7, Lesson 1 – Forces
- when the forces on an object produce a net force of 0 N, the forces are balanced & will NOT cause a change in motion for a moving/unmoving object
- when the net force on an object is not 0 N, the forces are unbalanced, which WILL produce a change in speed, direction, or both
Complete #12, pg.413

22. Unit 7, Lesson 1 – Forces

23. Unit 7, Lesson 1 – Forces What is Newton’s First Law of Motion?
- in the 1680s, Sir Isaac Newton discovered that force an motion are related
- Newton’s first law describes the motion of an object that has a net force of 0 N acting on it
“An object at rest stays at rest and an object in motion stays in motion at the same speed and direction, unless it experiences an unbalanced force.”

24. Unit 7, Lesson 1 – Forces
- Newton’s 1st law is also called the Law of Inertia
- inertia is the tendency of all objects to resist a change in motion
- forces are needed to move objects because of inertia
Complete #14, pg.414

25. Unit 7, Lesson 1 – Forces
Inertia caused the dishes to remain in place because the table cloth did not exert a force on the dishes great enough to make them move.

26. Unit 7, Lesson 1 – Forces
- this law also applies to moving objects in that unbalanced forces act on objects that are moving
- this is why when you slam on the brakes, the car stops, but you keep moving
- both parts of the law are really stating the same thing!
Complete #16, pg.415

27. Unit 7, Lesson 1 – Forces
The forces exerted by the airbag and seat belt acted on the dummy to stop its forward motion.

28. Unit 7, Lesson 1 – Forces What is Newton’s Second Law of Motion?
- when an unbalanced force acts on an object, the object accelerates
- objects that have different masses will have different accelerations if the same amount of force is used
“The acceleration of an object depends on the mass of the object and the amount of force applied.”

29. F = MA (Force = Mass x Acceleration)
Unit 7, Lesson 1 – Forces
- Newton’s 2nd law links force, mass, and acceleration
F = MA (Force = Mass x Acceleration)
- this equation tells us that a given force applied to a large mass will result in a small acceleration and a large acceleration for a small mass
Complete #18, pg.416

30. Unit 7, Lesson 1 – Forces (165 kg)(a) 150 kg  m/s² = (165 kg)(a)
a = 150/165  m/s²
a = 0.91 m/s²

31. Unit 7, Lesson 1 – Forces What is Newton’s Third Law of Motion?
- all forces act in pairs, which we call action forces and reaction forces
- both of these forces are present, even if there is no motion
“For every action, there is an equal or opposite reaction.”

32. Unit 7, Lesson 1 – Forces
- when an object pushes against another object, that object pushes back equally hard
- astronauts can maneuver in space because of compressed gas canisters, which applies Newton’s 3rd law to create an action/reactive force
Complete #22, pg.418

33. Unit 7, Lesson 1 – Forces

34. Unit 7, Lesson 1 – Forces
- even though action & reaction forces are equal in size, their effects are often different
- dropping a ball towards Earth allows you to easily see the action force, but the reaction force of Earth being pulled towards the ball is not as easy to see
- Earth has much more mass than the ball, so Earth’s acceleration is much smaller than that of the ball’s, as expressed in Newton’s 2nd Law!
Complete #23, pg.419

35. Unit 7, Lesson 1 – Forces
action force
reaction force

36. Visual Summary
field
inertia
mass
direction

37. Exit Question
What would happen to Earth if the Sun stopped exerting the force of gravity on it?
The Earth would travel in a straight line away from the Sun in the direction it was traveling when the force of gravity stopped acting on it!