Understanding the Relationships Between Surface Area, Mass, and the System of Forces Acting in a Body in Motion Across a Horizontal Surface
Laboratory No. 4
Introduction. When a smooth block is set sliding on a smooth horizontal floor, we note that it moves in a straight line but gradually slows down and comes to a stop. Why does it not obey the Law of Inertia (Newton's First Law) and continue at constant speed? The answer is that there is an unbalanced force acting on it. This is the force of friction exerted upon it by the floor. Friction may be defined as the force that resists a body when it moves over or through another body. Thus a projectile, once set in motion, is slowed down by friction as it moves through the air. The block in our example is therefore slowed down by friction as it moves over the floor.
Friction explains why a car, once set in motion, will not continue to move at constant speed on a horizontal road without further help from the engine. The frictional resistance of the road to its forward motion and of its axles to the motion of its wheels over them gradually brings the car to a halt once the engine is turned off. It follows that, to keep the car moving at constant speed, the engine must supply just enough force to overcome the retarding force of friction. With the force of friction neutralized in this manner, there are no unbalanced forces acting on the car and it obeys the Law of Inertia.
Equipment (See pictures below)
12.0 inch x 4.0 inch x 2.0 inch solid pine friction block
Threaded brass attaching hook
Ten mass-fastening screws
Set of assorted slotted masses
Spring scale
Triple beam balance
Meter stick
Friction surface(s)
Tasks
1. The rectangular friction blocks employed in this laboratory exercise are constructed of solid pine, each possessing a length of 12.0 inches, a width of 4.0 inches, and a height of 2.0 inches. Assuming that each and every block is internally consistent and uniform, and neglecting the attached hardware (mass-fastening screws and attaching hook),
a. measure the mass of your friction block
__________ (grams)
__________ (kilograms)
calculate the mass of your friction block with added 200 g mass
__________ (grams)
__________ (kilograms)
calculate the mass of your friction block with added 500 g mass
__________ (grams)
__________ (kilograms)
b. calculate the surface area of the greatest side
__________ (cmexp2)
__________ (mexp2)
c. calculate the surface area of the intermediate side
__________ (cmexp2)
__________ (mexp2)
d. calculate the density of your unladen block
__________ (g/cmexp3)
__________ (kg/mexp3)
2. Pressure (P) is defined as the amount of force (F) exerted by an object per unit area of surface (A). Based on your lab data and the results of your calculations in Task 1,
a. calculate the pressure exerted by the friction block on a horizontal surface
when resting on the side with the greatest surface area
unladen [no added mass] __________ (N/mexp2)
with added 200 g mass __________ (N/mexp2)
with added 500 g mass __________ (N/mexp2
b. calculate the pressure exerted by the friction block on a horizontal surface
when resting on the side with the intermediate surface area
unladen [no added mass] __________ (N/mexp2)
with added 200 g mass __________ (N/mexp2)
with added 500 g mass __________ (N/mexp2)
c. Under which situation is greater unit pressure exerted? Explain
5. Briefly explain the observed relationship between an object's surface area and frictional force. Name the principal factors that either enhance or diminish the force of friction which acts to retard
an object's movement in contact with and respect to another object across a horizontal surface.
6. Prepare graphs of your Fp (pulling force) versus Fn (normal force) data acquired in both experimental situations. One graph should present your data acquired from sliding the friction block along the side with the greatest surface area and another graph should present your data acquired from sliding the friction block along the side with the intermediate surface area. Pulling forces required to overcome kinetic (sliding) friction and maintain a constant rate of motion are to be plotted only. Organize your graphs so that pulling force (Fp, which also incidentally equals Ff in magnitude) appears on the ordinate and Fn (which also incidentally equals weight in magnitude) appears on the abscissa. How do the slopes of your graphed lines compare with one another? What does the slope of each line reveal about the relationship between mass, surface area, pulling force, and frictional force? To which numerical value that we have previously analyzed is the slope equal? Explain.
Friction explains why a car, once set in motion, will not continue to move at constant speed on a horizontal road without further help from the engine. The frictional resistance of the road to its forward motion and of its axles to the motion of its wheels over them gradually brings the car to a halt once the engine is turned off. It follows that, to keep the car moving at constant speed, the engine must supply just enough force to overcome the retarding force of friction. With the force of friction neutralized in this manner, there are no unbalanced forces acting on the car and it obeys the Law of Inertia.
