What is a Force?

Force

• Push or pull

• Typical Units

  • Newtons ( N = kg m/s² )

  • Pounds ( lbs. = slug ft/s² )

  • Dynes ( Dyne = g cm/s² )

• is a vector

  • #, unit, and direction

  • Recall: that we can represent the force vector as an arrow tipped line segment

Free-body Diagrams

• A diagram that shows all the forces that are acting on that object through the use of arrow tipped line segments

  • Concurrent forces

Four Fundamental Forces

• Naturally occurring

• Gravitational

  • Gravity

  • Attraction between masses

  • 4th strongest

• Electromagnetic

  • Gives materials strength

  • Results in compound bonding (atoms)

  • 2nd strongest

• Strong Nuclear

  • Holds nucleus of atom together

  • 1st strongest

• Weak Nuclear

  • Radioactive decay

  • 3rd strongest

Contact Forces

• forces in which an object provides a force to another object only when in “contact” with it

• Applied

  • Force that is applied to an object by a person or another object

  • F_a

• Tension

  • Force exerted by ropes, strings, cables, and chains

    • Only a pulling force; Never pushes

  • F_t

• Spring

  • Force exerted by a compressed or stretched spring upon any object that is attached to it

  • F_s

• Normal

  • Force that pushes the surfaces together

    • Always perpendicular to the surfaces in contact

  • F_n

• Friction

  • Force that opposes motion between two objects that are in contact with each other

    • Can sometimes be in the direction of the movement

  • F_f || f

• Drag

  • Air Resistance

  • Force that a fluid (gas or liquid) exerts on a moving object

  • Depends on:

    • Shape (C)

    • Air density (p)

    • Cross sectional area (A)

    • Speed (v)

  • F_D = ½ C p A v²

Field Forces

• Forces in which objects don’t touch. Force felt over a distance

• Long range forces

• Gravitational

  • Weight

  • Is the pull of gravity on a mass

    • Mass and weight are not the same thing. Mass is how much matter is in an object and weight is the pull of gravity on that mass

      • Varies with location

      • Depends on g ( g = 9.8m/s² on Earth )

      • F_g = w = mg

  • F_g || w

• Electrostatic

  • The attractive or repulsive interaction between any two charged objects

  • F_E

• Magnetic

  • The attractive or repulsive interaction between magnets or moving charge

  • F_B

History of Force

Aristotle

• Natural motion is to return to rest without force

• Violent Motion is when an object moves away from its natural state because of a force

Galileo

• Objects will continue to move if all forces are removed

• Inertia

  • The tendency of an object to resist a change in its motion

  • Property of matter

  • Inertia = mass

Sir Isaac newton

• In 1665, started work on his three laws of motion

• Studied relationships between forces and their resultant motion

Newton’s 1st Law

• An object at rest will remain at rest, and an object in motion will remain at constant velocity (same speed and direction) motion until acted upon by a net force

  • Newton’s 1st law = rest or constant velocity

• The Inertial Law

  • Mass is the quantitative measure of inertia

  • ΣF = 0

    • Net Force = 0

Newton’s 2nd Law

• The acceleration of an object is always directly proportional to the net force acting on it and inversely proportional to the mass of the object

  • a ∝ ΣF

    • More force = More acceleration

    • Direction of force = Direction of acceleration

  • a ∝ 1/m

    • More mass = Less acceleration

  • m ∝ ΣF

    • More mass = More force needed

• 2nd Law Equation

  • a = ΣF / m

  • ΣF = ma

Newton’s 3rd Law

• If object A exerts a force on object B, then object B exerts an equal force on object A but in the opposite direction

  • For every action force there is an equal and opposite reaction force

  • F_A = -F_B→A

Friction

• F_f or f

• The force that opposes motion between two objects that are in contact with each other

• F_f = μ F_n

Types of Friction

• Static

  • Before the object moves

• Kinetic

  • While the object moves

  • Sliding Friction

    • Rolling

  • Static friction is always greater than kinetic friction

• Depends on:

  • surface texture

    • μ

  • the force pushing the surfaces together

    • Normal Force

    • F_n