Question, this is engineering and physics (2 Viewers)

[Twickel]

Well, I put a model car down a ramp and I used a=gSinx to calculate the acceleration but I forgot that the wood provides a friction which effects the acceleration.

How can I calculate the constant friction provided by the wood.

 

[Aplus]

Aren't we taught in High School Physics (Preliminary at least), to ignore any external factors of influence when making calculations?

 

[Sandchairs]

If its engineering then they would have given you the coefficient of friction if they had wanted you to inlude it.

 

[Twickel]

I showed my teacher and she said thats wrong and I will lose marks thats BS how am I to calculate the friction.

 

[lolokay]

is friction constant? if it is, you can measure the time, and manipulate equations from there

net force = force from gravity - frictional force (f)
a = gsin@ - f/m
s = 1/2(gsin@-f/m)t^2

 

[Twickel]

I do not know the frictional force. Thats what im trying to figure out. Im just so annoyed I cant really think atm.

 

[lolokay]

I just gave you an equation that can (hopefully) determine the friction

 

[Twickel]

s= displacement?
-f/m whats that f meant to be?

 

[Twickel]

Look here is the thing im looking for.
A trolley rolls on a footpath at an angle of 30 find the force if the footpath exerts a constant friction force of 270N one the trolley.

I need that friction force.

 

[Twickel]

Fo F=un is for frictional force. Ok now what is the u and n in my case. I need to know how this will effect the velocity of the car as it hits the wall more then anything. I used 2gh=v^2 for final velocity, so how will this effect the final velocity. I know for force it will= Wx-friction then rearrange that to get a.

 

[Twickel]

The assignment is modeling a vehicle safety device so I used a model car.
So the frictional force = 0.2 x 0.35 ( mass of my model) x 9.8 x Cos 30
So now how can I find the final velocity using a.

 

[Twickel]

So the final velocity = acceleration x sqrt of 2 distance/a , I did this experiment down a ramp assuming I could calculate a=gsinx because I thought gravity was the only force acting on the car, but I forgot resistance.

 

[Twickel]

I wonder if my teacher will know that, but ummm I guess that goes under the reliability section of the discussion, but ATLEAST now this is a little more reliable, I mean before it was just no forces at all. No resistance from the wood.

 

[Twickel]

So as the angle increases the friction decreases?

 

[Twickel]

So a=gSin @ would have been better then me finding the frictional force and then acceleration from that? Today I told her that I used a=gsin@ and she said thats wrong because your not taking the friction into consideration.
The differences are big a using gsinx=4.9 using frictional force = 3.23.

God im nervous I put so much effort in this during the holidays and by the sounds of things from what the teacher said it looks like ill be getting a bad mark.

 

[Twickel]

In my textbook it has an example of a trolley down an ashphalt surface they say the friction force is 217 how come they do not take into consideration the rotational friction force.

 

[Twickel]

The aim is to model a safety device, I decided to do a crumple zone, I used software analysis to calculate the time of collision, it increased with the can attached, so the force decreased. BUT for me to calculate the force during the collision I needed F= m ( v-u) /t u is the speed just as its about to hit the wall and V is always 0. Then I calculate impulse and momentum. Hey is it possible to get a negative momentum?

 

[Twickel]

For some strange reason as the speed of the car increased the time of collision decreased, is that ok? Also the net force = the kinetic energy.

 

[Twickel]

The final knietic energy is always 0 , so work = kinetic energy, and the kinetic energy is equal to the net force.

 

[Mumma]

In the ideal case (no slip between the wheel and the ground) no energy is lost due to friction with the ground. The friction losses are mostly in the axle bearings and such (assuming the wheel is mostly inflexible). That's impossible to calculate so stick with experimental data (divide distance by time taken).

Kinetic energy is NOT equal to the net force ... not sure where you got that from. Consider something moving in space with no net force (constant velocity relative to something) - it will still have kinetic energy.

The NET WORK DONE by the net force is equal to the CHANGE in kinetic energy of the object. So, during the collision, the work done by the force would equal to the change in kinetic energy of the car.

Work = force * distance.
So the collision force * collision distance = work = change in kinetic energy
(You can ignore gravity since it acts perpendicular to the distance traveled, and the effects of air resistance during collision are negligible)

What is the collision distance? I suppose that can be approximated by how much your crumple zone deflects. The change in kinetic energy is simply your initial kinetic energy since the final kinetic energy would be zero (assuming its motionless after collision, that is it bounces very little)