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Unit 3: Forces

    This unit was all about forces; balanced forces, unbalanced forces, and friction. A force is any push or pull that an object experiences, which is denoted by F and is in newtons (N). The mass (m) or an object is the amount of matter in a object, which always has to be in kilograms. Weight is the force of gravity on an object in newtons and denoted by Fg. Weight is calculated by multiplying the mass of an object by the force of gravity, which is 9.8 m/s/s. 
    Balanced forces mean that all the forces acting on an object equals zero; the total/net force on a object equals zero. In order for forces to be balanced, the forces in both the y and x must be balanced. Balanced forces happen when an object is not moving or it is moving at a constant velocity. All of the following forces described must cancel out for an objects forces to be balanced. An applied force (Fa) is a push/pull that a person applies with their hand or other appendage. A normal/surface force is very similar to an applied force, but instead of a person holding up the object, a table or other thing holds up the object. The normal force (FN) is always perpendicular to surface of the object. Inertia is how easy/difficult it is to change and object's motion, which is directly proportional to the object's mass. The more mass an object has, the more inertia it has. A tension force (FT) is the force in a cable, wire, string, or other attachment holding an object. The forces acting upon an object are not always perpendicular to each other. Many times one or more of the forces are at angles to the object, requiring one to find the force of the x and y components of the angled forces using basic trigonometry in order to balance the forces.
    Newton discovered three laws to describe motion. Newton's first law of motion, sometimes called the law or inertia states that an object's velocity will remain constant unless there is a net force acting upon it. This describes balanced forces. Newton's second law has to do with unbalanced forces, which are exactly what their name implies: unbalanced. Unbalanced forces do not cancel out so there is a net force upon the object, meaning that the object has an acceleration and therefore a changing velocity. The second law states that the net force is equal to mass times acceleration (Fnet=ma). Newton's third law states that for every action force, there is an equal (in size) and opposite (in direction) reaction force. This brings in the concept of force pairs, which is one action force and its opposite reaction force. 
    As mentioned before, unbalanced forces have to do with Newton's third law of motion. Objects moving with an acceleration (non-constant velocity) are unbalanced, and the net force acting on those objects is determined by multiplying the object's mass by the acceleration on the object. A common situation involving unbalanced forces is a person is in an elevator. In an elevator, a person's apparent weight (FN) often feels different from their actual weight (Fg). This is important to consider when doing calculations involving circumstances such as this. If an object is on an incline, then it is important to find the forces on the individual x and y components before doing other calculations.
    Lastly, friction is a force that opposes the motion of an object, denoted by the symbol f. There are two types of friction: kinetic and static. Kinetic or sliding friction (fk) is the friction acting on a moving object. If the velocity of an object is constant, the pull force acting on the object is equal to the kinetic friction force. If the velocity is not constant (the object is accelerating/decelerating), then the pull force is either greater or stronger than the kinetic friction force respectively. Static friction (fs) is the friction that prevents an object from moving. Because the object is not moving, the pull force acting on the object is equal to the static friction force. The static friction force is not constant; it adjusts to the applied force acting upon the object. The max value of the static friction force is the force required to start the object moving. The equation used to determine friction is the coefficient of friction times the normal force acting upon the object (f=μFN). The coefficient of friction is a measure of the roughness of a surface; it is a number between 0 (frictionless) and 1 (very rough). As with unbalanced forces, if the object is on an incline or is being pushed/pulled at an angle, then the individual x and y components of the forces must be found before other calculations are done.


This unit started out as a tricky one for me. It took a while for me to incorporate the concepts of the x and y components of forces into my calculations. This of course made things tricky for me to get right. Once I was able to remember this, things started to fall into place and I got into the swing of things. Occasionally I had issues determining how to set up the problems. I had a couple of problems figuring out what equation to use at what time. By then end of the unit though, I had almost everything figured out; avoiding silly mistakes was my main concern going into the test. My test scores for both objectives are very reflective of the progress I made throughout the unit.

Unit 3 Objective Sheet



The purpose of the Friction lab was to determine which shoe has the greatest coefficient of kinetic and static friction. This lab was done by using a spring scale to pull three different types of shoes to the verge of motion, which gives the experimenter the static friction force. Each shoe was then pulled at a constant velocity by a spring scale and the newtons used measured, which are equal to the kinetic friction force. Each shoe was also massed on a balance. Then, by using the equation for friction, the coefficient of first static, and then kinetic friction was solved for. After calculations, it was concluded that the tennis shoe had the highest coefficient of both kinetic and static friction. This lab forced me to use much of the knowledge I gained throughout the unit and also my experimental design skills, making it an accurate representation of my mastery of the unit objectives.

Friction Lab


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