Lab 2: Types of Forces
on each surface collide, impeding each other’s mo on. A specialized fric on force when an object is in free fall is air resistance, which is affected by the speed of an object and its cross‐sec onal area. Though it can never cause an object to move, it can check or stop mo‐ on. As resistance, fric on wastes power, creates heat and causes wear. It has been shown
that the force required to slide one object over another is propor onal to the normal force pressing the surfaces together, expressed by the equa on shown below: Ff = μFN where μ is called the coefficient of fric on and represents the roughness of the surfaces in contact. There are two types of fric on, sta c (not moving) fric on and kine c (moving) fric on. They have unique coefficients of fric on, μs and μk, respec vely. In general, μs ≥ μk.
Tensile forces are transmi ed through an object when opposing forces pull at op‐ posite ends. The tension force pulls equally on the object from the opposite ends.
Spring forces are exerted on an object by a compressed or stretched spring. The spring acts to restore its original or equi‐ librium posi on. For most springs, the magnitude of the force is directly propor‐ onal to the stretch or compression of
the spring, expressed by the equa on below:
Fs=‐k∆x The SI unit for force is the Newton (N), where 1 N = 1 kg·m/s2 (the lb is the English unit). In other words, it takes 1 N of force to accelerate a 1 kg mass by 1 m/s2. If you are given a mass in kilograms, all you need to do to find the force (N) is to mul ply the mass by the accelera on due to gravity, g = 9.8 m/s2. Take a look at Figure 5 for an example. Another measurement of force you are familiar with is the pound (lb), but scien sts usually s ck with the SI units of measurement. When a number of forces act on an object at once, it is helpful to draw a free body diagram (FBD). Free body diagrams show all the forces ac ng on an object as arrows. For now, we will only talk about forc‐ es that point in the horizontal or ver cal direc ons. Since forces are vector quan es, when they add together we must take into account both magnitude and direc on. For example, if a 5 N force acts to the le on an object, and at the same me an 8 N force acts to the right, the total force or net force would be 3 N to the right. Using FBDs, you can visualize which forces will cancel others out. When you draw a FBD, each object of interest is drawn (you can draw the object, or even a box or point to represent the object), and each force is represented by an arrow. The length of the arrow rep‐ resents that magnitude of the force, and the direc on of the arrow indicates the direc on the force is ac ng upon the object. This way, you can visualize which forces will cancel out others, leaving a total net force in one direc on. If all the forces cancel each other out (for instance, equal but opposite forc‐ es in the ver cal and horizontal direc ons) the object is said to be in sta c equilibrium—the net force is equal to zero, even though there are many forces ac ng at once.
Figure 3: Despite gravity’s weakness as a force, it is responsible for the ball shape of planets and stars, and for the shape of galaxies. Masses within these structures a ract every other bit of mass within
the object, which creates their ball shape.
Lab 2: Types of Forces
Consider a book si ng on a table. If you apply a force to slide it across the table to your study partner, there are actually four forces involved in the mo on. The FBD would involve the normal force, gravity, the applied force and fric on, and the diagram is shown in Figure 4. The normal force arrow is drawn perpendicular to the surface, directly opposite the force of gravity in this case. We know the object is not moving in the ver cal direc on, so the ver cal forces are equal but in opposite direc ons and can‐ cel out on the net force diagram. Since enough force was applied to overcome fric on and move the book, we draw the applied force arrow longer than the fric onal force arrow that acts to resist mo on. The applied force is greater than the fric on force, so the net force is in the direc on of the applied force. This object will accelerate to the right. When an object is not moving in the horizontal or ver cal direc on, the sum of the forces must equal zero in that direc on (∑F=0).
Figure 4: The le figure is an example of a typical free body diagram (FBD) with a variety of forces labeled. The normal force (Fnorm) and the force due to gravity (Fgrav) must be equal and opposite because the object is not falling into the surfaces or accelera ng into the air. The applied force Fapp is larger than the force due to fric‐ on, so the net overall force Fnet points to the right‐‐shown on the reduced FBD on the right. The normal force
is not always directly opposite the force of gravity, as with an object res ng on an incline.
Figure 5: The 1 kg mass on the le is supported by a rope drawn around a pulley and anchored to a flat sur‐ face. The free body diagram on the right shows the case of sta c equilibrium: the force of gravity is balanced out
by the tension in the string. In FBDs only the forces ac ng direc onally on the object of interest ma er!
Figure 6: The two masses (weights labeled) are sus‐ pended by a single rope through a pulley wheel. The right side is a free body diagram for each mass; note that the tension in the string is the same on each side (in other words, the string does not stretch). The net force is upward on the 5 N mass and downward on the
8 N mass—which way will the assembly move?