## Lesson 7: Forces and Free Body Diagrams

## Overview:

In this lesson, you are reminded of the fact that forces are responsible for changes in motion. You will review the concepts of forces, free-body diagrams, and net force. You will develop skill in constructing free-body diagrams in two-dimensional situations including situations involving inclined planes and apply your skills to find the net force on objects.

## Curriculum Expectations:

**Overall Expectations:**

**B2.**Investigate, in qualitative and quantitative terms, forces involved in uniform circular motion and motion in a plane, and solve related problems.

**B3.**Demonstrate an understanding of the forces involved in uniform circular motion and motion in a plane.

**Specific Expectations:**

**B2.1**Use appropriate terminology related to dynamics, including, but not limited to: inertial and non-inertial frames of reference, components, centripetal, period, frequency, static friction, and kinetic friction.

**B2.3**Analyse, in qualitative and quantitative terms, the relationships between the force of gravity, normal force, applied force, force of friction, coefficient of static friction, and coefficient of kinetic friction, and solve related two-dimensional problems using free-body diagrams, vector components, and algebraic equations (e.g., calculate the acceleration of a block sliding along an inclined plane or the force acting on a vehicle navigating a curve).

**B2.4**Predict, in qualitative and quantitative terms, the forces acting on systems of objects (e.g., masses in a vertical pulley system [a “dumb waiter”], a block sliding off an accelerating vehicle, masses in an inclined-plane pulley system), and plan and conduct an inquiry to test their predictions.

## Success Criteria:

- What is a force and what are the SI units of force?
- Compare contact vs. non-contact forces.
- Describe the following forces: force of gravity, normal force, tension force, air resistance, and applied force.
- Compare static friction vs. kinetic friction.
- What happens to the tension force, when a string passes over a pulley?
- Explain why ropes can only pull and never push.
- Describe the process of drawing a Free-Body Diagram (FBD).
- Describe how to find the net force acting on an object. Also consider the case when the forces are not always parallel or perpendicular.

## Time Allocation: 1 hour

## Learning A

ctivities:**Read**pages 62 - 68 from Nelson 2.1

Force and MotionExplore the forces at work when you try to push a filing cabinet. Create an applied force and see the resulting friction force and total force acting on the cabinet. Charts show the forces, position, velocity, and acceleration vs. time. View a Free Body Diagram of all the forces (including gravitational and normal forces). |

In the playlist below, video:

- Will show you how to draw free body diagrams.

**Practice**questions 1 and 2 on page 65.

**Practice**questions 4 and 5 on page 68.

## Task:

**Solve**questions 4, 5, 6, and 9 from Nelson 2.1 Review on page 69.

*Optional Extension:*- Practice question 3 on page 68.
- Solve question 7 on page 69.

## Reflect:

How did your study of forces help you understand how objects move or do not move? For the various objects around you -- your computer, a table or desk, anything that you can move from one place to another -- ask yourself, "What forces act on that?" Try to explain these forces in terms of Newton's three laws of motion. In particular, think of how the various forces interact according to Newton's third law.