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Basic Mechanics: Forces and Moments
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Newton's First Law
An object at rest stays at rest and an object in motion stays in motion with the same speed and in the same direction unless acted upon by an unbalanced force.
Newton's Second Law
, where is force, is mass, and is acceleration.
Newton's Third Law
For every action, there is an equal and opposite reaction.
Force
A vector quantity that tends to accelerate an object; measured in Newtons ().
Mass
A scalar quantity representing the amount of matter in an object; measured in kilograms ().
Acceleration
The rate of change of velocity of an object; measured in meters per second squared ().
Weight
, where is weight, is mass, and is acceleration due to gravity.
Moment (Torque)
, where is the torque, is the position vector (distance from the pivot), is the force, and is the angle between and .
Hooke's Law
, where is force, is the spring constant, and is the displacement from equilibrium.
Friction
The resistance to motion of one object moving relative to another.
Kinetic Friction
, where is the kinetic friction force, is the coefficient of kinetic friction, and is the normal force.
Static Friction
, where is the static friction force, is the coefficient of static friction, and is the normal force.
Normal Force
The component of contact force perpendicular to the surface; it prevents objects from passing through each other.
Tension
The force transmitted through a string, rope, cable or wire when it is pulled tight by forces acting from opposite ends.
Pressure
, where is pressure, is the normal force, and is the area over which the force is distributed.
Work
, where is work done, is force, is displacement, and is the angle between the force and displacement vectors.
Power
, where is power, is work done, and is the time taken.
Potential Energy
, where is potential energy, is mass, is acceleration due to gravity, and is height above a reference point.
Kinetic Energy
, where is kinetic energy, is mass, and is velocity.
Mechanical Advantage
, where is mechanical advantage, is the output force, and is the input force.
Pulley System
A simple machine used to change the direction of a force and potentially multiply its magnitude.
Centripetal Force
, where is centripetal force, is mass, is velocity, and is the radius of the circular path.
Centripetal Acceleration
, where is centripetal acceleration, is velocity, and is the radius of the circular path.
Mechanical Equilibrium
A state where the sum of forces and the sum of moments (torques) on a body are zero.
Impulse
, where is impulse, is the average force applied, and is the time interval over which the force is applied.
Momentum
, where is momentum, is mass, and is velocity.
Angular Momentum
, where is angular momentum, is moment of inertia, and is angular velocity.
Moment of Inertia
A scalar measure of an object's resistance to rotational acceleration, dependent on the mass distribution with respect to the axis of rotation.
Shear Stress
, where is shear stress, is the force applied parallel to the surface, and is the area over which the force is applied.
Young's Modulus
, where is Young's Modulus, is stress, and is strain.
Bending Moment
The moment that induces bending in an object along an axis; occurs due to external forces or moments.
Torsion
The twisting of an object due to an applied torque that tends to produce rotational deformation.
Equilibrium of Forces
, indicating that the sum of all forces on a body is zero, hence the body is at rest or moving with constant velocity.
Equilibrium of Moments
, indicating that the sum of all moments (torques) about a pivot point is zero, and the object is in rotational equilibrium.
Density
, where is density, is mass, and is volume.
Buoyancy
The upward force exerted by a fluid that opposes the weight of an immersed object.
Thermal Expansion
The change in length, area, or volume of a material due to change in temperature.
Stress
, where is stress, is the force acting on an area, and is the area.
Strain
, where is strain, is the change in length, and is the original length.
Conservation of Energy
In a closed system, the total energy remains constant over time; energy can neither be created nor destroyed but can only change forms.
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