Objectives:
  • Define force
  • Classify forces
  • Define friction force
  • Define weight
  • Determine the resultant of two or more forces
  • Resolve a force into component forces acting at right angles to each other
  • Determine whether an object is in static equilibrium, if the forces acting on the object are known
  • Determine an unknown force acting on an object, if all the other forces acting on the object are known and the object is in static equilibrium

A force is a push or a pull.
A force is something that has the ability to accelerate an object.

Forces are described in units of Newtons or pounds
1N = 0.225lb
1lb = 4.45N

How much do you weigh?

Internal forces-Forces that act within the object or system whose motion is being investigated (Evan Jager).

External forces-Forces that act on an object as a result of its interaction with the environment surrounding it (Shalaya Kipp).

Types of Friction

Static friction: The force occurring between two dry surfaces that are not moving relative to each other.

Dynamic friction: The force occurring between two dry surfaces that are moving relative to each other.
Rolling friction: The force acting between a surface and a rolling object due to the deformation of one or both objects.

 

Static versus dynamic friction
Shoe friction in running


Normal force and surface area (book examples)

Running forces @ 4:00 min/mi

If a runner exerts a vertical force of 2000 N, and the coefficient of static friction between the shoe and the ground is 0.50, what is the maximum horizontal force he can generate under his shoe?

W=mg

Addition of Forces

What happens when there is more than one force?

Vectors involve magnitude and direction

Combining Vectors: Graphical and Additive Methods

A shot put is released with a velocity of 14 m/s at an angle of 42 degrees. What are the vertical and horizontal components of velocity? (Article example)

A baseball is thrown with a horizontal velocity of 47.0 m/s and a vertical velocity of 4.1 m/s. What is the resultant velocity (magnitude and direction)?

Another resultant force in running

A 70 kg runner experiences air resistance of 15 N. During the heelstrike, he exerts an A/P force of 100 N. The vertical ground reaction force at this time was 1300 N.

Draw a diagram of this situation and determine the resultant force (magnitude and direction relative to horizontal).

Static Equilibrium

When an object is at rest and the forces acting on the object are in equilibrium, they result in a net force of zero.

Weightlifter example: Person's mass=80kg, barbell=70kg, ground reaction force=1470N.
Is this system in static equilibrium?

Twister!

Gymnast

Kid's Running