Science—Physics—Newton's Laws

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Important Rules:

1. If an object is in "equilibrium" - it does not move at all, or the object has a constant velocity - acceleration is zero.
2. The standard unit for force is Newton [N]. $N=\frac{kg\cdot m}{s^2}$

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Common Forces Encountered in Newton's Law Problems

1. Normal Force
   • Always perpendicular to the surface of an object is resting on
   • If an object is suspended in the air by a cord/string/rope, there is NO normal force.
2. Tension
   • The tension originates at the point of contact and comes out of the object, pointing along the string.
   • When the mass of the string is ignored, the tension is the same on either end of the string.
3. Gravitational Force
   • Always points down to the ground.
4. Friction Force
   • Opposes or impedes motion.
   • If an object is static (not moving), use the static friction formula (f_s)
   • If an object is kinetic (moving), use the kinetic friction formula (f_k)

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Equations

${\boxed{Newton's\ Law:\ Static\ or\ Constant\ Velocity}}$

$\sum\overset{\rightharpoonup}{F}=m\overset{\rightharpoonup}{a}\\ \sum{F_x} = 0 \\ \sum {F_y} = 0 \\$

${\boxed{Newton's\ Law:\ Accelerating\ Object}}$

$\sum\overset{\rightharpoonup}{F}=m\overset{\rightharpoonup}{a}\\ \sum{F_x} = ma_x \\ \sum {F_y} = ma_y \\$

${\boxed{Friction\ Equations}}$

$f_s\le \mu _sn\\ f_k=\mu _kn$