Physics Term PaperAndrew Hensley11/28/18Dump Truck Physics Dump trucks are a common piece of construction equipment seen at virtually every major construction project for the removal of dirt and other debris. While the operation of a dump truck may appear no more complex than teaching a millennial to drive stick shift, the myriad physics concepts at play, specifically in the use of hydraulic lifts, tell a different story. Though understanding the hydraulics that power a dump truck’s massive bed are integral to its proper usage, a vehicle of this sort operates as a system comprised of many physics applications, such as angular momentum in steering and rotating the truck, pressure and equilibrium in the hydraulic lifts, and friction of the truck’s wheels against its terrain.
A dump truck’s locomotion is made possible through clever and effective design of its wheels, which have distinct tread patterns that allow for maximum area of contact on rough, irregular surfaces. In a scenario where a dump truck can move across a construction site unperturbed, the frictional force of the surface the truck is moving on, unlevel dirt for example, is not great enough to overcome the frictional force applied by the trucks hard, rubber tires.
When the truck moves, the wheels rotate, causing the grooves in the truck’s tires to contact the uneven surface of the construction site. When the coefficient of friction is high, the dump truck has significant contact with the ground surface, allowing for increased traction when torque is applied to the wheels. Deep tire treads are useful for dump trucks moving on especially uneven surfaces, such as freshly upheaved dirt, because the grooves in the tires give room for dirt and other softer material to pass into and through as the wheel rotates, again allowing for a maximum coefficient of friction between the dirt and the dump truck’s tires. A dump truck that can move through mud and dry, crumbled dirt is acted on by frictional forces. If the dump truck’s speedometer reads 50 mph but the vehicle is only moving at 25 mph, the truck is not being acted on equally by the surface it’s riding on. In this scenario, in fact, the truck’s tires are creating friction only 50% of the time, due likely to slipping of the wheels in uneven terrain. Newton’s third law, that in every interaction, there is a pair of forces acting on the two objects and that the size of the forces on the first object equals the size of the force on the second object (physics classroom) is evident in the way the mass of the dump truck pushes against the dirt of the construction site as well as the force the dirt exerts upward to resist the force of the dump truck. Once a dump truck has its footing on the ground, suppose it climbs a steep hill on a dig site, roughly 30є. The truck must overcome the force of gravity acting downward perpendicular to the hill as well as the weight of the vehicle acting down the slope in parallel with the hill. When the dump truck is put into gear and a torque is applied to the wheels, the wheels begin to rotate, changing from static friction to kinetic friction as the vehicle is in motion. Here, the wheels exert a force on the ground, and the ground exerts a force equal and opposite due to the friction that exists between the surface of the ground and the tread of the tires. So long as the truck does not slip, due to a low coefficient of friction, and the force acting on the wheels is great enough to overcome the force of gravity acting down the hill, the dump truck will accelerate up the hill. When the truck reaches the peak of the hill, the point on the construction site at which the dump truck has the most potential energy, the truck is at an appropriate position to eject whatever material is contained in its bed. It does so through the activation of its hydraulic cylinders, which move fluid (in this case oil) in order to transmit forces from location to location in the hydraulic system. The hydraulics in a dump truck’s bed function under Pascal’s principle, stating that pressure changes in a confined incompressible fluid are transmitted throughout the fluid, causing the same change in pressure to occur everywhere in the system. To activate the hydraulic cylinders, incompressible fluid is pushed into one end of the cylinder while fluid in the other end is simultaneously drawn out, increasing pressure in the cavity of the cylinder containing fluid, which forces the piston within the cylinder to move in the direction of the newly created low pressure end of the cylinder, resulting in the extension of the piston rod attached to the piston. The cap-end piston face of the cylinder (depicted below) has a greater surface area than the rod-end piston face, resulting in a greater extension force than retraction force. Since pressure is applied equally to all places of the piston head, a greater surface area will result in more force applied in that direction, because force is equal to pressure multiplied by the area of the surface. When the dump truck operator lifts the truck’s bed, they are causing fluid to flow into this cap-end port with enough force to overcome the force applied against the piston rod head by the material in the bed as well as the bed itself. Once the truck bed has reached an angle where the force of gravity is greater than the maximum force of static friction between the material in the truck bed and the metal surface of the truck bed, the material will slide out of the truck bed and fall into the designated collection area on the construction site. Through the application of physics concepts, relating specifically to fluid dynamics, friction, and torque, the deeper complexities of the operation of a dump truck in a construction site setting can be broken down and analyzed in order to better understand how to properly utilize the extensive capabilities of this seemingly simple machine. BibliographyA staff report | Jan 01. Engineering Essentials: Cylinders. Hydraulics & Pneumatics, Informa USA, 18 Jan. 2018, www.hydraulicspneumatics.com/200/TechZone/Cylinders/Article/False/6423/TechZone-Cylinders.Jones, and Childers. The Friction of Automobile Tires. Fahrenheit’s Thermometer, 2 June 2002, boson.physics.sc.edu/~rjones/phys101/tirefriction.html.Kurtus, Ron. Traction Friction of Tires. Three Lever Classes by Ron Kurtus – Succeed in Understanding Machines: School for Champions, School for Champions, 28 Mar. 2008, www.school-for-champions.com/science/friction_rolling_traction.htm#.W_4o92hKhEY.Newton’s Third Law. 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