Study Guide for Test 2
Newton’s first law: An isolated object (with no outside influences acting on it) moves in a straight
line at constant speed.
Newton’s second law: The acceleration of an object is determined by the vector sum of all the
forces acting on it. More precisely,
Fx=maxFy=mayFz=maz
Newton’s third law: When two objects, Aand B, interact, the forces they exert on each other are
always equal in magnitude and opposite in direction. Written as an equation,
Fon Afrom B=−
Fon Bfrom A
Newton’s laws work only in an inertial reference frame, that is, when all measurements are made
with respect to a reference frame that is not itself accelerating.
Every force on an object arises from an interaction with some other nearby object (called the
“agent” of the force). If a force has no agent, it’s not a real force.
Types of forces:
•Contact forces
1. compression (including “normal”)
2. tension (same at each end of an ideal string)
(for a stretched or compressed spring, Fx=−ksx)
3. friction (gripping, |
Ff|≤µs|
FN|; slipping, |
Ff|=µk|
FN|; also air resistance, etc.)
•Long-range forces
1. gravity or “weight”, |
Fg|=mg, where g=9.8 N/kg near earth’s surface
2. electromagnetic
(Center of mass of a system) = rcm =m1r1+m2r2+···
m1+m2+··· (weighted average of the positions)
Motion of center of mass:
Fexternal =Macm
Momentum = p =mv
Momentum conservation: pf=pi(for any isolated system) Note: This is a vector equation,
so it must be broken into components in solving problems: pf,x =pi,x, and similarly for y,z.
You should be able to do the following:
•Draw qualitatively accurate force diagrams, identify the type and agent of each force, identify
third-law partners.
•Solve “constrained motion” problems involving straight-line or circular motion of a single
object or multiple interacting objects.
•Recognize non-inertial reference frames (accelerating cars, elevators, etc.) and instead analyze
problems from the viewpoint of an inertial reference frame.
•Solve momentum-conservation problems.
Although this test will cover mainly the ideas listed here (and covered on Problem Sets 4–6),
you are still responsible for material covered on the first test (Problem Sets 1–3). Note that the
equations for constant acceleration and for uniform circular motion tend to come up frequently in
constrained motion problems.
