Math Is Fun Forum

careless25

Real Member

Registered: 2008-07-24

Posts: 560

Physics

Quick help

A car traveling at 90km/h is 100m behind a truck traveling 50km/h. How long will it take the car to reach the truck?

thx

integer

Member

Registered: 2008-02-21

Posts: 79

Re: Physics

Quick help

A car traveling at 90km/h is 100m behind a truck traveling 50km/h. How long will it take the car to reach the truck?

thx

careless25

Real Member

Registered: 2008-07-24

Posts: 560

Re: Physics

hahaha thx both of u...i needed a quick physics review.

k 1 more that i m confused about.

On the trip to Sydney, you get stuck behind a slow car traveling 90km/h in a 100km/h zone for 25km. How long must you travel at a speed of 110km/h to make up the lost time?

I feel stupid rite now cus i kno how to do it but just cant remember it...

quittyqat

Member

Registered: 2009-04-08

Posts: 1,215

Re: Physics

Both? There's only 1 post in between the 1st post and the 3rd post.

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mathsyperson

Moderator

Registered: 2005-06-22

Posts: 4,900

Re: Physics

Same general principles as the first question, just a bit different.
You travelled at 90 km/h for 25km, which means you did that for 25/90 = 0.277h = 16m 40s.

During that time you travelled 10km/h slower than you should have, so travelling 10 km/h faster than you should for the same length of time will get you back on schedule.

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Why did the vector cross the road?
It wanted to be normal.

careless25

Real Member

Registered: 2008-07-24

Posts: 560

Re: Physics

@ quittyqat: JFF replied with a hint but u already posted an answer so she deleted her post....thx tho
@ mathsyperson: i wasnt thinking straight yesterday.

careless25

Real Member

Registered: 2008-07-24

Posts: 560

Re: Physics

1 more question...

A person of mass 70kg jumps up and lands on a scale, causing the scale to read 750 N. Find the persons weight and acceleration.

I think i solved it but i m not sure if it is correct. All unit are SI so theres no constant k.
i did:

force = mass * acceleration

i used that formula to find the acceleration:
750 N = 70kg * a
a = 10.71m/s^2

to find weight i first found out the force if the person hadnt jumped.

F = m * a
F = 70kg * 9.8m/s^2
F = 686N

then i used that to find 'm' in the equation, but using the acceleration from above.

686 N = m * 10.71m/s^2
m = 64.1 kg

I m not sure if this is the right way to find the weight and acceleration of the person. Pls comment if u find anything incorrect.
I m unusure of my answer because i use the equation to find weight when the equation has mass in it. But thats the only equation we are to use to solve this except Fnet = Fa + Fg+....

Thx in advance,

C25

glenn101

Member

Registered: 2008-04-02

Posts: 108

Re: Physics

Hi careless25,
the way I go about motion questions is to first list my data; Oh and is this on Earth? or we don't know?
m= 70kg
Fscales=750N
W=686N
Now for weight we I don't like to write F=ma
I instead write W=mg
where W= weight of object
          g= gravitational field strength at his location
and hence W=70(9.8) or (10)
                   = 686N and that is his actual weight.
The scales read differently as he "jumped" on it however 686N is still his weight.
and so;
Weight = 686N
a should be= 9.8ms^-2
but since the scales read greater than his weight, his acceleration read by the scales as you have recalled would be
F=ma
750=70a
a=10.71ms^-2

oh and MASS NEVER CHANGES!

Last edited by glenn101 (2009-07-08 13:28:04)

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