Unit Two: Dynamics Section 1: Forces

What are we looking at in this unit? Take 10 minutes with the person next to you to answer these questions (just jot some ideas down) Write the questions down and leave some space to add to your answers! What is the difference between dynamics and kinematics? What is a force? What causes a force? What can a force do? What are the main forces we deal with everyday? (Create a table with Type of force, Symbol, contact or non-contact, and description) Newton’s 3 Laws of Motion

What Do I Already Know? Take 10 minutes to discuss with the person next to you. You can use your textbook and/or phones as well if you don’t know! What is dynamics? What is a force? What does a force do? What are some types of forces?

What is dynamics??? Kinematics: The study of how objects move (velocity, acceleration) Galileo performed experiments that allowed him to describe motion but not explain motion. Dynamics: The study of WHY objects move. The connection between acceleration and its cause can be summarized by Newton’s 3 Laws of Motion (published in 1687) The cause of acceleration is FORCE.

Forces What is a force? Force = mass x acceleration Symbol: F A push or a pull that causes a change in motion or shape Force = mass x acceleration Symbol: F Formula: F=ma Units: kg x m/s2 = Newtons (N)

What does a force do??? Some forces cause acceleration (like gravity) Some forces cause stretching, bending, squeezing (like spring force)

The 2 Main Types of Forces Contact Forces: - Force when two objects are physically in contact with one another Don’t copy the examples! Example: push/pull (APPLIED force or Fapp), normal force (FN), friction (Ff), spring force (Fe), tension (FT), air resistance Non-contact Forces: - Force when two objects are not touching Example: gravitational force (Fg), nuclear force, magnetic force, electrostatic force (electric charge) Contact forces: friction, normal force, tension (string), spring, air resistance Non-contact Force: gravity, magnetic force, electrostatic force (electric charge), nuclear force (holds the nucleus together)

Copy this down and add as we go! Name of Force Symbol Contact or Non-Contact Description

Friction Symbol: Ff A contact force that occurs because of electromagnetic force (between surface atoms of objects touching) Acts in the opposite direction to motion There must be motion for friction!!!

Spring Force Symbol: Fe Also called elastic force Contact force (spring is pushing something) Occurs when a spring or elastic is stretched and released We will look at this in the Energy unit!

Tension Symbol: FT Contact force Tension is the magnitude of the pulling force exerted by a string, cable, chain, or similar object on another object.

Air Resistance Symbol: Fair Contact force between the “air”’s molecules and the object Caused when things are moving through the air usual In physics 11 we generally ignore air resistance as it is very small in comparison to most of the other forces (ie, negligible) HOWEVER, you may need to include air resistance in free body diagrams (coming soon!)

Normal Force Symbol: FN Contact force A force that acts in a direction perpendicular to the common contact surface between two objects Example Diagram:

Gravitational Forces Symbol: Fg We typically look at Fg as a a non-contact force caused by the pull of the Earth’s centre on an object In reality, gravity acts as an attractive force between any two objects. The force is proportional to the objects so since Earth is so much larger than most objects we look at, we only think about it as Earth’s pull.

Can you answer most of the original questions now? What is the difference between dynamics and kinematics? What is a force? What can a force do? What are the main forces we deal with everyday?

Check Your Learning Identify the forces (with their direction) acting on the book in each of the following situations. A) A book is at rest on a table. Fg (acting downward) FN (acting up) B) A book is being pushed by a person horizontally to the right at a constant speed Fg (down) Fapp (right) Ff (left)

D) The book finally stops. C) The book in the previous part is let go, allowing it to slow down and come to rest. Fg (down) FN (acting up) Ff (left) Since there is no longer any contact between the person and the book, there is no longer an applied force. The other forces remain until it stops. D) The book finally stops. Ff is no longer present as there is no motion.

E) A book is falling through the air, accelerating downward. Fg (down) Fair (up)

Free Body Diagrams A free body diagram will be used in most dynamics problems in order to simplify the situation In a FBD, the object is reduced to a point and forces are drawn starting from the point FN Fa Ff Fg

Free Body Diagrams (summary) We usually use a box or small circle to represent the object. The size of the arrow is reflective of the magnitude (SIZE) of the force. The direction of the arrow reveals the direction in which the force acts. Each force arrow in the diagram is labelled to indicate the type of force. Use math symbols to show equality if needed.

What can you tell about these forces??? What else could we add?

Free Body Diagram Examples Draw a FBD for each of the Check Your Learning questions.

The Net Force The sum of all vector forces acting on an object. Label ALL forces that are acting on an object and also determine which would cancel each other out (use equality symbols) Ones that do not completely cancel out will be used to determine the net force

The Net Force In most situations we consider in Physics 11, the forces will be parallel (ie, up and down, etc) and perpendicular

Net Force Draw an FBD for each situation. Include the magnitude (how large) the forces are in comparison to other forces. Then describe the NET FORCE. Example 1: What are the forces acting on a stopped car? Example 2: What are the forces acting on a car moving at 100km/h [N] on cruise control? Example 3: What are the forces acting on a car speeding up? Example 4: What are the forces acting on a car slowing down?

Tug of War – Try this! What are the forces at work in a tug of war? What direction are the forces? If your team wins, what does that mean about the forces? If your team loses, what does that mean about the forces? What other forces are there on the players?

What about if we have actual numbers instead of just arrows???

Find the net force on each FBD

Find the net force on the FBD

FBD and Net Force Mini Worksheet