University of North Carolina at Chapel Hill
Newton's Laws Basics - Example 3

# Physics 101 Mechanics

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## Video Transcript

welcome to our third example video on Newton's laws. In this video, we're going to continue talking about Newton's second law, which last time we wrote as net force is equal to mass times acceleration. Remember the units of force our Newton's, which are equal to kilograms meters per second squared. Make sure you use kilograms and not grams. Okay, so this is Newton's second law. Let's go back to our example of a box on the floor. It's a boring example, but it's a very effective example because everyone has pushed boxes or something across the floor. Okay, so what if you have a big box and you decide that you're going to push this thing and you're going to push it with a force will say force equal to 30 Newton's. So if we push on this thing with the force due to force of 30 Newton's, how fast is it going to accelerate? Is the question. What's a going to be equal to? Well, we know that a is equal to F net over em in the horizontal direction. There aren't any other forces. There's only us pushing on it. We're going to ignore friction for now, which means that if we want to find acceleration in the X, we want to think about the net force in the X. Remember the Net force and the why is only gonna affect acceleration in the Y direction, so acceleration in the X direction then will be equal to 30. Newton's the only force in the horizontal direction divided by the mass of the box. Let's say the mass. The box has a mass of tang kilograms, giving us an acceleration of three meters per second squared. So this is the basic idea for how we're going to go about calculating accelerations now. We could also add in other forces here that would require us to do a sum of forces. For example, say that your buddy comes up and he decides he's gonna push to. So he grabs a long stick to reach past Houston's. We're on a two dimensional plane here, and he pushes as well. And so now your net force is equal to your own 30 Newton's plus 20 Newtons due to your friend with the stick, and we say, What's the new acceleration? The new acceleration is going to be 5 m per second squared. Remember, this is only true for a long as both of you are pushing on the box. So if you just were to walk up and give it one big shove, it wouldn't continue to accelerate it 5 m per second squared. In fact, it would, as soon as it leaves your fingertips continue at a constant velocity. Assuming that there is no friction here, If there were friction, then it would immediately start to slow down. It would have a negative acceleration, but hopefully you're comfortable looking at this set up here we have a sum of forces. We have a mass, we have an acceleration. It's pretty simple algebra to move all these things around. The only time it starts to get hairier is when we start to do funny things with some of forces. So let's go back to our friend with the stick and us pushing on the box and say that our F net is equal to our force, plus some mystery force. We're not actually sure how hard our friend with the stick is pushing it. So this is the force. We'll just call it Force One. For now. We're pushing with 30 Newton's in the eye hat. We want to add it to F one, and that's to be equal to mass. Times are acceleration. So in this case, if we were to measure the Accelerations Day, we measure the acceleration in the X as being equal to UH, 2.5 m per second squared. We could then reorganize this in order to solve ref one. So we would say F one is equal to mass times 2.5 m per second squared in the I had direction minus 30. Newton's in the eye hat direction, and that will give us the force that our friend is pushing on the box with the stick. So we did have to do one other step of algebra here, where, instead of just multiplying or dividing, we had to do some subtraction to move the 30 Newton's over to the other side. Now that's not particularly complex as faras, the algebra go still. But that's about as hard as the math gets here. Until we start adding in a system of equations, which is something we won't get to for a few more examples