1,000,000 Question Thread :D

[Aden]

Basically what you have there are two equations, the one on the left is the total distance of the speeding car, and the one on the right is the total distance of the police car (where x is the variable for time, although I guess it should be t). You make them equal to each other and find what 'x' is, because their distances will be the same when the policeman catches up to the car.

[kenhung123]

Thanks

Can we only ust d=st when acceleration=0 and speed is constant?

[Aden]

No, if you look at the graph that I provided with the question, there is acceleration (when the police car speeds up from stationary to 100km/hr) and we can still find the distance by multiplying speed and time (area).

[kenhung123]

So what conditions must be met to use d=st?

[Aden]

Whenever you have two of the three variables and you think it will apply to the question. If you can't figure out these pursuit questions using a self-made graph and equaling the disances, you can use the motion formulas instead (longer).

[kenhung123]

Can't get my head around d,e

[Aden]

What you have to realize here is that velocity is a vector and speed is a scaler; so whilst velocity has magnitude and direction, speed has no direction. So the change in speed of an object would just be final speed - initial speed. However, with velocity you have give it a direction (lets say down is positive), so its velocity when it is flying down to the ground is positive and when it is flying back up into the air is negative (change in velocity is still final - initial). Bearing that in mind, you should be able to do this question.

[kenhung123]

I understand that concept but it says as it landed. But they both land as same speed/velocity.

[Aden]

Not really, the tomato landed with a splat while the golf ball rebounded. If the tomato had an initial speed of 6.26m/s and then exploded onto the ground, its change in speed would be m/s. Whereas if the golf ball had an initial speed of 6.26m/s and then rebounded at 5.42m/s, then the change in speed would be m/s. The tomato therefore has a greater change in speed.

The change in velocity of the tomato is the same, since it didn't rebound. The golf ball, however, would be m/s or m/s upwards (since we called downwards as positive).

[kenhung123]

Fank you

[kenhung123]

Why does the normal reaction force and force of gravity vary during the bounce of a ball?
Fg=mg
FN=mg cos theta (since a flat surface theta=0 and cos 0=1)

Thus FN=mg


Is there such thing as Net Velocity? Are the components for a velocity vector Vh (horizontal velocity), Vv (vertical velocity)?

[stonecold]

Your scaring me with this thread ken,  brings back bad memories of physics from last year lol.

And aren't they the same because the normal is perpendicular to a flat surface, meaning it is just gravity that applies?

[Aden]

They wouldn't be velocity vectors, but force vectors instead, where there can be a horizontal and vertical acting force with a 'resultant' force.

[kenhung123]

Apparently not, the book says it varies which confuses me :S

[kenhung123]

They wouldn't be velocity vectors, but force vectors instead, where there can be a horizontal and vertical acting force with a 'resultant' force.

How would you resolve the unknown velocity of projectile motion without vectors?

E.g. Q4

What I did was I tried to use a vector to resolve for the velocity components.

So I used v=u+at
u=28, t=0, a=9.8, v=?

v=28m/s

So we know that the horizontal component is constant (found in question 3)
We know that the net velocity is 28 so we use pythag.
(28^2-24^2)^0.5=14m/s (which is correct)

If I follow the same method for b, it is wrong.