Correct statement about this motion

  • Thread starter songoku
  • Start date
  • #1
1,356
59

Homework Statement:

A particle moves along a straight line in such a way that its displacement during any given interval of 1 second is 3 meters larger than its displacement during the previous interval of 1 second. Which one of the following options is correct?
a. the particle moves with constant velocity 6 m/s
b. the particle moves with constant velocity 3 m/s
c. the particle moves with constant acceleration 3 m/s/s
d. the particle moves with constant acceleration 6 m/s/s
e. the particle moves with increasing acceleration over time

Relevant Equations:

Kinematics
Let say the displacement of the first interval of 1 second is 10 m, so the displacement of the next 1 second interval will be 13 m, and the next after this one will be 16 m and so on.

Option (a) and (b) are wrong because in any interval of 1 second the displacement will be constant

I think option (c), (d) and (e) are also all wrong because the increment of displacement of each 1 second interval won't be constant 3 meters.

Am I wrong or actually no correct option? Thanks
 

Answers and Replies

  • #2
Delta2
Homework Helper
Insights Author
Gold Member
2,990
1,046
I actually found that option c) is correct.

Define $$y(t)=v_0((t+1)-t)+\frac{1}{2}a((t+1)^2-t^2)$$, that is y(t) is the displacement between time t and t+1 (assuming motion with constant acceleration ##a## and initial velocity ##v_0##).
The problem tell us that ##y(t)=y(t-1)+3##. If you do the math you ll conclude that ##a=3##.
 
Last edited:
  • Like
Likes songoku
  • #3
PeroK
Science Advisor
Homework Helper
Insights Author
Gold Member
14,279
6,657
I think option (c), (d) and (e) are also all wrong because the increment of displacement of each 1 second interval won't be constant 3 meters.

Am I wrong or actually no correct option? Thanks
If it travels ##3m## more every second, then it's travelling ##3m/s## faster every second, and that is pretty much the definition of an acceleration of ##3m/s^2##.

Is there any reason you chose not to do a calculation?
 
  • Like
Likes songoku
  • #4
Delta2
Homework Helper
Insights Author
Gold Member
2,990
1,046
If it travels ##3m## more every second, then it's travelling ##3m/s## faster every second, and that is pretty much the definition of an acceleration of ##3m/s^2##.

Is there any reason you chose not to do a calculation?
I think your reasoning is correct but for the average velocity and for the average acceleration. To prove that the acceleration is constant one has to do the math as i suggest in post #2.
 
  • Skeptical
Likes PeroK
  • #5
1,356
59
I actually found that option c) is correct.

Define $$y(t)=\frac{1}{2}a((t+1)^2-t^2)$$, that is y(t) is the displacement between time t and t+1 (assuming motion with constant acceleration a).
The problem tell us that ##y(t)=y(t-1)+3##. If you do the math you ll conclude that ##a=3##.
If it travels ##3m## more every second, then it's travelling ##3m/s## faster every second, and that is pretty much the definition of an acceleration of ##3m/s^2##.

Is there any reason you chose not to do a calculation?
I did something like this:

Assume the object starts from rest and accelerates for ##3m/s^2##. Using kinematic formula:

##d = V_o ~ . ~ t + \frac{1}{2}at^2 = \frac{3}{2}t^2##

For ##t = 0 \rightarrow d = 0##

For ##t = 1 \rightarrow d = \frac{3}{2} m##

For ##t = 2 \rightarrow d = 12 m##

So I got that the object did not travel ##3 m## more each second
 
  • #6
Delta2
Homework Helper
Insights Author
Gold Member
2,990
1,046
First of all for ##t=2## it is ##d=6m##. The problem tell us that it should be $$(\frac{3}{2}-0)+3=6-\frac{3}{2}$$ which holds.
 
  • Like
Likes songoku
  • #7
PeroK
Science Advisor
Homework Helper
Insights Author
Gold Member
14,279
6,657
I did something like this:

Assume the object starts from rest and accelerates for ##3m/s^2##. Using kinematic formula:

##d = V_o ~ . ~ t + \frac{1}{2}at^2 = \frac{3}{2}t^2##

For ##t = 0 \rightarrow d = 0##

For ##t = 1 \rightarrow d = \frac{3}{2} m##

For ##t = 2 \rightarrow d = 12 m##

So I got that the object did not travel ##3 m## more each second
If we assume ##a = 3m/s^2## and look at the velocity after each second we get:
$$t = 1s, v(1) = 3m/s; \ \ t = 2s, v(2) = 6m/s; \ \ t = 3s, v(3) = 9 m/s$$
Now we take the average velocity ##v_{avg}## during each second:
$$v_{avg}(0-1) = 1.5 m/s; \ \ v_{avg}(1-2) = 4.5 m/s; \ \ v_{avg}(2-3) = 7.5 m/s$$
And, we have the distance travelled every second:
$$d(0-1) = 1.5m; \ \ d(1-2) = 4.5m; \ \ d(2-3) = 7.5m$$
And we can see the distance travelled per second increases by ##3m## every second.
 
  • Like
Likes songoku
  • #8
1,356
59
First of all for ##t=2## it is ##d=6m##. The problem tell us that it should be $$(\frac{3}{2}-0)+3=6-\frac{3}{2}$$ which holds.
If we assume ##a = 3m/s^2## and look at the velocity after each second we get:
$$t = 1s, v(1) = 3m/s; \ \ t = 2s, v(2) = 6m/s; \ \ t = 3s, v(3) = 9 m/s$$
Now we take the average velocity ##v_{avg}## during each second:
$$v_{avg}(0-1) = 1.5 m/s; \ \ v_{avg}(1-2) = 4.5 m/s; \ \ v_{avg}(2-3) = 7.5 m/s$$
And, we have the distance travelled every second:
$$d(0-1) = 1.5m; \ \ d(1-2) = 4.5m; \ \ d(2-3) = 7.5m$$
And we can see the distance travelled per second increases by ##3m## every second.
Thank you very much for the help delta2 and perok
 
  • Like
Likes Delta2 and PeroK
  • #9
jbriggs444
Science Advisor
Homework Helper
2019 Award
9,061
3,784
Many non-constant acceleration profiles are possible.

The conditions of the problem are consistent with, for instance, a particle that is at rest for t=0 through t=0.5 and accelerates at 6 m/sec2 for t=0.5 through t=1.0, ending the interval at v=3 m/sec. The particle then repeats the acceleration profile forever, coasting for half a second and accelerating hard for another half-second.

Indeed, any acceleration profile will work, provided only that the delta v between interval start and interval end is 3 m/sec2 and that the profile is periodic with period 1 second.

One might imagine scenarios where velocity and acceleration are undefined everywhere. The problem statement only talks about "displacement". But this is a physics, not math, so I will not go there.
 
  • Like
Likes songoku
  • #10
502
194
Many non-constant acceleration profiles are possible.
The problem does state:
A particle moves along a straight line in such a way that its displacement during any given interval of 1 second is 3 meters larger than its displacement during the previous interval of 1 second.
"during any given interval" rules out many interpretations.
 
  • Like
Likes songoku
  • #11
jbriggs444
Science Advisor
Homework Helper
2019 Award
9,061
3,784
"during any given interval" rules out many interpretations.
Yes. It was tempting to think that it would rule out everything but a constant acceleration profile. Such is not the case. Though it does enforce periodicity.
 
  • Like
Likes hmmm27 and songoku
  • #12
Delta2
Homework Helper
Insights Author
Gold Member
2,990
1,046
I don't think that any acceleration profile such that ##a(t)## is periodic with period 1sec and $$\int_t^{t+1} a(s)ds=3$$ (which simply means that the velocity is increasing by 3 units at the end of each second), would satisfy the condition of the displacement in any second being 3 units greater than the displacement in the previous second. It seems to work with piecewise constant accelerations but i dont think it will work with any acceleration curve.
 
  • Like
Likes songoku
  • #13
jbriggs444
Science Advisor
Homework Helper
2019 Award
9,061
3,784
I don't think that any acceleration profile such that ##a(t)## is periodic with period 1sec and $$\int_t^{t+1} a(s)ds=3$$ (which simply means that the velocity is increasing by 3 units at the end of each second), would satisfy the condition of the displacement in any second being 3 units greater than the displacement in the previous second. It seems to work with piecewise constant accelerations but i dont think it will work with any acceleration curve.
Why ever would it not work? If the acceleration is periodic (with period 1 second) and has an integral which is equal to 3 over any specific 1 second interval then it must have an integral which is equal to 3 over every 1 second interval.

It follows that the velocity at any time will be 3 m/s greater than the velocity one second prior.

It then follows that the displacement over any 1 second interval is 3 meters greater than the displacement in the 1 second prior.
 
  • Like
Likes songoku
  • #14
Delta2
Homework Helper
Insights Author
Gold Member
2,990
1,046
It then follows that the displacement over any 1 second interval is 3 meters greater than the displacement in the 1 second prior.
i have serious doubts about this logical step. The displacement during 1sec depends on the velocity curve for that 1sec and not only on the start and end velocity.
 
  • Like
Likes songoku
  • #15
jbriggs444
Science Advisor
Homework Helper
2019 Award
9,061
3,784
i have serious doubts about this logical step. The displacement during 1sec depends on the velocity curve for that 1sec and not only on the start and end velocity.
$$\int_{T+1}^{T+2} v(t)\ dt = \int_T^{T+1}3+v(t)\ dt = \int_T^{T+1} 3\ dt + \int_T^{T+1}v(t)\ dt = 3 + \int_T^{T+1} v(t)\ dt$$
By the time we get to this step we've already reasoned that v(t) = 3 + v(t-1) for all t. Which is to say we've already constrained the entire velocity curve over the interval, not just its endpoints.
 
Last edited:
  • Like
Likes songoku and Delta2
  • #16
Delta2
Homework Helper
Insights Author
Gold Member
2,990
1,046
yes well i now realized that the velocity curve will also be kind of periodic (same waveform just shifted by +3 on the vertical axis, every 1 sec.), thanks for this.
 
  • Like
Likes songoku and jbriggs444

Related Threads on Correct statement about this motion

  • Last Post
Replies
2
Views
124
Replies
25
Views
1K
Replies
2
Views
137
Replies
26
Views
467
Replies
15
Views
1K
Replies
4
Views
572
Replies
18
Views
622
Replies
10
Views
571
Replies
12
Views
420
Replies
5
Views
26K
Top