AP Physics Unit

Unit 3 - Circular Motion

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Mathematical

GQ

Find the net gravitational force on a 2.0 kg sphere midway between a 4.0 kg sphere and a 7.0 kg sphere that are 1.2 m apart.

The net gravitational force on the 2.0 kg sphere will be the vector sum of the forces exerted by the 4.0 kg and 7.0 kg spheres. Since the 2.0 kg sphere is midway, each force will act along the line connecting the centers of the spheres. See the FBD below.

Step Formula Derivation Reasoning
1 r_{\text{half}} = \frac{1.2 , \text{m}}{2} = 0.6 , \text{m} Half the distance between the two spheres.
2 F_{\text{4 to 2}} = G \frac{4 \times 2}{0.6^2} Gravitational force between the 4.0 kg and 2.0 kg spheres.
3 F_{\text{7 to 2}} = G \frac{7 \times 2}{0.6^2} Gravitational force between the 7.0 kg and 2.0 kg spheres.
4 F_{\text{net}} = F_{\text{7 to 2}} – F_{\text{4 to 2}} Net force is the difference between the two forces, as they are in opposite directions.

Let’s calculate the net gravitational force.

Step Formula Derivation Reasoning
4 F_{\text{net}} \approx 1.11 \times 10^{-9} , \text{N} Calculated net gravitational force.

The net gravitational force on the 2.0 kg sphere, located midway between the 4.0 kg and 7.0 kg spheres (1.2 m apart), is approximately \boxed{1.11 \times 10^{-9} , \text{Newtons}} . This force is directed towards the 7.0 kg sphere due to its larger mass.

 


  • The blue arrow represents the gravitational force ( F_{4 \to 2} ) exerted by the 4.0 kg sphere.
  • The green arrow represents the gravitational force ( F_{7 \to 2} ) exerted by the 7.0 kg sphere.

These forces indicate the gravitational pull exerted on the 2.0 kg sphere from each of the other two spheres. In this scenario, the net force is the vector sum of these two forces, with the direction towards the 7.0 kg sphere due to its larger mass.

Fnet = 1.11 x 10-9

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Fnet = 1.11 x 10-9

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Nerd-Notes.com
KinematicsForces
\Delta x = v_i t + \frac{1}{2} at^2F = ma
v = v_i + atF_g = \frac{G m_1m_2}{r^2}
a = \frac{\Delta v}{\Delta t}f = \mu N
R = \frac{v_i^2 \sin(2\theta)}{g} 
Circular MotionEnergy
F_c = \frac{mv^2}{r}KE = \frac{1}{2} mv^2
a_c = \frac{v^2}{r}PE = mgh
 KE_i + PE_i = KE_f + PE_f
MomentumTorque and Rotations
p = m v\tau = r \cdot F \cdot \sin(\theta)
J = \Delta pI = \sum mr^2
p_i = p_fL = I \cdot \omega
Simple Harmonic Motion
F = -k x
T = 2\pi \sqrt{\frac{l}{g}}
T = 2\pi \sqrt{\frac{m}{k}}
ConstantDescription
gAcceleration due to gravity, typically 9.8 , \text{m/s}^2 on Earth’s surface
GUniversal Gravitational Constant, 6.674 \times 10^{-11} , \text{N} \cdot \text{m}^2/\text{kg}^2
\mu_k and \mu_sCoefficients of kinetic (\mu_k) and static (\mu_s) friction, dimensionless. Static friction (\mu_s) is usually greater than kinetic friction (\mu_k) as it resists the start of motion.
kSpring constant, in \text{N/m}
M_E = 5.972 \times 10^{24} , \text{kg} Mass of the Earth
M_M = 7.348 \times 10^{22} , \text{kg} Mass of the Moon
M_M = 1.989 \times 10^{30} , \text{kg} Mass of the Sun
VariableSI Unit
s (Displacement)\text{meters (m)}
v (Velocity)\text{meters per second (m/s)}
a (Acceleration)\text{meters per second squared (m/s}^2\text{)}
t (Time)\text{seconds (s)}
m (Mass)\text{kilograms (kg)}
VariableDerived SI Unit
F (Force)\text{newtons (N)}
E, PE, KE (Energy, Potential Energy, Kinetic Energy)\text{joules (J)}
P (Power)\text{watts (W)}
p (Momentum)\text{kilogram meters per second (kgm/s)}
\omega (Angular Velocity)\text{radians per second (rad/s)}
\tau (Torque)\text{newton meters (Nm)}
I (Moment of Inertia)\text{kilogram meter squared (kgm}^2\text{)}
f (Frequency)\text{hertz (Hz)}

General Metric Conversion Chart

Example of using unit analysis: Convert 5 kilometers to millimeters. 

  1. Start with the given measurement: \text{5 km}

  2. Use the conversion factors for kilometers to meters and meters to millimeters: \text{5 km} \times \frac{10^3 \, \text{m}}{1 \, \text{km}} \times \frac{10^3 \, \text{mm}}{1 \, \text{m}}

  3. Perform the multiplication: \text{5 km} \times \frac{10^3 \, \text{m}}{1 \, \text{km}} \times \frac{10^3 \, \text{mm}}{1 \, \text{m}} = 5 \times 10^3 \times 10^3 \, \text{mm}

  4. Simplify to get the final answer: \boxed{5 \times 10^6 \, \text{mm}}

Prefix

Symbol

Power of Ten

Equivalent

Pico-

p

10^{-12}

Nano-

n

10^{-9}

Micro-

µ

10^{-6}

Milli-

m

10^{-3}

Centi-

c

10^{-2}

Deci-

d

10^{-1}

(Base unit)

10^{0}

Deca- or Deka-

da

10^{1}

Hecto-

h

10^{2}

Kilo-

k

10^{3}

Mega-

M

10^{6}

Giga-

G

10^{9}

Tera-

T

10^{12}

  1. Some answers may be slightly off by 1% depending on rounding, etc.
  2. Answers will use different values of gravity. Some answers use 9.81 m/s2, and other 10 m/s2 for calculations.
  3. Variables are sometimes written differently from class to class. For example, sometime initial velocity v_i is written as u ; sometimes \Delta x is written as s .
  4. Bookmark questions that you can’t solve so you can come back to them later. 
  5. Always get help if you can’t figure out a problem. The sooner you can get it cleared up the better chances of you not getting it wrong on a test!

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