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.
You've probably heard of 'The Force,' but this isn't quite the same as a force. A force is a push or pull on an object. This push or pull comes from the objects interacting with one another and only from such interactions. Once the interaction stops, there is no longer any force.
While forces come in pairs, there can be even more forces in an interaction. For example, when you throw a ball into the air, the ball is experiencing the force of gravity, friction, and the pushing force from you all at the same time!
Forces are important because, as we learned in another lesson, they are responsible for changes in motion. In fact, Isaac Newton describes this in his first law. This law of inertia states that an object continues in its state of rest or motion unless acted on by an outside unbalanced force. So, your cat sleeping on the couch isn't likely to move unless you apply a force (pushing on the cat).
Newton wasn't just great at scientific laws; he also got his own unit of measure! The standard unit of force is the Newton, or N. So, for example, if something is 5 N, this means 5 Newton force.
But force has both magnitude and direction, which makes it a vector quantity. The magnitude of the force is how much, and the direction is in which way. So, in order to fully describe the force, we would need to say the force is 5 N and in which direction (like downward or to the left). As with any vector quantity, we show forces with arrows. The arrow's length represents the magnitude, while the arrow's direction shows the direction of the force.
We can put forces into two different categories, contact and non-contact. Let's look at each one more closely to get a better understanding of them.
Contact forces are just what they sound like: forces that result from the interaction of two objects in contact with each other. Forces that belong in this category are friction, air resistance, normal force, applied force, tension force, and spring force.
Friction is a force that you are quite familiar with already. This force occurs when objects rub against each other. The burn you feel on your skin when you go down a slide? Friction! Your brake pads and rotors stopping your car? Friction again! Friction acts in a direction to oppose motion - when you pull a bag across the floor to the right, the force of friction on the bag is to the left.
Even objects falling downward through the air experience friction acting upward, this time from the air. This special type of frictional force is called air resistance. It's the friction that acts on an object as it moves through the air. Again, this force is in a direction opposite to the direction of the object's motion.
There's a really important force that keeps us from falling through the floor called the normal force. This is the upward force that balances the weight of an object (another force that we'll talk about later) on a surface. If the object is at rest on a horizontal surface, the normal force is the same as the object's weight. Gravity pulls you down, but the normal force pushes back up on you from the floor.
An applied force is a force that is applied to an object by another object. If you push a box across the floor, your force pushing on the box is the applied force. And remember how we said before that you can have multiple forces acting at once? You better believe that in addition to your applied force on the box, that box is also experiencing friction from the floor, gravity pulling it down, and the normal force pushing it up - all at the same time!
Tension force is the tension through a string or other fully-stretched object. If you tie something to a string and let it hang from your fingers, the tension force is the same for both the object at the end of the string and your fingers holding that string.
Spring force is the force exerted by a compressed or stretched spring. This force acts on the object that is compressing or stretching the spring. Push a spring together with your hands, and I bet you'll feel that spring force being exerted!
If contact forces are forces from objects in contact with each other, it makes sense that non-contact forces are forces that result when the two interacting objects are not in contact with each other. Forces in this category include gravitational, magnetic, and electrical forces.
Remember weight from before? Simply put, this is the force due to gravity. Gravity is the force of two particles pulling toward each other, which means they don't need to be in contact with each other to feel the effect. Gravity is what pulls you back down to Earth if you're in the air - you're certainly not in contact with the ground as you're racing toward it! It's also what keeps planets, moons, and satellites in orbital paths.
Electric force is the attraction or repulsion between two charged objects. For example, there is an electric force between the positive protons in an atom's nucleus and the electrons orbiting around it. Though the protons and electrons aren't in physical contact, there is a non-contact force pulling them toward each other.
You can easily see magnetic force when you put the ends of two magnets together. This is the force exerted between two magnetic poles, and it can be a push or a pull. Put the north end of one magnet to the south end of the other, and they pull together. But put both north ends together, and you'll feel them repelling each other very strongly!
A force, which is a push or a pull, can come in different sizes, directions, and types. They are the result of interactions between two objects. And while each object exerts a force on the other, there may be other forces involved in the interaction as well.
Force is a vector quantity because it has magnitude and direction. When we describe a force, we use the standard unit of measure, the Newton, as well as state which way the force is acting.
Forces can be contact or non-contact. Contact forces are those that result from the interaction of objects in contact with each other. These include things like friction, air resistance, applied force, tension force, and spring force.
Non-contact forces are those that result from the interaction of objects not in contact with each other. Gravity (or weight), electric force, and magnetic force are all forces that objects may experience during an interaction where they're not physically touching.
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Back To CourseAP Physics 1: Exam Prep
12 chapters | 136 lessons