Force, a push or pull, or a connection between force and motion. From long-range forces to contact forces, forces are everywhere in our life. Gravity, normal force, applied force, tension, spring force...
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Although we can name many of the forces, what do they exactly do? What is the relationship between forces and motion? The answers are all in Newton's laws! In this article, Newton's laws will be introduced. Read to learn more about forces!
Have you asked yourself, why is it so important to wear the seat belt during a ride? Imagine this - you are in your car and driving to go on a trip. Suddenly, a dog appears right in front of your car, and you have to step on the brake to make sure you don’t kill the dog. When the car decelerates and comes to a sudden stop, what will happen if you are not wearing the seat belt? Well, you will most likely lean forward and the steer may hit you hard. But why will that happen?
According to Newton’s first law, An object has no forces acting on it. If it is at rest, it will remain at rest. If it is moving, it will continue to move in a straight line at a constant speed. So when the car you were driving decelerated and stopped, there was no force acting on you, so you will continue to move forward.
We know that force is a quantity, but how do we measure it? Here, Newton’s second law may be introduced. An equation - F=ma can be used to calculate the net force. In the equation, “m” is the mass of the object, and “a” is the acceleration of the object. When solving force-related problems, where different forces are exerted on the same object, we can apply Newton’s second law, adding all the forces together, and set the expression equal to the object’s mass times its acceleration.
Here is an example problem related to Newton’s second law:
A 20 N horizontal force gives a stone on the ground acceleration of 4 meters per second squared (m/s^2). What is the weight of the stone?
(solve this problem on your own before you keep reading if you want to. The answer will be in the next few lines.)
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Answer: 49 N.
Explanation: There are three forces exerted on the stone: the force of gravity (weight), the normal force, and the applied force. According to Newton’s second law, F=ma, which in this case means that 20 N (the applied force) equals mass times 4 (the acceleration). After solving this equation, we know that the mass of the object is 5 kg. And because the force of gravity, or the weight, can be calculated by multiplying the mass of the object to gravity (9.8), we can calculate the weight of the object by using 5 times 9.8, and we get our final answer, which is 49 N.
Now after learning about Newton’s first and second law, let’s talk about Newton’s third law. According to Newton’s third law, every force occurs as one member of a pair of interacting forces. The two members of a pair of interaction forces act on two different objects, and the two members are equal in magnitude but opposite in direction. That being said, when you are pushing against the wall, the wall is pushing back on you. Another great example would be, when you slam someone (don’t do so though), the person will hurt, but you can feel the pain from your palm as well. (if you really want to try, just slam your thigh, and you will notice how both your thigh and palm feel the pain)
Here are some practice problems to get you familiar with the concept of Newton’s third law. In the following statements, mark the ones that you think demonstrate Newton’s third law.
You press on the surface of your table using your hand and the table presses back on your hand.
The Earth pulls down on you (your weight) and the floor pushes up on you.
A small car tows a large truck and they move at a constant speed. The truck pulls back on the car and the car pulls on the truck.
A small car tows a large truck and they speed up. The truck pulls back on the car and the car pulls on the truck.
You hold a book against the wall. You apply a force to the book and the wall applies a force on the book.
(the answer will be shown in the next few lines, decide on your answer before you continue to read if you want to solve the problem on your own)
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The correct answers are … 1, 3, and 4!
The reason why 2 and 5 are wrong is that multiple objects are in interaction. Notice how the objects in the other three statements are exerting forces on each other, but in 2 and 5 the forces are exerted on more than two objects and don’t come in pairs.
And that is all for Newton’s laws. Although this article covers all three of the laws, they are very basic, and Newton’s three laws definitely cover more than what is included in this article. So, continue to learn, and remember to keep practicing and exploring!
Written by: Benetta Wang
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