A-level physics multiple choice question

mc4.docx

For the first one:
I worked out the mass to be 0.2kg, what about the velocity to do mv. Or is there another way to work it out?

For the second one:
I know that the total momentum is unchanged because mu=mv ( momentum is always conserved). So it must be B or A, and the answer is B. But why does the kinetic energy decrease.

It would be much appreciated if someone can explain these questions!

Reply 1

Walker16

What's the question and the multi choice answers?

Reply 2

Dat1Guy

Original post by Mark jam

For the first question, you are told how much water passes through PER SECOND, and you correctly calculated the mass that passes per second.
The equation ft=mv-mu gives the momentum, but you have been given it for 1 second, i.e. t is 1, so you can calculate the force. But because the question says how much momentum leaves PER SECOND you can just use 1 second again. If it had of said how much momentum leaves in 5 seconds, you would times f by 5.

For the 2nd question, kinetic energy doesn't have to be conserved. Remember, in a collision there will be friction, which leads to work being done, and so heat loss - it is an inelastic collision (kinetic energy is lost). In almost every collision there is always a slight bit of heat loss due to friction, we just often say there is none lost to make it easy for ourselves.

Reply 3

Mb.68-72

Original post by BDunlop

You cant calculate the force as you from ft=mv-mu, you only have m and t

So?

Reply 4

have

Original post by Mark jam

You find the velocity be seeing how far the water travels in one second. In one second, if the cross sectional area is X, and the volume is Y, wouldn't you agree that the "length" of the water is Y/X, and therefore the distance travelled by the water in one second is Y/X

For the second question, you should know that a collision where the two bodies move together at the same velocity after the collision is called a totally inelastic collision. In this type of collision, the maximum amount possible of kinetic energy is lost. However, here all you need to know is that some kinetic energy is lost.

Reply 5

Dat1Guy

Original post by Mark jam

You cant calculate the force as you from ft=mv-mu, you only have m and t

So?

Oh, i'm sorry.

The units for speed are ms^-1
Let e=x10^
you are given 2e-4m³s^-1
and 7.2e-4 m²
if you divide the rate per second by CSA you'll get speed = 5/18 ms^-1

Reply 6

Anonymouspsych

Original post by Mark jam

1) Using dimensional analysis you know that the SI units of momentum will be kg m s^-1 as 1 Ns = 1 kg m s^-1
So one way you can arrive at the answer is trying to aim to get to these units from the quantities given in the question.
You know the density is 1000kg m^-3 and the rate at which the water is leaving is 2*10^-4 m^3 s^-1 so the mass of water leaving per second =1000*2*10^-4 = 0.2 kg. You can find the speed of the water flow by doing (2*10^-4)/(7.2*10^-4) = 0.27777... m/s . You know momentum = mv
so the momentum the water carries per second as it leaves the hose = mass of water leaving per second * speed of water = 0.2 *0.2777.... = 0.056 Ns

2) Momentum is always conserved in collisions so you can eliminate C and D immediately. It says the two trucks move together so you they are stuck after the collision so you know this is going to be an inelastic collision. This means Kinetic energy isn't conserved which leaves you with one answer to pick..