Newton

1
Newtons 2nd Law

• Chapter 5

2
Acceleration and velocity

• Remember v d/t and a v/t
• An object traveling at constant velocity has zero
  acceleration
• An object at rest has zero acceleration
• In terms of force, the forces acting on these
  objects are balanced

3
Acceleration and velocity

• Balanced forces means that the net force zero.
• Zero net force zero acceleration
• Net force is directly proportional to
  acceleration
• F a

4
Acceleration and velocity

• Mass resists acceleration
• Acceleration is inversely proportional to mass.
• This has to do with the inertia of the object.
• Remember, inertia is the objects ability to
  resist change (Newton 1).
• a 1/m

5
Newtons 2nd Law

• Newtons 2nd Law states The acceleration
  produced by a net force on an object is directly
  proportional to the magnitude of the net force,
  is in the same direction as the net force, and is
  inversely proportional to the mass of the object.

6
Newtons 2nd Law

• Restated, the law says F ma
• The units for force
• F 1N 1kgm/s²

7
Forces

• Forces occur in pairs.
• Gravity is paired with normal force.
• On an airplane, there are four forces involved.

8
Forces - Plane

• Thrust moves the plane forward
• Drag pulls it backward
• Lift moves it up
• Weight holds it down

9
Example

• How much thrust must a 30,000 kg plane develop to
  achieve an acceleration of 1.5 m/s² ?
• F ma
• F (30000kg)(1.5 m/s²)
• F 45,000 N

10
More forces

• Friction is a force that affects motion.
• It acts on materials that are in contact with
  each other.
• It is dependent on surface irregularities.
• Friction always acts in a direction to oppose
  motion.

11
Friction

• Friction occurs in solids, liquids, and gases.
• In air, it is called air resistance.
• In gas, it is called fluid friction.
• Fluids include both gas and liquid. They both
  flow.

12
Free-body diagrams

• Free body diagrams show only the forces.
• They are merely vector diagrams of all the forces
  acting on an object.

13
Free body diagram
14
Pressure

• Pressure is the amount of force applied over a
  unit of area.
• P F/A
• The unit is the Pascal
• 1 Pa 1N/m²

15
Terminal velocity

• When air resistance weight, then net force
  zero.
• When net force zero, acceleration zero.
• This is called terminal speed or velocity.

16
Terminal velocity

• For humans skydiving, terminal velocity is
  roughly 150 to 200 km/hr.
• For a penny, terminal velocity is approximately
  60 km/hr.

17
Example

• What is the minimal force required to lift a 5kg
  baby out of a crib?
• F ma
• F (5kg)(10 m/s²)
• F 50 N