Work, Energy, and Momentum Flashcards
Work
When a constant force acts on a body to move it a certain distance: W = F d cosθ, where θ is the angle between F and d. Note, when the force and displacement are perpendicular, the work done is zero, so centripetal force does no work. Units: Joule = N m = kg m^2/ s^2
Power
Power is the rate of work or rate of change of energy, where P = W/ Δt with units Watt = J/ s = N m/ s = kg m^2/ s^3
Kinetic Energy
Energy of an object in motion, where KE = 1/2 mv^2, with units of Joule = N m = kg m^2/ s^2
Potential Energy
The energy of an object dependent upon its position, where U = mgh and U = 1/2 kx^2 (springs), with units of Joule = N m = kg m^2/ s^2
Total Mechanical Energy
E = KE + U, mechanical energy is not conserved when friction is present
Nonconservative Force
A force whose work depends on the path taken, such as friction
Conservative Force
A force whose work is independent of the path taken, such as gravity, spring forces, and electrostatic forces
Work-Energy Theorem
W = ΔKE = KE(final) - KE(initial)
Conservation of Energy
W(done by nonconservative forces) = ΔE = ΔKE + ΔU
In the presence of a nonconservative force, E(initial) > E(final)
What is the significance of and which equations would you use in a pulley system?
Pulley systems allow for the same amount of work to be done by exerting a smaller force over a greater distance.
Efficiency = W(out)/ W(in) = (Load X Load Distance)/ (Effort X Effort Distance)
The distance through which the effort must move is equal to how much the supporting ropes must shorten.
Momentum
A vector quantity: p = mv, with units kg m/ s = N s
Impulse
J = F t = mv(final) - mv(initial) = Δp, with units kg m/ s = N s
Conservation of Momentum
When no net external forces on a system, then p(initial) = p(final), therefore Δp = 0
Completely Inelastic Collision
The bodies stick together after colliding, where p conserved, but KE(initial) > KE(final)
Completely Elastic Collision
When p and KE (KE(initial) = KE(final)) conserved
Center of Mass
X = (m1x1 + m2x2)/ (m1 + m2) in all coordinates
Center of Gravity
The point on some object where the entire force of gravity acts.
