Newton's Laws Legacy Problem #15 Guided Solution

A good problem solver seldom rushes to their calculator before getting a good visual and conceptual understanding of the physical situation described in the problem. Here in this unit the visual representation is usually going to take the form of a free body diagram in which we represent the forces which act upon the object. Here in this question we read about Earl who is on the ground sliding towards the second base and we observe as we read the question that there is a friction force acting upon Earl as he slides across the dirt into the base. So we have to think about what forces are acting upon Earl during the sliding portion of the motion. Well obviously there is a friction force so I would draw a free body diagram and represent it by an arrow. I just put a box like you have seen on some of the other problems and as you have seen in the physics classroom tutorial and on the box I would draw a force off to the left presuming that Earl maybe is moving to the right and the force acts leftward like it does in all sliding friction situations. I would label this as F friction and I would even record the value of the magnitude of the force as 249 newtons. Now there are two other forces that are not explicitly stated here in the problem but we can reason that they are present. There is on every object a force of gravity that pulls directly downwards upon the object. That force of gravity can always be calculated if you know the mass of the object as we do here. If you wish you can take that mass and you can multiply it by 9.8 and get the down force acting upon Earl's body as he is sliding in the second base. And then there is an up force and the up force is supplied by the ground again. It supplies what we know as a normal force. It's what supports objects and prevents them from falling down under the influence of gravity. The up force F norm is simply going to be equal to the down force. We know this is the case whenever we have a situation in which there is no vertical acceleration and certainly when you are sliding in the second base you are not accelerating up nor down. Now you have a free body diagram with the only three forces acting upon Earl. It's bothersome to some people that there is no rightward force for this rightward moving object. You probably need to, if that is the case, need to read the physics classroom tutorial the big misconception in the Newton's laws unit. Now what we can do is we can determine the net force and the acceleration. This is the strategy plotting phase of our problem. We are going to look for the net force because we know we need it in order to calculate the acceleration. And we are going to look for the mass as well. Now the acceleration is the net force divided by the mass. So the way we would get the net force is we would add up all these forces. Easy does it here because the ups and the downs will balance each other and cancel out. And that leaves us with one force remaining and that is the 249 newtons. That's not just the friction force. That's also what all the forces add up to. So now we take 249 newtons as our net force and we divide it by the mass and we get our acceleration.