To solve the problem using principles of work and energy, we consider the system comprising the two masses. We know that one mass will move up while the other moves down, and we’re given the displacement d = 0.8 meters for each mass.
Assuming the 12 kg mass moves downward and the 9 kg mass moves upward, we need to find the common velocity v after they have moved 0.8 meters, taking into account that energy is conserved in this isolated system (ignoring air resistance and friction). Here’s the step-by-step analysis:
| Step | Derivation/Formula | Reasoning |
|---|---|---|
| 1 | m_1 = 12 \, \text{kg} m_2 = 9 \, \text{kg} |
Define the masses where m_1 is the 12 kg mass and m_2 is the 9 kg mass. |
| 2 | g = 9.81 \, \text{m/s}^2 | Acceleration due to gravity is g . |
| 3 | \Delta h = 0.8 \, \text{m} | Both masses displace by 0.8 m, one moving up and the other down. |
| 4 | U_{\text{initial}} = 0 \, \text{J} | Initial potential energy is set to zero for simplicity as we’re only interested in changes. |
| 5 | U_{\text{final}} = -m_1 g \Delta h + m_2 g \Delta h | Final potential energy obtained by considering the gain in height by m_2 and loss in height by m_1 . |
| 6 | U_{\text{final}} = -12 \times 9.81 \times 0.8 + 9 \times 9.81 \times 0.8 | Substitute the known values to find U_{\text{final}} . |
| 7 | U_{\text{final}} = -23.544 \, \text{J} | Calculate the final potential energy. |
| 8 | KE_{\text{initial}} = 0 \, \text{J} | Initial kinetic energy, assuming they start from rest. |
| 9 | KE_{\text{final}} = \frac{1}{2} (m_1 + m_2) v^2 | Kinetic energy for the system of the two moving masses at the final state. |
| 10 | KE_{\text{final}} = \frac{1}{2} (12+9) v^2 | Expression for the final kinetic energy of both masses. |
| 11 | KE_{\text{final}} = 10.5 v^2 | Simplifying the expression for final kinetic energy. |
| 12 | KE_{\text{final}} = -U_{\text{final}} | By conservation of energy, change in potential energy equals change in kinetic energy. |
| 13 | 10.5 v^2 = 23.544 | Substitute computed value of U_{\text{final}} . |
| 14 | v^2 = \frac{23.544}{10.5} | Solve for v^2 . |
| 15 | v = \sqrt{\frac{23.544}{10.5}} | Calculate v . |
| 16 | v \approx 1.49 \, \text{m/s} | Final velocity of the masses after traveling 0.8 m. |
This result indicates the common speed of both masses after they have moved 0.8 meters, based on the conservation of mechanical energy.
Phy can also check your working. Just snap a picture!
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A horizontal force of 110 N is applied to a 12 kg object, moving it 6 m on a horizontal surface where the kinetic friction coefficient is 0.25. The object then slides up a 17° inclined plane. Assuming the 110 N force is no longer acting on the incline, and the coefficient of kinetic friction there is 0.45, calculate the distance the object will slide on the incline.
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1.49 m/s
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| Kinematics | Forces |
|---|---|
| \Delta x = v_i t + \frac{1}{2} at^2 | F = ma |
| v = v_i + at | F_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 Motion | Energy |
|---|---|
| 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 |
| Momentum | Torque and Rotations |
|---|---|
| p = m v | \tau = r \cdot F \cdot \sin(\theta) |
| J = \Delta p | I = \sum mr^2 |
| p_i = p_f | L = I \cdot \omega |
| Simple Harmonic Motion |
|---|
| F = -k x |
| T = 2\pi \sqrt{\frac{l}{g}} |
| T = 2\pi \sqrt{\frac{m}{k}} |
| Constant | Description |
|---|---|
| g | Acceleration due to gravity, typically 9.8 , \text{m/s}^2 on Earth’s surface |
| G | Universal Gravitational Constant, 6.674 \times 10^{-11} , \text{N} \cdot \text{m}^2/\text{kg}^2 |
| \mu_k and \mu_s | Coefficients 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. |
| k | Spring 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 |
| Variable | SI 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)} |
| Variable | Derived 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.
Start with the given measurement: \text{5 km}
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}}
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}
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} |
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