1. 2.2 Forces Q1: What is the philosophical nature of motion?
N1: ”The ’natural’ state of an object is to continue at constant speed in a straight line (or if at rest stay at rest)”
Q2: OK, but if an object naturally wants to keep moving with constant velocity, what then makes the velocity change?
N2: 𝐹=𝑚𝑎
”The sum of all forces acting on an object equals the mass times the acceleration”
Force (F) in Newtons (N), N = kg m s-2
Q3: OK, that works for a single object, but how then do two different objects interact with each other?
N3: ”For every action there is an equal and opposite reaction”

2. Common forces: Weight (W = mg or G = mg)
Normal force (N), or Reaction force (R) (90° from a supporting surface)
Force of friction (f = μR or Fμ = μN), μ is the coefficient of friction
Tension force (T or S) (in a supporting string)
Bouyant force (B) (for things that float)

3. Mass (m) is measured in kg and is the same everywhere
Weight (W) is a force measured in Newtons and depends on what planet you are on
A free-body diagram (FBD) is a figure where all the forces acting on a specific body are drawn to scale
If there is no acceleration, the forces must cancel in both x- and y-directions

4. Draw a free-body diagram for
A book on a table
A chair pushed by a person at constant speed
A skier accelerating down hill
A street light hanging from two wires
Sketch
FBD

5. Equilibrium: no acceleration ( 𝐹=0 )
Three tug boats move a ship at constant velocity. One pulls to the South ( N) and a second one to the West ( N). What is the size and direction of the third force?

6. Accelerated motion ( 𝐹=𝑚𝑎 )
A wagon of puppies has the mass 35 kg. Peter pulls the wagon using 180 N and Lisa pushes the wagon using 230 N, while the grass resists the motion with 340 N. What will the acceleration of the wagon be?

7. Friction example: A car (m = 1500 kg) traveling at constant speed (50 km/h) is driven forward with a force of 850 N.
How big is the force of friction?
What is the coefficient of friction?

8. Friction Static friction: to get something moving …is bigger than …
Kinetic friction: to keep something moving
Simple formula: 𝐹 𝜇 =𝜇𝑁
Coefficient of friction 𝜇 is a constant depending only on the surfaces
Air resistance/fluid resistance important, but much more complicated

9. 2D force calculations:
Draw FBD
Split forces along x- and y- axes
Solve 𝑭=𝒎𝒂 for each direction
Sometimes easier to tilt the coordinate system!
Ex 1: A picture (m = 2.0 kg) hangs on the wall from a string, forming a 90° angle over the nail. What is the tension force in the string?
Ex 2: A block (m = 2.0 kg) accelerates down a slope (𝞱=32°, a = 0.35 m s-2).
What is the force of friction?
What is the coefficient of friction?