Physics Help Please!

Wats_in_a_name

The panicking is starting to set in!

Physics & Static Electricity Question has been doing my head in and I'd appreciate some help!

Find the magnitude & direction of the electric field strength at a distance of 2m from a charge of +4uC

Thanks in advance!

fungie

E field has a q/r^2 dependence, so just pop it into the formula for electric field and the direction of E field is the direction of positive charge would go, so the charge is positive so the field will point away from it.

Wats_in_a_name

Thanks for that! Got it worked out.

However new problem on a different chapter!

A car of mass 725kg is travelling along a straight level road at a uniform speed of 30m/s. The total resistance to its motion is 600N. If the engine is capable of exerting a force of 1000N, hat is the least time it would take the car to reach a velocity of 100 km/h if it starts from rest and the forces opposing it remain the same.

I know its and easy question but I can't even work out the formula to use.

jumpguy

Wats_in_a_name wrote: »

Thanks for that! Got it worked out.

However new problem on a different chapter!

A car of mass 725kg is travelling along a straight level road at a uniform speed of 30m/s. The total resistance to its motion is 600N. If the engine is capable of exerting a force of 1000N, hat is the least time it would take the car to reach a velocity of 100 km/h if it starts from rest and the forces opposing it remain the same.

Question is a bit unclear. Does the engine exert it's full force of 1000N as it accelerates to 100km/hr? And the reason it tells you the car is travelling at 30m/s is beyond me, when the question asks about the car starting at rest...

Anyway, assuming it does, the total forward motion is 400N (as 1000-600 = 400). The mass of the car is 725kg. F=ma, you have the mass and you have the force. a = F/m. Once you find acceleration, you now know what a is, u is (0 as it starts from rest), v (100km/hr, which you must convert to m/s), and you're looking for t (time)

v = u + at
(v-u)/a = t

Wats_in_a_name

Thank you so much!

Wats_in_a_name

I've got another question! Feel bad for keep asking people so Ill have a look at other threads and see if I can pass on some wisdom (not on physics though!)

A car and lorry are travelling along roads that intersect at right angles. The car of mass 1000KG is travelling at 50m/s and the lorry of mass 4000KG is travelling at 20m/s when they reach the intersection. If the vehicles coalesce on impact, find the magnitude and directoin of the velocity of the wreckage immediately after impact.

I know you need to draw a diagram and vectors but all I have is a right angle and not sure where the resultant goes?!

B_Fanatic

Basically draw two arrows perpendicular to each other and write the momentum of the lorry (mass x velocity) on one and the momentum of the car (mass x velocity) on the other. It might help you to visualise it better if you make the arrow with the bigger value (The lorry in this case) bigger and the one with the smaller value smaller. After you do this, connect the other two points of the line to make a triangle.

1. In order to get the resultant momentum of the two combined you can just use pythagoras' theorem as it will be a right angle triangle.
2. I'm not entirely positive about this one: In order to get the resultant direction you label one of the angles (But not the right angle) θ (You can label it anything really) and use the formula sinθ=o/h where o = the opposite angle, and h = the hypotenuse. Bring across sin to get θ = sin-1(o/h). insert in the opposite and hypotenuse and you've got θ. Don't know how to explain this part, but basically you should have a vertical line and a horizontal line on your triangle. Label them North or South, and East or West as appropriate and depending on where you took the angle θ from, you can say that the resultant momentum acts θ degrees north of east or θ south of east or whatever. Really don't know how to explain this without diagrams.

fungie

Im not going to do that but from reading it, it seems like a preety standard conservation of momentum problem

Wats_in_a_name

Thanks for all the help. I'm going to have a lot of questions today and tomorrow though. Some of this stuff I just can't get my head around.

For example

The decay constant of a certain radioactive isotope is 8x10^8. A sample of this isotope undergoes 3 x 10^3 disintegrations per second. What is the number of undecayed atoms in this sample?

brownlad

3.75x10-6




i think:p

magicianz

think it must be 2.67x10^5, just because the only other answer it 0.00000375 of an atom left :P

Donerkebab

Should the decay constAnt not be 8x10^-8? Pretty sure it cant be such a large number. So the answer is 3.75x10^10

magicianz

above makes more sense