Day 1
Newton's Laws
Problems
FW = mg
&
1 N = 0.2248
lb
Force: a push or pull
Spring scales are used to measure force.
Use Spring Scales to measure
the force of things
•
•
•
•
Shoes
Pencils
Books
etc.
Units
Force is measured in Newtons
Newton = kg • m/s2
1 N = 1 kg m/s2
British Units
mass = slug
force = pound (lb)
g = 32.2 ft/s2
How much is a Newton?
1 N ≈ ¼ lb’er
1 N = 0.225 lb
Mass vs. Weight
Mass- measure of the inertia of a body
Weight- depends on gravity
SI Units
Mass -grams, kilograms
Weight- Newtons
Newton's Three Laws of Motion
1. Law of Inertia
2. F = ma
3. Action/Reaction
Newton's First Law of Motion
Law of Inertia
Newton’s 1st Law of Motion
Every body continues in its state of uniform
speed in a straight line unless it is compelled
to change that state by a net force acting on
it.
Newton’s 1st Law of Motion
Basically....
• An object moving at a constant speed will
continue at a constant speed unless an outside
force acts on it.
• An object at rest will continue to be at rest
unless an outside force acts on it.
Law of Inertia
Physics at Home page 31
Examples of Inertia
Newton's 2nd Law of Motion
F = ma
Newton’s 2nd Law of Motion
sometimes written as...
sigma
ΣF = ma
ΣF= sum of forces
m = mass
a= acceleration
Newton's 3rd Law of Motion
Action - Reaction
Newton’s
rd
3
Law of Motion
Whenever one object exerts a force on a second
object, the second exerts an equal and opposite force
on the first.
Appropriately Named:
Equal and Opposite
rd
3
Law of Motion cont'd
NOTE:
The equal and opposite forces
act on different bodies!
Balloon Rockets Example
This is NOT
Newton's
3rd Law
Calculating Weight
FW = mg
Units of weight ⇒ Newtons
g = 9.8 m/s2
(near the Earth’s surface)
Conversion to
remember!
1 N = 0.2248 lb
Calculating Weight
What is the weight of a 2 kg mass?
How many pounds
is this?
1 N = 0.225 lb
Problem 1
Dr. Gerla weighs 168 lbs.
What is his weight in newtons?
747 Newtons
1 N = 0.2248 lbs
Problem 2
What is the weight (in Newtons) of 150 grams of notebook
paper?
1.47 N
Problem 3
A child has a mass of 35.0 kilograms. What is her weight
in newtons? What would her weight be in pounds?
343 N, 77 lbs
1 N = 0.2248
Problem 4
A woman weights 0.70 kN on Earth. What is her mass in
kilograms?
71.4 kg
Problem 5
A woman has a mass of 50-kg.
a) What is her weight on Mars, where g=3.7 N/kg;
b) on Venus, where g=1.6 N/kg.
a) 185 N
b) 80 N
Free Body Diagrams
&
Equilibrium
Next time………..