# Conceptual general physics help?

I have some concept problems that I cannot seem to master. I have them narrowed down to what I think the answers are but I keep getting them wrong.
I'm supposed to choose all the statements that are false.

A motorcycle moved along a straight road at a constant speed. The magnitude of its acceleration was 0 m/s2. - false...how can it be moving if it isn't accelerating.

A rabbit moved a total distance of 11 m and had a total displacement (magnitude) of 8 m. -true, the rabbit moved from point a to point b and it took 11 m, but point a is 8 m from point b.

It is observed that car's velocity and acceleration magnitudes are parallel. During this time, the car's speed increased by 4 mph. - true, the car's speed increased so the velocity increased because velocity is delta-displacement over time, and acceleration is velocity over time.

An automobile is speeding up along a straight stretch of road. At one moment during this time it is observed that xcar = -27 mi, vcar,x = -16 mph, & acar,x = -9 mph/s. - have no clue...i know that vcar is velocity, and acar is acceleration.

The truck traveled a distance of -78 mi. - false, it seems stupid to say a truck traveled a negative amount, but at the same time i just did a problem where distance was negative but thats because I was measuring a rock that was thrown in the air and then went below the initial point that it was thrown from.

A rock was thrown straight up into the air. At the peak, its acceleration was 9.8 m/s2 down.- false, at the peak it's acceleration would be 0, then as it fell its acceleration would be 9.8 m/s2.

An SUV was at rest for 10 s. During this time its acceleration (magnitude) was changing. - false, how could it be accelerating if it's at rest?

An airplane went from New York to Boston. During the trip its average speed was 67 mph and its average velocity (magnitude) was 51 mph. - false, speed is a scalar of velocity ( a vector), and a scalar cannot be greater than its vector, correct?

This is an online question (must choose all that are false to get it right), and this is what I think but it keeps saying I'm wrong.

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phinds
Gold Member
a couple of them seem to be obviously wrong, but you seem to be just asking to be spoon fed the answers. Tell us your reasoning and perhaps we can help you learn something.

okay, i will edit the original question

lewando
Homework Helper
Gold Member
It is observed that car's velocity and acceleration magnitudes are parallel. During this time, the car's speed increased by 4 mph. - true
These questions are weird. It could be true. But what if it's speed increased 5 mph? What's undeniably true is that the speed increase is not negative.

An automobile is speeding up along a straight stretch of road. At one moment during this time it is observed that xcar = -27 mi, vcar,x = -16 mph, & acar,x = -9 mph/s. - have no clue?
What if the car was moving to the "left" of the origin of your reference frame (in the negative range of the x-axis)?

A rock was thrown straight up into the air. At the peak, its acceleration was 9.8 m/s2 down.- false, at the peak it's acceleration would be 0, then as it fell its acceleration would be 9.8 m/s2.
It's velocity would certainly be zero. Did gravity stop working when the rock was at its peak?

An airplane went from New York to Boston. During the trip its average speed was 67 mph and its average velocity (magnitude) was 51 mph. - false, speed is a scalar of velocity ( a vector), and a scalar cannot be greater than its vector, correct?
Maybe your having trouble with definitions. Average speed is the total distance travelled divided by the total time. Average velocity is the final displacement divided by total time.

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This is a similar question that gave me the answers (those with a asterisk (*) are false:
*A motorcycle moved along a straight road at a constant speed. The magnitude of its acceleration was 9.8 m/s2.

*A chipmunk moved a total distance of 7 m and had a total displacement (magnitude) of 11 m.

It is observed that the velocity magnitude and acceleration magnitude of a car are parallel. During this time, the car's speed increased by 2 mph.

An automobile is speeding up along a straight stretch of road. At one moment during this time it is observed that xcar = 95 mi, vcar,x = -28 mph, & acar,x = -6 mph/s.

*The truck traveled a distance of -27 mi.

*A rock was thrown straight up into the air. At the peak, its speed was 9.8 m/s.

*An SUV was moving for 10 s. During this time its velocity was zero.

A train went from New York to Boston. During the trip its average speed was 134 mph and its average velocity (magnitude) was 62 mph.

^lewando
for my lack of being exposed to physics, only three days, I am going to respond to your last 2 statements.
... no gravity did not stop working, it never stops, tossing the rock in the air vertically would give an acceleration of -9.8 m/s2, and as it comes back down it would be 9.8 m/s2 before it hits the ground.
...and from what you said about displacement over time, then im gussing the airplane statement is true, the airplane could not take a direct flight path.

you know what. I just came up with some different answers and explanations. I uploaded the questions this time.

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lewando
Homework Helper
Gold Member
Now I see why some of the questions seemed strangely worded--they are possible/impossible questions--not "true/false". Thanks for clearing that up.

... no gravity did not stop working, it never stops, tossing the rock in the air vertically would give an acceleration of -9.8 m/s2, and as it comes back down it would be 9.8 m/s2 before it hits the ground.
You are then saying that gravity is acting differently upon the object depending on what it's velocity is. The force due to gravity is independent of the displacement or velocity of an object. Hopefully, you have been exposed to the equation F = ma (force = mass * acceleration, bold items are vector quantities), if not, consider yourself exposed. When you throw an object up into the air, the moment it leaves your hand there is only one force acting on the object (neglecting air resistance) and that is the force due to gravity. This force is a vector pointing towards the center of the Earth at all times. Since mass is a (positive) constant, the acceleration vector is also pointing in the same direction.

...and from what you said about displacement over time, then im gussing the airplane statement is true, the airplane could not take a direct flight path.
Don't guess. Use your knowledge of the definitions for average speed and average velocity in conjunction with your indirect flight scenario.

Here is my final thoughts.
-if the motorcycle is moving at a constant speed and changing position at a constant (linear) fashion, the it is not accelerating. So this is possible.
-This is possible, the distance travelled can always be greater than the displacement.
-This is possible, because velocity is displacement over time, and acceleration is change in velocity over time, and if the displacement changes by 4 mph, then the acceleration will change slightly as well, and the lines will stay parallel.
-this is possible, the automobile went past its point of origin and started moving in the negative direction, and if the position is negative then the velocity and acceleration will be negative.
-this is impossible, we don't just say something traveled a negative distance, now if a point of origin were included then this would be possible.
-the up direction is chosen as positive, so the rock reaches +9.8 m/s2 then reaches 0, then goes to -9.8 m/s2. This is possible.
-impossible, if the truck is resting then logically it is not accelerating.
- average speed is total distance traveled over time elapsed, and velocity is the speed of something in a given direction over time. So this is possible, the total distance travelled can be greater than the speed the object is moving in over time.

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