28. A car traveling at 60 km/hr brakes to a stop in 7.0 s. The acceleration of the car is___.
A. +1.00 m/s2
B. -2.38 m/s2
C. +3.00 m/s2
D. -8.57 m/s2
(B) because the formula to find a is: a = vf-vo/t. So you plug in for velocity final is zero and velocity original is 60, but you must convert it into m/s so it is 16.66 m/s divided by 7s which is -2.38m/s2.
29. If the average velocity of an object is the same for all time intervals, then the object moves at a(n) ____.
A. constant velocity
B. relative speed
C. increasing acceleration
D. changing speed
(A) because the object's velocity had no acceleration between time intervals; In other words, the object's velocity was constant and unchanging.
30. On a position vs. time graph, the velocity equals ____.
A. the run divided by the rise
B. the x-intercept
C. the y-intercept
D. the rise divided by the run
This is proven by the equation V= X/T. The X is the distance (position/Y-axis/Rise) and the T is the time (X-axis/Run).
Your explanation should say change in position divided by change in time (delta x/delta t)
Correction, X, is the change in position(not the distance) and the Y is the change in time) coming to be X/Y or delta x/delta t.
31. For an accelerating object, the slope on a position vs. time graph ____.
A. is zero
B. is the change in time divided by the change in position
C. changes with time
D. is constant
Choices (A) and (D) were the first two answers I spotted incorrect because they define zero acceleration and a constant velocity. Choice (C) says that after some time, the acceleration would be different than what it was before. Also, knowing that any acceleration on a position vs. time graph shows a parabola, (C) is the correct answer. I did not choose (B) because the position should be divided by the time rather than what is stated above.
Just explain why this is the answer. No need to explain the incorrect choices.
After each unit of time, the velocity changes. If you were to plot the points, you will find that a parabola is formed, and the slope is changing as time passes.
32. Ignoring air resistance, if a 10 kg ball and a 200 lb crate were both dropped from the top of a building, the acceleration of the crate would be ____ the acceleration of the ball.
A. less than
B. equal to
C. greater than
The answer is B (equal to) due to the law that all bodies in free fall near the earth's surface have the same downward acceleration. But only if neglecting air resistance. If there is air resistance, then the objects would fall at a slightly different speed from friction. Weight doesn't matter.
33. The acceleration due to gravity of a free-falling object ____.
A. points up or down depending on the velocity vector
B. always points up
C. depends on its mass
D. always points down
(D) Gravity is always down. A would not be correct because gravity is the only vector of an object in free fall, and it is always pointing down.