hey all

So I have a mechanics (kinematics) test coming up and while I’m pretty good at physics, a few of the problems trip me up a lot

I have a few questions if someone could please clarify!!

1. In projectile motion, when would you double the time for horizontal SUVAT? I understand why you have to spilt initial velocity into components but why would this affect time?

2. Is vertical initial velocity always 0?

Any other tips on revision or approaching mechanics problems ?!! Any help would be super appreciated

So I have a mechanics (kinematics) test coming up and while I’m pretty good at physics, a few of the problems trip me up a lot

I have a few questions if someone could please clarify!!

1. In projectile motion, when would you double the time for horizontal SUVAT? I understand why you have to spilt initial velocity into components but why would this affect time?

2. Is vertical initial velocity always 0?

Any other tips on revision or approaching mechanics problems ?!! Any help would be super appreciated

Original post by Nat4695

hey all

So I have a mechanics (kinematics) test coming up and while I’m pretty good at physics, a few of the problems trip me up a lot

I have a few questions if someone could please clarify!!

1. In projectile motion, when would you double the time for horizontal SUVAT? I understand why you have to spilt initial velocity into components but why would this affect time?

2. Is vertical initial velocity always 0?

Any other tips on revision or approaching mechanics problems ?!! Any help would be super appreciated

So I have a mechanics (kinematics) test coming up and while I’m pretty good at physics, a few of the problems trip me up a lot

I have a few questions if someone could please clarify!!

1. In projectile motion, when would you double the time for horizontal SUVAT? I understand why you have to spilt initial velocity into components but why would this affect time?

2. Is vertical initial velocity always 0?

Any other tips on revision or approaching mechanics problems ?!! Any help would be super appreciated

A bit of guessing about what the original questions are but

1) Im presuming you want something like time of flight but you calculate the time to peak. In that case, the time flight is double the time to peak as the projectile motion is a parabola so symmetric about the peak. Thats assuming the initial height is zero etc.

2) You usually say something like "drop" if the initial vertical velocity is zero. But more generally the initial vertical velocity is not zero.

A tip for the basic suvat equations is to understand them in terms of what they represent, so

constant acceleration <-> linear velocity <-> parabolic (quadratic) displacement

So all of the messing around with linear and quadratic (completing the square, roots, ...) functions you did at gcse is relevant to understanding them. Similarly

v = u + at

is effectively a momentum equation as multiplying through by mass gives

mv - mu = (ma)t

where the force ma is applied for a time so the impulse gives the change in momentum. Similarly

v^2 = u^2 + 2as

is effectively energy conservation as multiplying through by m/2 gives

mv^2 /2 - mu^2 / 2 = (ma)s

or change in KE = work done (force*displacement).

So really the 5 suvats should make a lot of sense? You should have covered the key stuff about how to answer the questions like a (linear/parabolic) sketch with variables marked on, decide the positive direction to make the calcs as simple as possible (few negatives), write down the suvat variables known/need to calculate and hence determine which suvat equation(s) you need ....

(edited 4 months ago)

Hey

Thanks for the detailed explanation!

So I’d say after about 3 hours more of doing problems + your explanation everything has pretty much clicked!

I still don’t quite get the part about initial velocity , maybe I didn’t phrase my question properly. When we are splitting the horizontal and vertical parts of a projectile, every problem I’ve done so far sets U for Vertical as 0 , why is this?

thanks again for the help : )

Thanks for the detailed explanation!

So I’d say after about 3 hours more of doing problems + your explanation everything has pretty much clicked!

I still don’t quite get the part about initial velocity , maybe I didn’t phrase my question properly. When we are splitting the horizontal and vertical parts of a projectile, every problem I’ve done so far sets U for Vertical as 0 , why is this?

thanks again for the help : )

Original post by Nat4695

Hey

Thanks for the detailed explanation!

So I’d say after about 3 hours more of doing problems + your explanation everything has pretty much clicked!

I still don’t quite get the part about initial velocity , maybe I didn’t phrase my question properly. When we are splitting the horizontal and vertical parts of a projectile, every problem I’ve done so far sets U for Vertical as 0 , why is this?

thanks again for the help : )

Thanks for the detailed explanation!

So I’d say after about 3 hours more of doing problems + your explanation everything has pretty much clicked!

I still don’t quite get the part about initial velocity , maybe I didn’t phrase my question properly. When we are splitting the horizontal and vertical parts of a projectile, every problem I’ve done so far sets U for Vertical as 0 , why is this?

thanks again for the help : )

If the particle is projected horizontally (2d) or dropped (1d) then the initial vertical velocitiy is zero. If its projected at a speed u at an angle theta to the horizontal then the resolved horizontal and vertical initial velocities would be

u*cos(theta), u*sin(theta)

So if theta=0 (projected horizontally), then the initial vertical velocity is zero.

(edited 4 months ago)

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