12.1: Rigid Body Translation

Last updated
Save as PDF

Page ID: 54789

\( \newcommand{\vecs}[1]{\overset { \scriptstyle \rightharpoonup} {\mathbf{#1}} } \) \( \newcommand{\vecd}[1]{\overset{-\!-\!\rightharpoonup}{\vphantom{a}\smash {#1}}} \)\(\newcommand{\id}{\mathrm{id}}\) \( \newcommand{\Span}{\mathrm{span}}\) \( \newcommand{\kernel}{\mathrm{null}\,}\) \( \newcommand{\range}{\mathrm{range}\,}\) \( \newcommand{\RealPart}{\mathrm{Re}}\) \( \newcommand{\ImaginaryPart}{\mathrm{Im}}\) \( \newcommand{\Argument}{\mathrm{Arg}}\) \( \newcommand{\norm}[1]{\| #1 \|}\) \( \newcommand{\inner}[2]{\langle #1, #2 \rangle}\) \( \newcommand{\Span}{\mathrm{span}}\) \(\newcommand{\id}{\mathrm{id}}\) \( \newcommand{\Span}{\mathrm{span}}\) \( \newcommand{\kernel}{\mathrm{null}\,}\) \( \newcommand{\range}{\mathrm{range}\,}\) \( \newcommand{\RealPart}{\mathrm{Re}}\) \( \newcommand{\ImaginaryPart}{\mathrm{Im}}\) \( \newcommand{\Argument}{\mathrm{Arg}}\) \( \newcommand{\norm}[1]{\| #1 \|}\) \( \newcommand{\inner}[2]{\langle #1, #2 \rangle}\) \( \newcommand{\Span}{\mathrm{span}}\)\(\newcommand{\AA}{\unicode[.8,0]{x212B}}\)

With rigid bodies, we have to examine moments and at least the possibility of rotation, along with the forces and accelerations we examined with particles. Some rigid bodies will translate but not rotate (translational systems), some will rotate but not translate (fixed axis rotation), and some will rotate and translate (general planar motion). Overall, we will start with an examination of translational systems, then examine fixed axis rotation, then pull everything together for general planar motion.

A white car braking heavily on a country road, with smoke coming from the tires. — Figure \(\PageIndex{1}\): The braking car in this picture is an example of a translational system. Though the car experiences a significant deceleration, it does not experience any significant rotation while it slows down. Public domain image by Sgt. Amber Blanchard.

As the start of our analysis, we will go back to Newton's Second Law. Since this is a rigid body system, we include both the translational and rotational versions.

\[ \sum \vec{F} = m * \vec{a} \]

\[ \sum \vec{M} = I * \vec{\alpha} \]

As we did with particles, we can break the vector force equation into components, turning the one vector equation into two scalar equations (in the \(x\) and \(y\) directions respectively). As for the moment equation, a translational system by definition will have zero angular acceleration. With the angular acceleration being zero, the sum of the moments must all be equal to zero. This is similar to statics problems; however, there is one big difference we must take into account. The moments must be taken about the center of mass of the body. Setting the moments to zero about other points will lead to invalid solutions for any body experiencing an acceleration. Putting these specifics into action, we wind up with the three base equations of motion below. To solve for unknown forces or accelerations, we simply draw a free body diagram, put the knowns and unknowns into these equations, and solve for the unknowns.

\[ \sum F_x = m * a_x \]

\[ \sum F_y = m* a_y \]

\[ \sum M_G = 0 \]

Video lecture covering this section, delivered by Dr. Jacob Moore. YouTube source: https://youtu.be/V4Gy-tyHupE.

Example \(\PageIndex{1}\)

A refrigerator is 2.5 feet wide and 6 feet tall, and weighs 80 lbs. The center of mass is 1.25 feet from either side and 2 feet up from the base. If the refrigerator is on a conveyor belt that is accelerating the fridge at a rate of 1 ft/s², what are the normal forces at each of the feet?

Front view of a refrigerator with 4 feet, on a conveyor belt moving it towards the right. Due to the belt's motion, the fridge is accelerating to the right. — Figure \(\PageIndex{2}\): problem diagram for Example \(\PageIndex{1}\). A refridgerator of the dimensions described above is on a conveyor belt that accelerates the fridge to the right at a rate of 1 ft/s².

Solution: Video \(\PageIndex{2}\): Worked solution to example problem \(\PageIndex{1}\), provided by Dr. Majid Chatsaz. YouTube source: https://youtu.be/59GRMn_ZvJk.