1. Physics 1 – Oct 27, 2016 P3 Challenge –
Two masses are connected by a cord and pulled to the right by another cord. (Assume frictionless surface.)
Draw free body diagrams for a) green mass, b) red mass, c) the two blocks together
Today’s Objective: Second Law Problem Solving T F
Get out Forces homework for check

2. Agenda, Assignment IB 2.2 Forces Agenda Assignment:
Forces, Newton’s Laws, etc…
Agenda
Common forces
Newton’s Laws
Free body diagrams
Dynamics w/ kinematics
Assignment:
Forces Worksheet

3. Forces Motion is caused by a net force.
Kinematics (both 1D and 2D) are used to describe and predict motion
Dynamics answers the question WHY does motion occur (or not occur)
Motion is caused by a net force.
A force is a vector quantity and is measured in Newtons, N
1 N = 1 kg∙m/s2
A net force is the vector sum of all forces ON a body of interest

4. Common Forces
Weight, W = mg mass times the acceleration due to gravity
Distinguish between mass and weight
Present anytime you are on or near the surface of the earth
Always directed downward, toward center of earth
Tension, T in a string connected to an object
Pulling force applied through a string
Always directed along the string , away from the body of interest

5. Common Forces Applied force, FA or just F Generic force Friction, f
May be a push or a pull resulting from direct contact
Could be directed anywhere
Friction, f
Present anytime two surfaces move/slide (or might move) relative to one another
Always directed opposite the direction of motion
Often set to zero (frictionless) to simplify a problem

6. Common forces
Normal reaction force, R or N (avoid confusion with unit N)
Present anytime there is a surface that something is resting on.
Always directed perpendicular to the surface
Spring force, Ts = kx a variable force that depends on x
k is called spring constant; represents the “stiffness” of a spring
Proportional to the amount of displacement from equilibrium, x
May be a compression or extension of the spring
Present anytime there is a spring
Always directed to restore the spring to its equilibrium position.

7. Common forces Drag force, Fdrag
Present anytime an object is moving through a fluid (liquid or air)
Is often negligible (especially in air with compact objects)
Always directed opposite the direction of motion
Upthrust, Fup Also known as buoyant force (in water) or lift force (in air)
Present anytime an object is floating in a fluid
Always directed upward, opposite gravity

8. Newton’s Laws – the First
Newton’s First Law: A body at rest stays at rest and a body in motion stays in motion with a constant velocity unless acted upon by a force.
Known as the Law of Inertia
IB Language: When the net force on a body is zero, the body will move with constant velocity (which may be zero).
Converse is also true. If a body is not moving with constant velocity, then there must be a net force.

9. Examples of the First
Release from a rotation: a mass rotating on the end of a horizontal string on a table top is released. The mass will continue moving in the last direction it was going and trace out a tangent to the circular motion.
Need for a safety belt: If you don’t have a safety belt on and the car stops, inertia will send you through the windshield.
Swaying on the subway: When the subway starts you fall back while hands and feet try to exert enough forward forces to create the acceleration. When the subway stops, you fall forward while hands and feet exert a stopping force.

10. Newton’s Laws – the Second
The acceleration of an object produced by a net force is directly proportional to the magnitude of the net force, in the same direction as the net force. The proportionality constant is the mass of the object.
In math: Fnet = ma
Most problem solving will be applications of the second law:
w/ kinematics 1D Forces 2D without friction
Multiple bodies 2D w/ Friction Inclines
More on this later…

11. Newton’s Laws – the Third
Newton’s Third Law: For every action there is an equal and opposite reaction
Warning: Not all equal and opposite forces form an action reaction pair!!
For an action-reaction pair, the two forces involved must act on different bodies of interest.
IB Language: When a body A exerts a force on body B, body B will exert and equal but opposite force on body A.

12. Free body diagram
A free body diagram (FBD) is a key part of solving dynamics problems
Includes any and all forces acting on your body of interest
To create an FBD:
Sketch the system
Decide on your body of interest. Turn into a point body.
Are you on earth? Add a Weight, W force pointing down.
Is the body of interest sitting on a surface? Add a normal force, R perpendicular to the surface.
Are there sliding surfaces? Add a friction f opposite the direction of motion
Are there other things in the environment? Add a T or a FA or …. for each

13. Kinematics and Dynamics
Variable in common to both systems is acceleration.
Ex: What average force is needed to accelerate a 7.00 g bullet from rest to 175 m/s over a distance of m along the barrel of a rifle?
Ex: A 10. kg bucket is lowered by a rope in which there is 63 N of tension. What is the acceleration of the bucket? Is it up or down? How long before the bucket hits the water 13 meters below?

14. Exit Slip - Assignment
List three types of forces and the symbol most commonly used to represent that type of force.
What’s Due on Thurs Oct 27? (Pending assignments to complete.)
Forces Worksheet
What’s Next? (How to prepare for the next day)
Read p57-75