Some physics qns!

davidche SG

Ok i got quite some qns. This is the result of my revision for today. XD

1) As the speed of car(whose brakes are spoilt) decreases, would the friction between wheels and land decrease too?

Wheels are hard to picture. If i decrease my horizontal force applied on a moving rubber on a table, does the friction acting against my decreasing movement decrease?

2)Air passing through a chamber with cold tubes are cooled by conduction or convection?

3) For a non-conductor, are the negative charges able to move ard? Eg if a positively charged metal bar is placed near to a non-conductor, would the negative charges be attracted to that side?

4)(hard one here XD)

A car of mass 700kg with initial kinetic energy of 560KJ and initial speed of 40m/s approaches a slope of height 3m. Note that the car does not brake.

a) Find the change in gravitational potential energy.(this is not impt i am just posting a ''fuller'' qn.)

b) Find the average frictional force on the car if the car comes to rest after 40m.

Thanks!

skythewood SG

1) think the other way. as the car speed up, would the friction increase? yes.

2) conduction. convection has a different definition.

3) no. that's what you should answer at least.

4) smiley face

skythewood SG

4 b) work done = force x distance.
work done = frictional force x distance.

davidche SG

ok i understand thats what i did initially.

this is what i did. (560000- 21000(GPE)) = F (40)

SBS1984E SG

but this is assuming frictional force stays constant.

skythewood SG

but this is assuming frictional force stays constant.

nope.

this is just finding average frictional force.

davidche SG

but this is assuming frictional force stays constant.

they asked for average frictional force. DOes this info have sth to do with like "you shldnt use energy method to solve"?

davidche SG

btw the answer for my inital method is wrong. XD

skythewood SG

sigh... is the slope upwards or downwards?

SBS1984E SG

they asked for average frictional force. DOes this info have sth to do with like "you shldnt use energy method to solve"?

Oh yea... paiseh.

davidche SG

yo err sky, can solve?

davidche SG

oh do you want the model solutions? again there is a reason why i asked this qn even tho i have the solutions.

The first step of the solution is t=2s(just like tat) How does he know t=2s!!

Also, i tot could use energy method to solve XD

jlzk SG

Urgent Help!!

Calculate the density of the ice at 0 degree...Er..Someone help pls..!! I get 1 g/cm^3 but the answer in the book say 0.909g/cm^3.

davidche SG

what! ok i like this.

upwards. It is supposed to slow the car down XD

skythewood SG

sorry, was talking on the phone with girlfriend.

just give me the final answer.

t=2s? where did that come from?

MyNameisNas SG

Urgent Help!!

Calculate the density of the ice at 0 degree...Er..Someone help pls..!! I get 1 g/cm^3 but the answer in the book say 0.909g/cm^3.

Since u posted the ans. i do it in reverse.

Take volume of 0 deg. at 11cm^3

Use D=M/V= 10/11= 0.909...91

Hence density is 0.909cm^3

MyNameisNas SG

0.909g/cm^3

skythewood SG

the other q, the force thingie

skythewood SG

from the graph, ice has a volume of 11 cm cube at 0 degree celcius.

density = 10g divide by 11cm cube
= 0.909

the key is the diagram

jlzk SG

Since u posted the ans. i do it in reverse.

Take volume of 0 deg. at 11cm^3

Use D=M/V= 10/11= 0.909...91

Hence density is 0.909cm^3

But why u take 11 but not 10?Pretend you did not see the answer how u know is take 11 and not 10 cause in 0 degree they are at 11-10