Back To CourseAP Physics 1: Exam Prep
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Sarah has two Master's, one in Zoology and one in GIS, a Bachelor's in Biology, and has taught college level Physical Science and Biology.
Take a look around you. There are all sorts of stationary and moving objects. All of these objects have forces acting on them, but they may be different for each object. How do we know which forces are acting on any given object? The first thing we need to do is review the types of forces that can act on an object. There are some basic types to be aware of for the purpose of this lesson.
Gravity is a force you are all too familiar with because this is an object's weight. Gravity is what pulls you down to Earth, but you certainly don't need to be in contact with the ground for gravity to have an effect on you. Opposite to gravity is the normal force. This is the force that balances the weight of an object on a surface. The ground supports your feet with the normal force so you don't fall through the ground, just like a table supports a book with the normal force as it rests there. The normal force is always equal in magnitude and opposite in direction to gravity.
An object may also have friction acting on it. Friction is a force that occurs when two objects rub against each other, restricting or preventing motion. Air resistance is a friction that acts on an object in the air, like when a feather slowly floats to the ground instead of dropping quickly.
Applied force is the force applied to one object from another, like you pushing on a piece of furniture to move it across the floor. Tension is the force exerted through a fully stretched object, while spring force is the force exerted by a compressed spring.
Two other forces are similar to gravity in that they act on objects that aren't touching. One of them, electric force, is the attraction or repulsion between two charged objects, like what we see between atomic particles such as protons and electrons. Finally, magnetic force is the force exerted between two magnetic poles, like when you put the opposing ends of two magnets together.
Most of the time, you'll know if a magnet or electric force is being exerted because those are pretty specific situations.
But for other objects, we can identify the forces present by drawing free-body diagrams. These are force vector diagrams that provide a visual representation of an object and the forces acting on it. When drawing free-body diagrams, each force is represented by an arrow (a vector). The size of the arrow shows the relative magnitude of the force, while the direction the arrow is pointing tells us in which direction the force is acting. In a free-body diagram, a box is used to represent the object and arrows (the forces) are drawn outward from its sides.
Once you put all of this together, it's pretty simple to figure out which forces are acting on any given object. Let's apply our knowledge of forces to some examples to see how this works.
Say we have an object at rest on the ground. We know that gravity acts downward as the object's weight, and we also know that since the object is resting on a surface, the normal force opposes the gravitational force, right? So in this case, we can see that the individual forces acting on this object are gravity and the normal force. It's not moving, and there are no other objects exerting forces on it, so we can probably assume that there aren't any horizontal forces acting in this situation.
How about another example? This time, someone is pushing our object across the floor. The object still has weight, so we know gravity is at work here. Since the normal force opposes gravity, we also know that this force is being exerted on the object as well. The person pushing the object applies a force to the object, so there is an applied force moving the object across the floor. And since the object is moving across a surface, friction must also be present, working against the movement of the object. Makes sense, right?
Let's try another one, but let's complicate it a little. This time, a girl is hanging motionless by holding onto two ropes attached to the ceiling. Each hand holds a rope so that her weight is evenly distributed between them. Which forces are at work here? Well, she still has weight, so there's gravity pulling her down to Earth. But this time she's suspended from the ceiling by the stretched ropes, so the force acting against her weight is tension instead of the normal force. Each rope has the same amount of tension, and our free-body diagram represents this by the size of the arrows that are pointing upward. Don't forget that you need two arrows for this tension because each rope exerts half of the total tension force holding the girl!
And what if that girl lets go of the ropes? Which forces act on her as she falls toward the ground? Gravity definitely acts on her because it's pulling her down to the ground (and let's hope that she doesn't have too far to fall!). But as she falls, she encounters friction in the air, so air resistance acts on her but in the opposite direction to gravity.
Once on the ground, we see again how gravity and the normal force are acting on her while she recovers from her fall.
Identifying which forces are acting on an object is pretty straightforward. You need to have an understanding of the different types of forces and how they affect objects, such as gravity pulling objects down or friction acting against an object's motion. Then, if it helps, you can draw a free-body diagram. Illustrating the vector forces along with the object of interest can provide a good visual representation of the individual forces and in which direction they are acting.
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Back To CourseAP Physics 1: Exam Prep
12 chapters | 137 lessons