1. Newton’s Laws of Motion
The “Cause” Behind Kinematics

2. Aristotle’s Ideas
Aristotle believed that objects had two different types of motion
“Natural” motion – up and down
Objects return “home”
“Violent” motion – horizontal motion
Motion due to a push or pull
Also believed that objects moved only if a force was continually applied
Therefore, it was “natural” for objects to be at rest
Oct. 31, 2007
AF Carpinelli

3. Galileo’s Contribution
Challenged many of Aristotle’s ideas
Law of Falling Bodies (for example)
Realized that objects did not need a continual application of force to remain in motion
It only APPEARS that this is the case due to friction
Tested this will extremely smooth surfaces and inclined planes
Saw that it was NOT the nature of the objects to come to rest
Oct. 31, 2007
AF Carpinelli

4. Galileo’s Contribution
Realized that in the absence of friction, objects keep moving at constant velocity
Said that instead of a natural rest state, objects have a natural resistance to a change in their motion – inertia
This idea became Newton’s First Law of Motion
Oct. 31, 2007
AF Carpinelli

5. Sir Isaac Newton Most famous physicist of all time
Most of the laws of physics can be traced back to Newton’s work
Landmark book – The Principia
Invented Calculus (although this is disputed – could also have been von Leibniz)
Worked in many areas of science, including optics, chemistry, botany, and astronomy
Oct. 31, 2007
AF Carpinelli

6. Newton’s First Law of Motion
Law of Inertia
Objects at rest remain at rest, objects in motion remain in motion at constant velocity unless acted upon by an outside unbalanced force.
KEY POINTS
Remain – objects cannot change their own motion
Constant velocity – no change in speed or direction with the application of a force
Outside unbalanced force – a force that is applied to the object AND not canceled out by another force
Oct. 31, 2007
AF Carpinelli

7. Newton’s First Law of Motion
A balanced force
5 N N
No change in motion
An unbalanced force
3 N 5 N
Change in motion
Oct. 31, 2007
AF Carpinelli

8. Newton’s 2nd Law of Motion
Law of Acceleration
The acceleration of an object is directly proportional to the force applied to it and inversely proportional to its mass.
Basically, the greater the force, the greater the acceleration BUT the greater the mass, the less the acceleration
Expressed mathematically as F=ma [Force = mass X acceleration]
Oct. 31, 2007
AF Carpinelli

9. Newton’s 3rd Law of Motion
The “Action-Reaction Law”
If Object A exerts a force on Object B, Object B exerts the same force on Object A but in the opposite direction.
Known as Action-Reaction Pairs
Although the force on the pairs is the same, the acceleration of the objects will be different, depending on their masses
For example, the gravitational force that the earth exerts on you when you jump is the same as the force you exert on the earth, but because the earth is so much more massive, its acceleration is essentially zero compared to yours
Oct. 31, 2007
AF Carpinelli

10. Working with Newton’s Laws
New Quantities, Symbols and Units:
Mass (m) – unit is the kilogram (kg)
Force (F) – unit is the Newton (N)
Conversions –
1 kg = lbs
1 kg = 9.8 N
1 N = lbs
Oct. 31, 2007
AF Carpinelli

11. Conversion practice!! Convert the following – 3.2 kg = ____________ N
120 lbs = ___________ kg
65 kg = ____________ lbs
45,000 g = __________ kg
82,420 g = __________ N
72,500 g = __________ lbs
Oct. 31, 2007
AF Carpinelli

12. Conversions Answers Convert the following – 3.2 kg = 31.4 N
120 lbs = 54.4 kg
65 kg = 143 lbs
45,000 g = 45 kg
82,420 g = 808 N
72,500 g = 160 lbs
Oct. 31, 2007
AF Carpinelli

13. 2nd Law Sample Problem
A force of 10 N is applied to a 20 kg crate. What is its acceleration?
Oct. 31, 2007
AF Carpinelli

14. Net Force
The net force is the sum of all the forces acting on an object.
Direction of the forces is very important.
For example:
4 N
3 N
2 N
Oct. 31, 2007
AF Carpinelli

15. Net Force Problem
A force of 17 N directed east is applied to a 5 kg brick. At the same time, a force of 22 N directed to the west is applied. What is the net force on the brick? In what direction will it move? What is its acceleration?
Oct. 31, 2007
AF Carpinelli

16. Newton’s 2nd and Kinematics
A 1250 kg car brakes from 20 m/s to rest in 5 s. What force did the brakes exert on the car?
Oct. 31, 2007
AF Carpinelli