Forces Concepts Reading

Newton’s Laws and Forces

1. Inertia – Law #1

• Objects have a tendency to retain their motion (velocity).

• More massive objects resist changes in motion more than less massive ones.

• Inertia is often mistaken for a force; however, a force is needed to overcome it and change an object's motion.

2. Acceleration – Law #2

• Expressed mathematically as F = ma (or ΣF = ma for net force).

• F represents the net force acting on an object, m is its mass, and a is its acceleration.

• Greater force results in greater acceleration; higher mass with the same force yields less acceleration.

3. Interaction – Law #3

• Forces exist in action-reaction pairs; they are equal in magnitude and opposite in direction.

• Interaction occurs when two objects exert forces on each other.

Summary of Laws

• F = ma correlates with three variables in Newton’s three laws.

Types of Forces

1. Weight

• Weight (F_w) is the force due to gravity: F_w = mg (mass times the acceleration of gravity).

• Always acts downward.

2. Normal Force

• Acts perpendicular to surfaces in contact; it corresponds to the weight.

• Cannot be quantified with a static equation since it is a reactionary force.

3. Tension

• Found in ropes, strings, and chains; it pulls towards the center along the length of the object.

• Again, there is no formula for tension as it is reactive.

4. Friction

• Two types of friction: Static Friction and Kinetic Friction.

• Static Friction: Occurs when surfaces are stationary relative to one another. The force is variable up to a limit and opposes the applied force.

• Kinetic Friction: Acts when surfaces slide across one another; it is a constant value regardless of speed.

Friction Formulas

• Static Friction: Fs ≤ µS FN (where µ is the coefficient of static friction)

• Kinetic Friction: FK = µK FN (where µ is the coefficient of kinetic friction)

5. Spring Force

• Described by F = -kx (where k is the spring constant).

• More force results in more stretching or bending of the spring.

Free Body Diagrams (FBDs)

• Visual representation of forces acting on an object, indicating direction and relative size.

• Example scenarios:

  • A book on a table: Weight downward and normal force upward; forces are equal.

  • A block pulled by a rope: Tension force, weight force, and kinetic friction illustrated.

Applying the Second Law

• ΣF = ma allows for the calculation of acceleration through force summation.

• Example problems show how to determine net forces and resulting accelerations.

Forces on Inclined Planes

• Weight acts downward but can be resolved into components: one along and one perpendicular to the incline.

• Steeper inclines increase the component of weight parallel to the incline, impacting friction and movement.

Vertical Acceleration (Elevators)

• Acceleration observed when riding in an elevator affects perceived weight.

• An increase in normal force results in a feeling of ‘heaviness.’

• Upon reaching constant speed, weight and normal forces equalize, resulting in no further acceleration.

• Slowing down as the elevator reaches a floor produces downward acceleration.

Check Question

• If moving down and accelerating up in an elevator, the direction of motion indicates whether you’re leaving or arriving at a floor.