The foot of a 55 kg sprinter is on the ground for 0.25 s while her body accelerates from rest to 2.0 m/s. (b) What is the magnitude of the friction force?

Video transcript

Hey, everyone. So this is a Newton second law problem. Let's see what it's asking us. A 125 kg motorcycle in contact with the road accelerates from rest to eight m per second. In 1.2 seconds, we're asked to determine the magnitude of the friction force in this motion. So if we were to draw, Oh sorry. Our multiple choice answers here are a 1000 newtons. B 1226 Newtons C 18.8 Newtons or D 833 Newtons. So if we were to draw a free body diagram with the motorcycle, the only force acting on it in the X direction is our static friction force as it is trying to accelerate. So our, when we recall Newton's second law, which is the sum of the forces is equal to mass multiplied by the acceleration, we are left with that static force is equal to mass multiplied by the acceleration. Now, we aren't given the acceleration of this problem, but we are given speed and time. So if we recall that acceleration is defined by the change in speed over the change in time or delta V divided by delta T, we can plug that in to our first equation. And so for our static force, so that gives us our static force is equal to the mass multiplied by the change in velocity over our change in time. The mass is given to us in the problem as 125 kg. Our change in velocity is eight m per second. Right? We go from rest of 0 to 8 m per second and then our change in time is 1.2 seconds. We start at time zero and we reach that eight m per second speed in 1.2 seconds. So from there, we can plug that into our calculator And we are left with 833 newtons. And that is the answer to this problem. It, it aligns with answer choice D. So that's all we have for this one. We'll see you in the next video.