1. Newton’s Laws of Motion
1 & 2
Ch. 6

2. Force
A push or a pull

3. Kinematics – the study of motion
Dynamics – the study of the motion of objects acted on by forces

4. Forces can be... CONTACT FORCES Forces caused by touching.
ex. A cue stick on a billiard ball.
FIELD FORCES
Forces caused by not touching.
ex. Gravity, magnetism, electric

5. Force Facts A force causes motion in the same direction.
ex. push to the right, the object moves right.
Forces are Vectors, with magnitude & direction.
All changes in motion are caused by unbalanced or net forces.

6. Force Math Basics: Force Vocabulary:
The SYSTEM: The object of interest.
EXTERNAL WORLD: all around the system
AGENT: Causes the Force
Force Math Basics:
Forces acting on the same object and in the same direction, ADD.
Forces acting on the same object in opposite directions, SUBTRACT.
*They add, but one is (+) and one is (-), due to opposite directions, which is subtraction.

7. Newton’s Laws of Motion
I Inertia
II F=ma
III Action/Reaction
Not the Cookie.

8. 1ST LAW: LAW OF INERTIA
An object in motion will stay in motion and an object at rest will stay at rest unless acted on by a net outside force.
There is no fundamental difference between an object at rest and an object moving at a constant velocity: The forces on both are BALANCED. There is NO NET FORCE causing it to change its motion.
INERTIA: THE RESISTANCE TO A CHANGE IN MOTION. More Mass = More Inertia.

9. Newton's first law of motion contains the involves "no net force
Newton's first law of motion contains the involves "no net force." What does this mean?
The net force is the vector sum of all the individual forces acting on an object.
When you sit in a chair, there are essentially two forces acting on you: the upward push of the chair and the downward pull of gravity. Since you are at rest, the two forces must cancel out. Therefore, the vector sum of the forces, or net force, acting on you is equal to zero.

10. FNet = 0 Equilibrium FNet = 0FA FF W W
FNet = Equilibrium FNet = 0
If force is unbalanced, then the car will accelerate (Newton’s Second Law).

11. Our experience tells us that an object, such as a box being pushed across the floor, will stop moving if you stop pushing on it.
This occurs because of the force of friction acting between the box and the floor.
What would happen if the force of friction could be eliminated?

12. While friction cannot be eliminated completely, it can be greatly reduced.
An air track provides a cushion of air on which a cart can ride with virtually no friction.
When placed at rest on a level track, the cart will remain at rest until given a push.
Once the cart is in motion, it will remain in motion until acted on by a net force. In theory, if the track could be made infinitely long and perfectly frictionless, the cart would continue moving with a constant velocity forever.

13. 1st Law of Motion: Law of Inertia

14. Newton's first law is sometimes referred to as the law of inertia.
Loosely speaking, inertia means laziness. Objects may be thought of as lazy because they don't change their motion unless forced to do so.
The tendency of an object to resist any change in its motion is referred to as its inertia.

15. An Object in Orbit Has Inertia

16. Newton's 2nd Law
Newton's second law of motion tells how a force changes an object's motion.
Throwing a baseball requires less force than pushing a car and giving it the same speed as the baseball. Why?
The car has more a lot more matter than does a baseball.

17. An object's mass is a measure of the amount of matter it contains.
The unit of mass is the kilogram.

18. How does an object's acceleration depend on the force?
Acceleration is doubled when the force acting on a cart on an air track is doubled.

19. How does an object's acceleration depend on the mass?
Acceleration is halved when the force acting on the cart is doubled.

20. An object's acceleration is directly proportional to the force and inversely proportional to the mass.

21. When an unbalanced, net force acts on an object, it accelerates the object.
A force that causes acceleration is called the NET FORCE. NET FORCE is the sum of all force vectors.
Mathematically Fnet = ma
Force Unit N = kg * (m/s2)
1 Newton = the force that will give a 1 kg mass an acceleration of 1 m/s2

22. Fnet = ma Force is directly proportional to mass
More force is needed to move a greater mass
Force is directly proportional to acceleration
More force is needed to give a mass greater acceleration
But mass and acceleration are inversely proportional to each other.
Less acceleration is achieved when the force acts on a greater mass.

24. An 1800-kg car has an acceleration of 3.8 m/s2.
What is the force acting on the car?

25. A 6 kg bowling ball is rolled down the bowling alley with a force of 96 N.
What is the acceleration of the ball?

26. If a block is pushed with a force of 6 N and accelerates at 2 m/s2, what is the mass of the block?

27. The second law also applies to situations in which several forces are acting on an object.
When several forces act on an object, the Fnet in the equation Fnet = ma is replaced with the sum of the force vectors:
Fnet = ΣF = ma
Only a net force causes acceleration
If an object is accelerating: Fnet = ΣF ≠ 0
Watch direction/signs! Some force vectors will be subtracted (adding a negative)

28. Moe, Larry, and Curly push on a 752 kg boat floating next to a dock
Moe, Larry, and Curly push on a 752 kg boat floating next to a dock. They each exert an 80.5 N force parallel to the dock.
What is the boat’s acceleration if they all push in the same direction?
Fnet = ΣF = ma
Rearrange for a

29. Moe, Larry, and Curly push on a 752 kg boat floating next to a dock
Moe, Larry, and Curly push on a 752 kg boat floating next to a dock. They each exert an 80.5 N force parallel to the dock.
What is the boat’s acceleration if Larry and Curly push in the opposite direction from Moe?

30. A 3 kg box is sitting at rest on a table.
What is the force of gravity on the box? (Choose downward as negative)
What is the force of the table on the box? In what direction (+ or –) does this force act?
What is the net force (sum of forces) acting on it?

31. Mass vs Weight Mass – the amount of matter in an object
Doesn’t change with position (same on earth, moon, deep space…)
unit = kg
K48, right, came from the second batch of kilogram replicas to be produced. It was delivered to Denmark in 1949 with an official mass of 1 kg+81 µg. Like all other replicas, it is stored under two nested bell jars virtually all the time. Still, its mass and that of the IPK diverged markedly in only 40 years; the mass of K48 was certified as 1 kg+112 µg during the 1988–1992 periodic verification

32. Fg = mg Weight is a Force!!! Force of gravity = Wt = Fg
Weight – gravitational force exerted on an object
- More Matter = more gravitational Force exerted on it.
Changes with position
Force of gravity = Wt = Fg
Unit = Newton (like other forces)
Force of gravity = mass x acceleration due to gravity
Fg is towards the center of the Earth
g = acceleration due to gravity = 9.80 m/s2 on Earth
Fg = N
m = mass (kg)
Fg = mg

33. If a person has a mass of 68 kg, what is the person’s weight on Earth?
What would be this person’s weight on the moon? The acceleration due to gravity on the MOON is 1.62 m/s2.

34. Newton's 3rd Law According to Newton's third law:
Forces always come in pairs. That is, there are no isolated forces in the universe.
The forces in a pair are equal in magnitude and opposite in direction.
The forces in a pair act on different objects.
The third law is commonly stated in an abbreviated form: For every action, there is an equal and opposite reaction.

35. Law of Action & Reaction
FAB = -FBA
FAB = the force exerted by object A on object B
FBA = the force exerted by object B on object A

36. As a result, the two forces do not cancel.
Note that in the three examples in the figure, the paired action- reaction forces act on different objects.
As a result, the two forces do not cancel.
If they did, there may not be any motion (depending on other forces)

37. The Horse and Cart Scenario:
The horse says: “I cannot pull the cart. Newton’s 3rd Law says that I pull on the cart and the cart pulls back on me with the same, equal and opposite, force.”
Yet...we all know that a horse CAN pull a cart...

38. Identify the pairs of forces and what they are acting on.
Hint: Think of everything in contact with another surface.
2) We know objects accelerate. According to the horse’s reasoning, nothing in our world would ever change its motion..
3) An unbalanced force or NET FORCE is needed to cause a change in motion. One pair of forces must be greater than another pair of forces.

39. Revisiting the horse and cart:
The horse’s hooves put a force on the ground and the ground puts an equal force back. As long as this pair of forces is greater than the force the horse puts on the cart and the equal force the cart gives back, then the horse can move the cart.

40. It is the strength of the push on the ground that returns a force strong enough to move you and the truck forward.