Unit Two: Dynamics

Section 1: Forces

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.

Summary Slide

Dynamics: The study of WHY things move.

Forces (Summary Slide) What is a force?

A push or a pull that causes a change in motion or shape. It depends on the mass of an object and its acceleration.

Force = mass x acceleration

Symbol: F

Formula: F=ma

Units: kg x m/s2 = Newtons (N)

What does a force do???

causes acceleration (like gravity) OR

causes stretching, bending, squeezing (like spring forces)

The 2 Main Types of Forces

Contact Forces: - Force when two objects are physically in contact with one

another

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 touchingExample: gravitational force (Fg), nuclear force, magnetic

force, electrostatic force (electric charge)

Summary Slide: Types of Forces

Contact Forces: - Force when two objects are physically in contact

with one another

Non-contact Forces:

- Force when two objects are not touching

Copy this down and add as we go!

Name of Force

Symbol Contact or Non-

Contact

Description/Important Ideas/Formula

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

Spring Force

Symbol: Fe

Also called elastic force Contact force (spring is pushing something) Occurs when a spring or elastic is stretched

and released or compressed 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 In physics 11 we generally ignore air resistance

when completing force equations 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?

Identify the forces and their direction… for each situation (Copy down and leave a few extra lines for each)

A) A book is at rest on a table.

B) A book is being pushed by a person horizontally to the right at a constant speed

C) The book in the previous part is let go, allowing it to slow down and come to rest.

D) The book finally stops.

E) A book is falling through the air, accelerating downward.

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) FN (acting up) Fapp (right) Ff (left)

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. Fg (down) FN (acting up)

Ff is no longer present as there is no motion.

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

What have you noticed about Fg?

What have you noticed about FN?

What have you noticed about Ff and Fapp?