Physics Help Thread (1 Viewer)

[BlueGas]

Setting up this thread for questions that I need help with

For this question ------- A motorist travels 3 km North and 4 km East. What is the displacement from the starting position?

Do I just use Pythagoras's Theorem?

How about this? A student walks 1.0 kilometer due east and 1.0 kilometer due south. Then she runs 2.0 kilometers due west. The magnitude of the student' s resultant displacement is closest to..... why is the answer 1.4?

 

[HeroicPandas]

Yes.

Draw a diagram. "Magnitude of displacement" is just the distance between starting point and end point. In this case, it's √2 = 1.4142... ≈ 1.4.

 

[InteGrand]

Setting up this thread for questions that I need help with

For this question ------- A motorist travels 3 km North and 4 km East. What is the displacement from the starting position?

Do I just use Pythagoras's Theorem?

How about this? A student walks 1.0 kilometer due east and 1.0 kilometer due south. Then she runs 2.0 kilometers due west. The magnitude of the student' s resultant displacement is closest to..... why is the answer 1.4?

Make sure for the first one to also give a direction, since displacement is a vector . You can use basic right-angled trig. to provide a direction (kind of like a compass bearing like in HSC maths if you remember the bearing stuff).

(For the second one, it's only asking for the magnitude of the displacement, so we don't need to give the direction.)

 

[BlueGas]

Make sure for the first one to also give a direction, since displacement is a vector . You can use basic right-angled trig. to provide a direction (kind of like a compass bearing like in HSC maths if you remember the bearing stuff).

(For the second one, it's only asking for the magnitude of the displacement, so we don't need to give the direction.)

Is there a different between magnitude of displacement and displacement? What's the difference other than not giving the direction?

 

[InteGrand]

Is there a different between magnitude of displacement and displacement? What's the difference other than not giving the direction?

Well essentially the displacement is the magnitude together with the direction. The magnitude on its own would be the distance of the motorist from the original position. So displacement is basically when we also care about the direction (unlike for distance).

E.g. If you walked 1 km North from a building, whilst your friend walked 1 km South from the same building, you guys would end up the same distance from the building, but different displacements.

 

[BlueGas]

For Practice A, why do you add 90 not 180? How do I know which to add?

 

[InteGrand]

For Practice A, why do you add 90 not 180? How do I know which to add?

What do you mean add 90? If you wanted the direction from the positive horizontal axis, then yes it'd be 90 deg + theta, but is that really what it wants? What type of answer does it want? Some type of compass bearing?

 

[BlueGas]

What do you mean add 90? If you wanted the direction from the positive horizontal axis, then yes it'd be 90 deg + theta, but is that really what it wants? What type of answer does it want? Some type of compass bearing?

It's something about vector direction...

 

[HeroicPandas]

Here:

What do you mean add 90? If you wanted the direction from the positive horizontal axis, then yes it'd be 90 deg + theta, but is that really what it wants? What type of answer does it want? Some type of compass bearing?

 

[BlueGas]

Here:

Howcome for Practice B you add 180? What's the difference between the two? How do I know whether to add 90 or 180?

 

[InteGrand]

Howcome for Practice B you add 180? What's the difference between the two? How do I know whether to add 90 or 180?

 

[BlueGas]

I know how to find the average velocity for the first 2 seconds and 4 seconds but how do I find the average velocity for the last 2 seconds?

Consider a car starting at rest at the beginning of a 250 m street begins to move with constant acceleration. At time t = 2.0 s it is 20 m along the street. At time t = 4.0 s it is 80 m along the street.

 

[InteGrand]

I know how to find the average velocity for the first 2 seconds and 4 seconds but how do I find the average velocity for the last 2 seconds?

Consider a car starting at rest at the beginning of a 250 m street begins to move with constant acceleration. At time t = 2.0 s it is 20 m along the street. At time t = 4.0 s it is 80 m along the street.

Average velocity is the change in displacement divided by the time interval.

In the last two seconds (time interval being 2 s), the change in displacement is 60 m along the street. So the average velocity for this time interval is (60 m along the street)/(2 s) = 30 m/s along the street.

(Main calculation is just that 60/2 = 30.)

 

[BlueGas]

Average velocity is the change in displacement divided by the time interval.

In the last two seconds (time interval being 2 s), the change in displacement is 60 m along the street. So the average velocity for this time interval is (60 m along the street)/(2 s) = 30 m/s along the street.

(Main calculation is just that 60/2 = 30.)

Did you find 60m by doing 80 - 20? So they meant last 2 seconds as in the last 2 seconds before 4 seconds? LOL I was thinking the last 2 seconds before reaching 250m.

 

[InteGrand]

Yeah I did 80 – 20.

 

[BlueGas]

Yeah I did 80 – 20.

Howcome we don't convert the km to m? The table below metres for length.

 

[InteGrand]

Howcome we don't convert the km to m? The table below metres for length.

We don't need to, giving it in m/s is perfectly fine (and a common thing to do. In fact, m and s are SI units).

 

[BlueGas]

We don't need to, giving it in m/s is perfectly fine (and a common thing to do. In fact, m and s are SI units).

In my lecture notes, it says: All equations use SI units. Convert all values to SI before doing calculations. So don't we convert km to m?

 

[InteGrand]

In my lecture notes, it says: All equations use SI units. Convert all values to SI before doing calculations. So don't we convert km to m?

Correct, since m is already SI.

Edit: sorry misunderstood you. For this Q. there's no km involved. (Thought you were talking about that last Q.)

 

[BlueGas]

Correct, since m is already SI.

Edit: sorry misunderstood you. For this Q. there's no km involved. (Thought you were talking about that last Q.)

Ohhh it seems I confused myself...... nevermind lol.

But if say a question had km instead of m and asked to find the average velocity, do I have to change km to m before plugging the numbers into the formula?