1. Section 12-2
Newton’s Laws

2. On Page 101, Write the Questions and Answer the Following:
12.2 objectives
1. How does Newton’s 1st law relate change in motion to a zero net force?
Before:
After:
2. How does Newton’s 2nd Law relate force, mass, and acceleration?
3. How are weight and mass related?

3. Newton’s 1st Law
Every Motion you Observe or Experience is Related to a Force.
Sir Isaac Newton described the relationship between motion and force in three laws that we now call Newton’s laws of motion.
If you slide a book across a rough surface, the book will eventually come to a rest.
If you slide a book across a smooth surface, the book will slide much farther before stopping.

4. Friction causes the book to come to stop
Friction causes the book to come to stop. Without friction, the book would continue to slide forever.
This is an example of Newton’s First Law…..
An object at rest remains at rest and an object in motion maintain its velocity unless it experiences an unbalanced force.

5. You experience Newton’s first law every time you ride in a car and it comes to a stop. You feel your body continue to move forward. The seatbelt acts as an unbalanced force needed to bring you to a stop as the car stops.
Inertia is the tendency of an object at rest to remain at rest or, if moving, to continue moving with a constant velocity.

6. All objects have inertia because they resist changes in motion
All objects have inertia because they resist changes in motion. An object with very little mass can be accelerated with a small force. But it takes a much larger force to accelerate a large mass.

7. Newton’s Second Law
Newton’s Second Law Describes What Happens When the Net Force Acting on an Object is Zero.
The object either remains at rest or continues moving with constant velocity.
But what happens when the net force acting on an object is not zero?

8. Newton’s Second Law describes the effect of this unbalanced force on the motion of an object.
His second law describes the relation between mass, force, and acceleration:
The unbalanced force acting on an object equals the object’s mass times its acceleration.

9. Mathematically it is written as follows:
F = ma
Force = mass x acceleration
Think of pushing an empty shopping cart versus a full shopping cart. Which one takes more force to push? The one with more mass takes a much larger force to accelerate.
When two masses are the same, a greater force provides a greater acceleration.
The acceleration is always in the direction of the net force.

10. Force is Measured in Newtons.
The SI unit for force is the Newton (N).
1 Newton = 1 Kg x 1m/s2
1 lb. = N
1 N = lbs.

11. Examples:
1. Zookeepers lift a stretcher that holds a sedated lion. The total mass of the lion and the stretcher is 175 kg, and the lion’s upward acceleration is m/s2. What is the unbalanced force necessary to produce this acceleration of the lion and stretcher?
Fnet = ?
m = 175 kg
a = m/s2
Fnet = ma
Fnet = (175)(0.657)
Fnet = N

12. Examples:
2. What is the net force necessary for a 1.6 x 103 kg car to accelerate forward at 3.5 m/s2?
Fnet = ?
m = 1.6 x 103 kg
a = 3.5 m/s2
Fnet = ma
Fnet = (1600)(3.5)
Fnet = 5600N

13. Examples:
3. A softball accelerates downward at 9.8m/s2. If the gravitational force acting on the ball is 2.9 N, what is the softball’s mass?
Fnet = 2.9N
m = ?
a = 9.8 m/s2
M = F/a
M= 2.9 / 9.8
m = kg

14. Examples:
4. A cruise ship and its passengers have a combined mass of 12,875kg. If the cruise ship experiences an unbalanced force of 762 N pushing it forward, what is the cruise ship’s acceleration?
Fnet = 762 N
m = 12,875 kg
a = ?
a = F / m
a = 762 / 12,875
a = m/s2

15. Free Fall and Weight
When Gravity is the Only Force acting on an Object, it is said to be in Free Fall.
The free fall acceleration of an object is directed toward the center of the Earth.
Because Free Fall acceleration results from the force due to gravity, it is often abbreviated as the letter g.
Near Earth’s Surface, g is approximately equal to 9.8 m/s2.

16. Free Fall Acceleration near Earth’s Surface is Constant.
All objects near Earth’s Surface accelerate at the same rate, regardless of their mass.
This means you can drop two objects with different masses from the same height and they will hit the ground at the same time.
We will assume that all objects on Earth accelerate at exactly 9.8 m/s2.

17. Newton’s Second Law shows us that acceleration depends on both the force acting on an object and its mass.
A heavier object experiences a greater gravitational force than a lighter object. But heavier objects are also harder to accelerate because it has more mass. The extra mass of the heavy object exactly compensates for the additional gravitational force.

18. Weight Equals Mass Times Free-Fall Acceleration.
The force on an object due to gravity is called its weight.
On Earth, your weight is simply the amount of gravitational force exerted on you by Earth.
If you know the free-fall acceleration, g, acting on a body, you can use F = ma to calculate the body’s weight.

19. Weight equals mass times free-fall acceleration.
Weight = mass x free-fall acceleration
W = mg
The SI unit for weight is the Newton, (N).

20. Weight is Different from Mass.
Mass and Weight are easy to confuse.
Mass is a measure of the amount of matter in an object.
Weight is the gravitational force an object experiences due to its mass. The weight of an object depends on gravity, so a change in an object’s location will change the object’s weight.
(Think outer space vs. earth vs. other planets)

21. Gravity influences the shapes of living things
Gravity influences the shapes of living things. On land, large animals must have strong skeletons to support their mass against gravity.
For organisms living in water, the downward force of gravity is balanced by the upward forces of the water. For many creatures, strong skeletons are not necessary. Think of a jellyfish.

22. Go back to the objectives on page 44 for 12-2 and answer them.
How does Newton’s 1st law relate change in motion to a zero net force?
How does Newton’s 2nd Law relate force, mass, and acceleration?
How are weight and mass related?