1. Introduction to Forces
Dynamics
Introduction to Forces

2. What is a force? Simply put, a force is a push or a pull.
Forces are vectors.
Forces are measured in Newtons (N), named after the English physicist, Sir Isaac Newton.
(more on this later…)

3. Weight vs. Mass
A common misconception, these two terms mean different things:
Mass: How much matter is contained within a body.
Measured in kg.
A constant value no matter where in the universe.
Weight: The measure of the force of gravity on an object.
Measured in N.
A value that changes depending on gravity.
*g = 9.8 m/s2

4. The Normal Force
Ever wonder why we don’t get pulled straight down to the centre of the Earth because of gravity?
The normal force!
A force provided by a surface that counter- acts gravity.

5. Free-Body Diagrams (FBDs)
Simple way to graphically represent forces on an object.
Eg.1: Draw a FBD of a mass hanging on a string.
(tension (string) force)
Centre of mass
of object
(force of gravity)

6. Example #2 Draw a FBD of a mass sitting on a desk.
(normal (surface) force)
Stationary surface
(force of gravity)

7. The Net Force
Forces, as vectors, add like all other vectors… tip to tail.
If all the forces are balanced, we say that the system is in equilibrium and the net force = 0.
If the forces are unbalanced, we have an acceleration in the direction of the net force.

8. Example #3 Find the net force in each of the following sets of forces.
15 N [W], 20 N [E]

9. Example #3 cont’d…
b) 12 N [S], 5 N [W] θ
12 N
5 N

10. Example #3
c)100 N [E], 150 N [S 200E]
100 N θ
200
1100
150 N