Find the type of motion in the following case

Question

a) A car travelling along a straight road, changes its speed.
b) An athlete running a 100m race.
c) Moon revolving around the earth.
d) An Ant moving on the floor.

Answer 1

I would modify those answers somewhat:

 

A.  This is definitely accelerated motion (although, technically, there is no distinction in terminology between speeding up and slowing down.  Both are acceleration, only in the first case, the acceleration is in the same direction as velocity.  In the second case, acceleration is in the opposite direction from velocity.

 

B.  The only information we have here is about change in position.  We do not know what the accelerations may have been.  What I would add is that change in position is called displacement, if direction is considered (because displacement is a vector).  However, we are not told anything about direction.  Assuming the person ran only that race, we can assume the runner covered a distance of 100 m.  Distance is the amount of ground covered without regard to direction.  So, if the person ran in a straight line for 100 m to the east, for example, his or her distance covered would be 100 m, and the displacement would be 100 m east.  If the person ran 100 m north, the distance would still be 100 m, but the displacement would now be 100 m north.

 

One thing to note about displacement is that it only depends on where you start and where you finish.  It's as if you mark the person's initial spot, close your eyes until they are done moving, mark their final position, and draw a vector from the starting point to the ending point.  So, if the person ran 100 m around a circular track, and ended up where they started, the distance (ground covered) would still be 100 m, but the displacement would be zero, because there is no change between the initial and final position.

 

C.  This situation is very nearly uniform circular motion (it is actually an ellipse, but a nearly circular ellipse).  Uniform circular motion occurs when an object moves with a constant speed around some center.  Even though the speed is constant, this is accelerated motion (just like the first case).  In this case, though, the acceleration does not come from a change in the magnitude of the object's velocity, but rather from a change in the direction of the object's velocity.  Because vectors (like velocity) have magnitude AND direction, changing either one constitutes an acceleration.  In this case, the acceleration points toward the center of the circle, and is caused by a centripetal force.

 

D.   As mentioned before, we know the same thing about this as Part B, which is not much.  In fact, we know less, because we know nothing about the ant from what we are told.  This is a very vague set of questions, admittedly.

 

Hope this helps!

Answer 2

a)

 

Change in speed indicates acceleration.  If you speed down, this is called deceleration.  This describes Newton's second law of motion.

 

 

b)

 

This is a change in position.

 

 

c)

 

When an object revolves around another object, the changing direction causes that revolving object to change direction.  However, the object that is center of revolution exerts an equal but opposite force as the revolving object.  According to Newton's third law of motion, all forces come in pairs.

 

 

d)

 

This is the same situation as part b.