Daniel's Physics Questions

[danieltennis]

Hey guys,
A car is travelling with a constant speed of 80kmh passes a stationary motorcycle policeman. The policeman sets off in pursuit accelerating uniformly to 80kmh in 10 seconds and reaching a constant speed of 100kmh after a further 5seconds. At what time will the policeman catch up with the car?
What is the best way to approach a question like this and how would I solve it? Thanks

EDIT: Sorry I forgot to put "accelerating uniformly to 80kmh in 10 seconds"

[phagist_]

hint: You have to find when their displacements are the same.

[danieltennis]

hint: You have to find when their displacements are the same.

Obviously, but is sketching a graph the most effective way?

[cara.mel]

Two methods:
-draw a velocity-time graph and work out the time when the areas under the graph are equal
-use maths + straight line motion equations
It's essentially the same way, just some people need to visualise it

Your question is missing a piece of information though: "The policeman sets off in pursuit accelerating uniformly to 80kmh" - doesn't say how long this will take (or something similar), you can imagine the answer would change significantly if it accelerated at 1m/s vs reaching 80km/hr almost instantaneously

[phagist_]

I'd get two equations -

one describing the displacement of the car and one describing the displacement of the police car in terms of t.

then set them equal and solve for t.

[danieltennis]

I'd get two equations -

one describing the displacement of the car and one describing the displacement of the police car in terms of t.

then set them equal and solve for t.

The acceleration must be constant inorder to use the equations. So it's not possible to use equations in this case?

[phagist_]

Acceleration is uniform in this case.
for the first case, just use d=s*t1
second case, break it up into three parts - the acceleration parts;

using constant acceleration formulae;
find an expression for the distance traveled in each part;
d1 = from 0-80 (as cmel pointed out, you need more information)
d2 = from 80-100


and since he's going at 100 at constant speed, you can use d3=s*t2

so the TOTAL distance the police car travels is d1+d2+d3

Once you have an expression for distance for both vehicles, find the relationship between t1 and t2 then solve!

[Damo17]

Hey guys,
A car is travelling with a constant speed of 80kmh passes a stationary motorcycle policeman. The policeman sets off in pursuit accelerating uniformly to 80kmh (in 10 seconds) and reaching a constant speed of 100kmh after a further 5seconds. At what time will the policeman catch up with the car?
What is the best way to approach a question like this and how would I solve it? Thanks

Here's my working out. Sorry if it is hard to read. I find the best way to do this is draw the graph, but either way works. All depends on what way you were taught.

[cara.mel]

* caramel finds it amazing you can post a solution 4s before the question is fully written out

[Damo17]

* caramel finds it amazing you can post a solution 4s before the question is fully written out

I remember this question because I did it in transition. We must use the same text book.

[danieltennis]

hey,
A senior physics class conducting a research project on projectile motion constructs a device that can launch a cricket ball. The launching device is designed so that the ball can be launched at ground level with an initial of 28m/s at an angle of 30 degrees to the horizontal.
Calculate the vertical component of the velocity of the ball:
a/ intially
B/ after 1.0s
C/ after 2.0s

Thank you

[vce08]

initial vertical component is equal to 28 x sin 30 = 14m/s

after 1s this becomes 4m/s upwards since g= 10m/s^2 (in VCE physics at least)
after 2s this becomes 6m/s downwards. This is done by using the formula  v = u + at, where u is the initial vertical velocity

[methodsboy]

hey,
A senior physics class conducting a research project on projectile motion constructs a device that can launch a cricket ball. The launching device is designed so that the ball can be launched at ground level with an initial of 28m/s at an angle of 30 degrees to the horizontal.
Calculate the vertical component of the velocity of the ball:
a/ intially
B/ after 1.0s
C/ after 2.0s

Thank you

are u crazy
doing physics at like 2 am
WTF???!!!!

[danieltennis]

Hey,
A softball of mass 250g is thrown with an initial velocity of 16 m/s at angle (theta) to the horizontal. When the ball reaches its maximum height, its kinetic energy is 16J.

a. What is the maximum height achieved by the ball from point of release?

Thank you.

[Flaming_Arrow]

whats the answer?