Ch. 4

1
Ch. 4

• Resistance

2
Ch. 4.1

• Resistance in Mechanical Systems

3
Resistance

• Mechanical resistance opposes motion between
  solid surfaces. This resistance is called
  friction.
• Fluid resistance opposes the flow of fluids. The
  resistance is caused by friction between
  molecules in the fluid. When a solid moves
  through a fluid, the resistance is called drag.

4
Resistance - 1

• Electrical resistance is a conductors resistance
  to the flow of electrons. This resistance is
  caused by electrons colliding with atoms in the
  conductor.
• Thermal resistance opposes the flow of heat.

5
Newtons Second Law

• According to Newtons Second Law, the net force
  acting on an object causes the object to
  accelerate in the direction of the net force.
  The acceleration of the object is determined by
  the size of the net force and the mass of the
  object according to the equation
• acceleration (a) net force (F)/mass(m)
• a F/m

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Newtons Second Law - 1

• We can rearrange the formula for Newtons 2nd law
  to solve for force. The formula to solve for
  force is
• Force(F) mass(m) x acceleration(a)

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Force Calculations

• When calculating acceleration due to force, the
  unit for mass is kilograms(kg) and the unit for
  acceleration is meters/second/second (m/s2).
• This means that when you calculate force, you are
  multiplying kg x m/s2.
• The unit kg x m/s2 is called a newton, therefore,
  the newton (N) is the SI unit of force.

8
Force Calculations (Sample)

• You are pushing a friend on a sled. You push
  with a force of 40 N. Your friend and the sled
  have a combined mass of 80 kg. What is the
  resulting acceleration of the sled?

a F/m
F 40 N
40 N/80 kg
m 80 kg
0.5 m/s2
a ?
9
Force Calculations (Sample)

• Nick throws a football, causing it to accelerate
  at a rate of 10 m/s2. If the football has a mass
  of 2 kg, how much force does Nick apply?
• F ma
• F ?
• m 2 kg 2 kg x 10 m/s2
• a 10 m/s2 20 N

10
Force Calculations (Sample)

• Ashley hits a tennis ball with her racket. If the
  ball has a is hit with a force of 35 N and
  accelerates at a rate of 25 m/s2, what is the
  mass of the ball?
• m F/a
• F 35 N
• m ? 35 N/ 25 m/s2
• a 25 m/s2 1.4 kg

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Friction

• Friction is the force that opposes motion between
  two objects in contact with each other.
• Friction is caused by microwelds which are formed
  where the hills and valleys on the surface of the
  objects stick together.
• The greater the force between the objects the
  more the microwelds stick together, causing an
  increase in friction. This force is called the
  normal force.

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Types of Friction

• Static Friction is the friction between two
  surfaces that are not moving past each other. In
  this case, the applied force is not enough to
  break the microwelds, so the object does not
  move.
• Sliding Friction is the force that opposes motion
  between objects sliding past each other. Sliding
  friction is caused by the microwelds breaking and
  forming again as the surfaces slide past each
  other.

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Types of Friction - 1

• Rolling Friction is the friction between a
  rolling object and the surface it rolls on.
  Rolling friction is due partly to the microwelds
  between a wheel and the surface it rolls over.
  The microwelds break and then reform as the wheel
  rolls over the surface.