# Lesson 8

Chapter 2 Forces
Section 3 Driving forces and resisting forces
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Slide 1: Slide
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This lesson contains 13 slides, with text slides and 1 video.

Lesson duration is: 45 min

## Items in this lesson

Chapter 2 Forces
Section 3 Driving forces and resisting forces

#### Slide 1 -Slide

Chapter 2 Forces
Section 3 Driving forces and resisting forces
This Lesson
• Double helical springs
• Newtons first law
• Air resistance
• Practice exercises

Lesson 8

#### Slide 3 -Slide

Force and extension
C=xF
C: Spring constant (in N/m)
F: Force (in N)
x: Extension (in m)

#### Slide 5 -Video

Newtons first law
If the resultant force of all the forces is zero (0N), the object is either stationary or it moves along a straight line at a constant speed

Driving forces

#### Slide 7 -Slide

Resisting forces
Friction
Air resistance
Rolling resistance

Air resistance
Fr,a=21CdρAv2

#### Slide 9 -Slide

Air resistance
Fr,a=21CdρAv2
• Fr,a: Force, resistance, air resistance (n)
• C: resistance coefficient (no unit)
• ρ: The density of air (or whatever medium the object is traveling through) (kg/m3)
• A: area of the object (m2)
• v: the speed of object (m/s)

#### Slide 10 -Slide

Practice exercise
A skydiver is falling from the sky at terminal velocity of 60 m/s. This mean the forces acting on him are balanced. His weight is 735 N (downwards force). This force is balanced by the upwards force of air resistance. He has a frontal area of 0.33 m2. The density of air is 1.207 kg/m3. Calculate his drag coeficient (Cd).

Fr,a=21CdρAv2