Multiple Choice Question 10 from HSC 2013 (1 Viewer)

[kashkow]

Can someone explain why the answer is D? This is from the HSC 2013 past paper.

10.

A particle is moving along the x-axis. The displacement of the particle at time t seconds is x metres.
At a certain time, x{dot} = -3 m s^-1 and x{dot,dot} = 2 m s^-2
Which statement describes the motion of the particle at that time?
(A) The particle is moving to the right with increasing speed.
(B) The particle is moving to the left with increasing speed.
(C) The particle is moving to the right with decreasing speed.
(D) The particle is moving to the left with decreasing speed.

 

[Squar3root]

wouldn't it be (B)?

 

[DatAtarLyfe]

Can someone explain why the answer is D? This is from the HSC 2013 past paper.

10.

A particle is moving along the x-axis. The displacement of the particle at time t seconds is x metres.
At a certain time, x{dot} = -3 m s^-1 and x{dot,dot} = 2 m s^-2
Which statement describes the motion of the particle at that time?
(A) The particle is moving to the right with increasing speed.
(B) The particle is moving to the left with increasing speed.
(C) The particle is moving to the right with decreasing speed.
(D) The particle is moving to the left with decreasing speed.

F = ma. Assuming that travelling towards the left is negative and right is positive, the force due to acceleration is towards the right (acceleration is positive), whereas the particle is travelling towards the left (velocity is negative). Thus, the particle would be experiencing a resisting force and hence slow down (decreasing speed)

 

[Squar3root]

F = ma. Assuming that travelling towards the left is negative and right is positive, the force due to acceleration is towards the right (acceleration is positive), whereas the particle is travelling towards the left (velocity is negative). Thus, the particle would be experiencing a resisting force and hence slow down (decreasing speed)

Newton's 2nd Law isn't taught in 2U iirc. And also in 2U iirc particles were meant to be massless.

 

[Squar3root]

My bad the answer wouldn't be (B), the correct answer is (D).

This is because at this instance the particle is travelling towards the left and the velocity is negative. However as it continues moving it is gaining positive velocity so hence it is slowing down.

Another way to look at it. At this instance the velocity is -3m/s but in the next second it would be (-3 + 2×1) = -1m/s so it is slowing down

 

[InteGrand]

It's useful to note in general that if the particle's velocity and acceleration are of opposite signs, it's slowing down (speed decreasing), whilst if the velocity and acceleration are of same sign, it's speeding up.

 

[Esse]

It's useful to note in general that if the particle's velocity and acceleration are of opposite signs, it's slowing down (speed decreasing), whilst if the velocity and acceleration are of same sign, it's speeding up.

Took the words right out of my mouth xd

 

[kashkow]

Wow thanks guys for all your responses! Very helpful!