Physics Prelim 2 Important Concepts

Friction force

Parallel force component of an object at rest or sliding, always opposes the direction of an objects motion

Kinetic friction

Friction acting on an object in motion

How does the coefficient of friction relate to friction force?

Smaller coefficient of friction = less friction = more slippery surface

Static friction

Friction acting when an object isn’t moving, or when the friction force is greater than the applied force

Critical point

The point where applied force > max static friction

How do static and kinetic friction coefficients compare?

coefficient of static > coefficient of kinetic because it is harder to get an object moving than it is to keep an object moving

Fluid resistance

Force that a fluid (gas or liquid) exerts on an object moving through it

Direction of fluid resistance

Opposite direction of object’s velocity relative to fluid

What traits does drag depend on?

Shape and size of object (surface area) and air density

Direction of radial acceleration in circular motion

Points towards the center

Direction of net force in circular motion

Points towards the center

Direction of velocity in circular motion

Tangential to object’s path

What force balances an airplane’s weight?

Lift (L)

Why do planes take banked turns?

So that the lift force has a horizontal component

Work

Force applied to an object that undergoes a displacement (change in position)

What is the SI unit for work?

Joule (J)

Is work scalar or vector?

Scalar

When is work positive?

When the force is acting in the same direction as displacement

When is work negative?

When the force is acting in the opposite direction of displacement

When is there no work?

When force is perpendicular to displacement

Traits of kinetic energy

• Scalar

• Cannot be negative

• Only zero when object is at rest

Work-Kinetic Energy Theorem

Wtot = Delta(K) = change in kinetic energy

What is the SI unit for energy?

Joule (J)

Kinetic energy

The total work done to accelerate an object from rest to its present speed

Hooke’s Law

Fx = kx (k = spring constant, x = distance), the force required to stretch a spring

Potential energy

Energy associated with position

What is the total mechanical energy of a system composed of?

Kinetic Energy + Potential Energy

Elastic potential energy

Process of storing energy in a deformable object

Elastic object

Object that returns to its original shape and size after being deformed

Elastic potential energy

Uel = 1/2(k)(x²)

Conservative force

A force that offers two-way conversion between KE and PE, work is reversible

Properties of work done by conservative forces

• Can be expressed as change in potential energy

• Reversible

• Independent of path

• Work = zero when start point and end point are the same

When is total mechanical energy constant?

When only conservative forces do work

Internal energy

Energy associated with the change in state of materials of a system

Point of unstable equilibrium in PE graph

Local/global max - point when object has most PE

Point of neutral equilibrium in PE graph

Range of x where PE is constant

Point of stable equilibrium in PE graph

Local/global min where PE is lowest

Power

Time rate at which work is done

Is power scalar or vector?

Scalar

Average Power

Change in work / change in time

SI unit for power

watt (W)

Momentum

Time rate of change of the product of object’s mass and velocity

Is momentum scalar or vector?

Vector

SI unit for momentum

kg * m/s

Impulse

Product of net external force and time interval

SI unit for impulse

N*s

Impulse-momentum theorem

Impulse = change in momentum

Isolated system

System with no external forces

Collision

Strong interaction between objects that lasts a relatively short time, momentum is conserved

Elastic collision

Collision where no mechanical energy is gained or lost

Inelastic collision

Collision where the final kinetic energy differs from initial kinetic energy

Two types of inelastic collisions

• Collision where objects stick to each other

• Collision where objects experience deformation

Completely inelastic collision

Collision where objects stick to each other

Center of mass

Mass-weighted average position of particles

What propels rockets forward?

Rearward ejection of burned fuel

Can Newton’s second law apply to rockets?

No, because the mass of the rocket decreases as fuel is ejected

Linear vs Angular velocity

• If an object has linear velo, whole object is moving along an axis

• If an object has angular velo, object is rotating around an object

How does linear speed depend on radius?

Linear speed increases as you get farther away from axis

Circular Orbit

Closed orbit where only force acting on object is earth’s gravitational attraction

What happens as orbital radius increases?

• Total mech energy increases

• Potential energy increases

• Kinetic energy decreases