Lesson 8

• Physics
• Middelbare school
• vwo
• Leerjaar 3

Chapter 2 Forces

Section 3 Driving forces and resisting forces

Chapter 2 Forces

Section 3 Driving forces and resisting forces

This Lesson

• Double helical springs
• Newtons first law
• Air resistance
• Practice exercises

Lesson 8

Force and extension

C=​x​​F​​

C: Spring constant (in N/m)

F: Force (in N)

x: Extension (in m)

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

Resisting forces

Friction

Air resistance

Rolling resistance

Air resistance

F​r,a​​=​2​​1​​C​d​​⋅ρ⋅A⋅v​2​​

Air resistance

F​r,a​​=​2​​1​​⋅C​d​​⋅ρ⋅A⋅v​2​​

• F_r,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)

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 (C_d). 

F​r,a​​=​2​​1​​C​d​​⋅ρ⋅A⋅v​2​​