Transcript AP Review Cont

AP Review Cont.
Forces
• If there is a force at an angle, make the
free body diagram appear level.
– Tilt the FBD so that the normal force is
pointing straight up.
– Then separate out the weight (which is now at
an angle) into the x and y components
– Normal force is the y component of the weight
– The x component is the x component of
gravity (why a car slides down hill even if
there is no other force being exerted on it)
Work and Power
• Work is measured in Joules (J)
– J = N *m
• W = FcosΘs
– F is ΣF unless otherwise specified
– F and s MUST be parallel to each other if no angle is
mentioned. If there is an angle, it MUST be the angle
that will make the force and displacement parallel to
each other.
– If the force and displacement are perpendicular, there
is no work being done on the object.
• Power is measured in Watts (W)
– W = J/s or N*v
• P = W/t or P = Fv
Energy
• Mechanical energy is the TOTAL energy of
a system (unless your are considering
heat too)
– ME = KE + PE
• Conservation of energy: the total energy
before equals the total energy after
– MEi = MEf
– Kei + PEi = Kef + PEf
• If you see mass, height, and velocity in the
same problem, it is probably a
conservation of energy problem
Energy
• Energy is measured in Joules (J)
• Since both work and energy are measure in J,
energy is the amount of work done on an object.
– W = ΔKE (ΔK on your salmon sheet)
– W = - ΔPE (ΔU on your salmon sheet)
• The initial and final speeds for KE are the actual
speeds or overall speed, NOT the speed in the x
or y direction.
• The height for PE is the height from the “zero
point,” wherever the problem states the
measurement of the height starts (usually the
ground).
Momentum
• Momentum is like inertia, but in motion
p = mv
• The SI unit for momentum is kg*m/s
• Impulse is the same as change in
momentum and is measured in the same
units… J = Δp
and since J = FΔt
Thru substitution, FΔt = mΔv
Momentum
• Conservation of momentum: momentum
before a collision equals the momentum
after the collision
pbefore = pafter
– Two types of collisions: elastic (bounce off
each other with no warping) and inelastic
(usually stick together…“perfectly
inelastic”…but they all deform in some way
– Collisions in physics are when two or more
objects come in contact with each other
(coming together or coming apart)
Differentiating types of problems
1. As soon as you see the word “force,”
draw a FBD
2. If you see m, v, and h, it will more than
likely be a conservation of energy
problem
3. If you see just m and v and there is more
than one object involved, it will probably
be a conservation of momentum
problem.
Differentiating types of problems
4.