Glossary
Buoyant Force (Fb)
The upward force exerted by a fluid on an object, equal to the weight of the fluid displaced by the object. It is crucial for understanding floating and sinking.
Example:
A submarine uses its ballast tanks to adjust its density, allowing the Buoyant Force to either lift it towards the surface or let it sink deeper.
Equilibrium
A state where the net force acting on an object is zero. This means the object is either at rest or moving with a constant velocity.
Example:
An object floating at the surface of water is in Equilibrium because the upward buoyant force perfectly balances its downward weight.
Free-body diagrams (FBDs)
Essential tools for solving physics problems by visualizing all forces acting on an object. They simplify complex scenarios into a clear representation of forces.
Example:
When analyzing a boat floating on water, drawing a Free-body diagram helps you clearly see the buoyant force pushing up and the weight pulling down.
Net Force
The vector sum of all individual forces acting on an object. It determines the object's acceleration according to Newton's Second Law.
Example:
If the upward buoyant force and resistive force on a sinking object equal its downward weight, the Net Force is zero, and it moves at a constant velocity.
Newton's Laws (ΣF = ma)
Fundamental principles of classical mechanics that describe the relationship between an object's motion and the forces acting on it. ΣF = ma states that the net force on an object is equal to its mass times its acceleration.
Example:
To calculate the acceleration of a sinking object, you would apply Newton's Laws by setting the sum of forces (weight, buoyant, resistive) equal to its mass times acceleration.
Normal Force (N)
A contact force exerted by a surface on an object, acting perpendicular to the surface. It is only present when an object is resting on or pressed against a surface.
Example:
When a rock rests on the bottom of a pond, the pond's bed exerts an upward Normal Force on the rock, preventing it from sinking further.
Resistive Force (Fr)
A force that opposes motion through a fluid, often called drag force. Its magnitude typically increases with the object's velocity and the fluid's viscosity.
Example:
A skydiver experiences a significant Resistive Force from the air, which eventually balances their weight, leading to a constant terminal velocity.
Terminal Velocity
The constant velocity achieved by a falling object when the resistive force and buoyant force (if applicable) balance the force of gravity. At this point, the net force is zero.
Example:
A raindrop falling through the atmosphere eventually reaches its Terminal Velocity when the upward air resistance equals its downward weight, causing it to fall at a steady speed.
Weight (W or Fg)
The force of gravity acting on an object, always pointing downwards towards the center of the Earth. It is calculated as mass times the acceleration due to gravity (mg).
Example:
A diver experiences their Weight pulling them towards the Earth's center, even when submerged in water, which is a constant downward force.