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**careless25****Real Member**- Registered: 2008-07-24
- Posts: 560

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

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**integer****Member**- Registered: 2008-02-21
- Posts: 79

careless25 wrote:

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

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**careless25****Real Member**- Registered: 2008-07-24
- Posts: 560

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...

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**quittyqat****Member**- Registered: 2009-04-08
- Posts: 1,215

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

I'll be here at least once every decade.

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**mathsyperson****Moderator**- Registered: 2005-06-22
- Posts: 4,900

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.

Why did the vector cross the road?

It wanted to be normal.

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**careless25****Real Member**- Registered: 2008-07-24
- Posts: 560

@ 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.

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**careless25****Real Member**- Registered: 2008-07-24
- Posts: 560

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

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**glenn101****Member**- Registered: 2008-04-02
- Posts: 108

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)*

"If your going through hell, keep going."

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