1. 12.2 Key Concepts
How does Newton’s first law relate change in motion to a zero net force?
How does Newton’s second law relate force, mass, and acceleration?
How are weight and mass related?

2. Aristotle _______________ thought that a force was required to __________ an object moving
Almost 2000 years later ______________ work helped to correct this misconception
incorrectly
keep
Galileo
Galileo Galilei
(1564 – 1642)

3. Aristotle, Galileo, and Newton
By rolling balls down wood ramps, Galileo studied how gravity produces _________________ acceleration.
He concluded that moving objects NOT subjected to ______________ or any other force would continue to move ___________________.
constant
friction
indefinitely

4. Aristotle, Galileo, and Newton
Newton built on the work of scientists such as Galileo.
He published his results in book called _________________
In this book, Newton defined _________ and _____________ and introduced his laws of motion.
Principia
mass
force

5. Newton’s First Law of Motion
According to Newton’s first law of motion, the state of motion of an object does ______ ______________ as long as the net force acting on the object is ________.
So an object at rest ______________ at rest
A moving object ________________ moving
This law is also called the law of ______________ -
the tendency of an object to resist a change in
motion
not
change
zero
remains
continues
inertia

6. Newton’s First Law of Motion
A strong kick or ___________ will overcome static friction and make the ball move.
Without friction,
the ball would
continue to move
push
push
friction
Unbalanced forces
change motion

7. Newton’s First Law of Motion
This crash sequence illustrates inertia. The test dummy continues its ______________ motion as the car slows and stops.
forward

8. Newton’s Second Law of Motion
According to Newton’s second law of motion, the acceleration of an object is equal to the ________ _________ acting on it divided by the object’s ___________.
net
force
mass

9. Newton’s Second Law of Motion
So the acceleration of an object is ______________ proportional to the net force acting on it.
Example: The _____________ you throw a ball, the more it accelerates
directly
harder

10. Newton’s Second Law of Motion
The acceleration of an object also depends ______________________upon its mass - measure of the ____________ of an object.
Example: Which will have more acceleration when you throw it…a baseball or a bowling ball?
inversely
inertia
A baseball – less mass

11. Newton’s Second Law of Motion
The acceleration of an object is always in the _____________ direction as the net force.
When a net force acts in the direction opposite to the object’s motion, the force produces a _____________________
same
deceleration

12. Newton’s Second Law of Motion
An automobile with a mass of 1000 kilograms accelerates when the traffic light turns green. If the net force on the car is 4000 newtons, what is the car’s acceleration?
4000 N
= 4 N/kg
1000 kg

13. Newton’s Second Law of Motion
Recall that one newton is the force that causes a _______ mass to accelerate at a rate of _________ each second
1 N =
So 4 N = =
1 kg
1 m/s
kg·m s2
kg·m s2
= m/s2 kg kg

14. Newton’s Second Law of Motion
1. A boy pushes forward a cart of groceries with a total mass of 40.0 kg. What is the acceleration of the cart if the net force on the cart is 60.0 N?
60. 0 N
= 1.5 N/kg
= 1.5 m/s2
40.0 kg

15. Newton’s Second Law of Motion
2. A 25-N force accelerates a boy in a wheelchair at 0.5 m/s2. What is the mass of the boy and the wheelchair? (Hint: Solve Newton’s second law for mass.)
(M)
0.5 m/s2 =
_25 N_
(M)
(M)
0.5 m/s2 =
25 N M
0.5 m/s2
0.5 m/s2 =
M =
50 kg

16. Mass and weight are related but are NOT the same.
Weight and Mass
Mass and weight are related but are NOT the same.
Mass is the measure of the ____________ of material an object contains.
Weight is the force of gravity _____________ on the mass of an object.
amount
acting

17. Weight and Mass
If an astronaut has a mass of 112 kilograms, what is his weight on Earth where the acceleration due to gravity is 9.8 m/s2?
Weight =
112 kg x 9.8 m/s2
= kg·m/s2
= 1098 N

18. Weight and Mass
On the moon, the acceleration due to gravity is only about ___________ that on Earth.
1/6
Astronaut on Earth Mass = 88.0 kg, Weight = 863 N
Astronaut on Moon Mass = 88.0 kg, Weight = 141 N

19. Assessment Questions What is inertia?
the force of gravity acting on an object
forces of friction slowing an object’s motion
the mass of an object
the tendency of an object to resist change in its motion

20. Assessment Questions
A 3600-N force causes a car to accelerate at a rate of 4 m/s2. What is the mass of the car?
600 kg
900 kg
14,400 kg
1200 kg

21. Assessment Questions
How would your mass and weight change if you were on the moon’s surface?
They wouldn’t change.
Your mass would remain constant, and your weight would increase.
Your mass and weight would decrease.
Your mass would remain constant, and your weight would decrease.