1. In your Journals: Quickwrite on Forces
Define the following:
Force
Mass
Weight
Inertia

2. Force: Newton’s Laws

3. What is a Force???

4. What is a Force? A force is a push or a pull on an object
Unbalanced forces cause an object to accelerate…
To speed up
To slow down
To change direction
A Force is a vector and the units are Newtons

5. Types of Forces Contact forces involve contact between bodies.
Normal, Friction
Field forces act without necessity of contact.
Gravity, Electromagnetic, Strong, Weak
Question: Is there really any such thing as a contact force?

6. Forces and Equilibrium
If the net force (SF) on a body is zero, it is in equilibrium
An object in equilibrium may be moving relative to us (dynamic equilibrium). But we can only have inertial reference frames without an acceleration – so constant v is ok
An object in equilibrium may appear to be at rest (static equilibrium)

7. Galileo’s Thought Experiment

8. Galileo’s Thought Experiment
This thought experiment lead to Newton’s First Law

9. Newton’s First Law Newton’s 1st Law is also called the Law of Inertia
A body in motion stays in motion in a straight line unless acted upon by an external force
This law is commonly applied to the horizontal component of velocity, which is assumed not to change during the flight of a projectile

10. Newton’s Second Law
Newton’s 2nd Law says that a body acted upon by a net external force will accelerate
The acceleration is proportional to the net force and inversely proportional to the mass. It is in the direction of the net force
SF = ma

11. Newton’s Third Law Newton’s 3rd Law is commonly stated:
“For every action there exists an equal and opposite reaction”
If A exerts a force F on B, then B exerts a force of -F on A

12. Commonly Confused Terms
Inertia: the resistance of an object to being accelerated
Mass: the same thing as inertia (to a physicist)
Weight: gravitational attraction
inertia = mass  weight

13. Sample Problem: Three forces act upon a 3
Sample Problem: Three forces act upon a 3.0 kg body moving at constant velocity. F1 = (4i – 6j + k) N F2 = (i – 2j - 8k) N Find F3.
F3 = (-5i + 8j +7k) N

14. Sample Problem:
Two forces act upon a body of mass 3.0 kg
F1 = (4i – 6j + k)N and F2 = (i – 2j - 8k)N
No other forces act upon the body
What do you know must be true?
Find the Net Force and the acceleration of the object (both in unit vector notation and as magnitude and direction).
Fnet= (5i - 8j – 7k) N or 11.7 N at 32° towards –j from +i and 127° down from +k
a =1.67i – 2.67j – 2.33k or a = 3.9 m/s2 at 32° towards –j from +i and 127° down from +k

15. Determine the magnitude and direction of the puck’s acceleration
Sample Problem:
A hockey puck has a mass of .30 kg and slides horizontally of the frictionless surface of an ice rink as shown below.
Determine the magnitude and direction of the puck’s acceleration
F2 = °
a = 34 m/s2 at 31°
F1 = °