When the velocity of a moving object changes, we say that the object is accelera

Question

When the velocity of a moving object changes, we say that the object is accelerating. We can change the velocity by changing either the magnitude of the motion (the speed) or the direction of the motion. The rate of change of velocity is called acceleration. Thus we define acceleration as Acceleration = change of velocity / change in time Acceleration = ( final velocity - initial velocity ) / change in time = (Vf - Vi)/ (tf - ti) When straight line motion is being considered, speed is the same as velocity. When motion is just along a straight line, acceleration is along a straight line. Acceleration ( in a straight line ) = ( change of speed ) / ( change in time ) When the acceleration is constant, we can easily find the velocity acquired as a function of time. Velocity acquired = acceleration X time Note the units of acceleration are ( meters / sec) / sec or meters / (sec X sec). Note that acceleration may be either positive or negative. In ordinary language, a negative acceleration is called a decceleration. However, in this class we will call it only negative acceleration. A car accelerates from rest for 5 seconds until it reaches a speed of 20 m/s. What is the car's acceleration in meters per second per second (also stated equivalently as m/s/s or m/s squared or m/s^2) Question 31 options: 1 m/s^2 2 m/s^2 3 m/s^2 4 m/s^2 5 m/s^2

Explanation / Answer

The only statement that MUST be true is (C) the net force on the object is zero. That doesn't mean there aren't ANY forces acting on the object; it just means they are all canceling out. So there COULD be friction, but if there is, there's also some force that just matches the friction in the opposite direction. I hope that helps. Good luck! EDIT: There seems to be some confusion among the other respondants, so let me clear a few things up. Acceleration is a change in velocity over time, and velocity can be thought of as speed in a certain direction. So, when something is accelerating, its speed or direction (or both) must be changing. However, nothing can accelerate without the application of a NET force. The word "net" is very important in this situation. If something is moving along with NO force acting on it, it will continue to travel in a straight line at constant speed. If there are many forces acting on it, but they all balance out, it's the same story. Consider an airplane that flies west at a constant 400 km/h. There are several forces acting on the plane: gravity pulling down, lift pushing up, thrust pushing forward, and drag pushing backward. If the plane's velocity is constant, we know that all four of these forces MUST cancel each other out; the lift being produced by the plane's wings is equal to the weight of the plane, and the thrust is equal to the drag. If any of those forces were greater than any others (for example, if the lift were GREATER than the weight) then the plane would accelerate in the direction indicated by the net force (upward in this example). So we cannot say that an object moving at constant velocity has a constant force being applied in the direction of motion; that's only true if the object is moving in a situation where friction must be considered. And we cannot say there are NO forces acting on the object. There may be many forces, but they cancel out. All we can say for sure is that whether there are no forces or many, the NET force is zero.

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