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# A few questions... watch

1. Ok, erm, not sure how to answer these, anyone know:

A trolley of mass 2kg travelling at 3m/s collides with a stationary 1kg trolley. If the trolleys stick together calculate:
a) The total momentum before the collision
b) The speed of the two trolleys after the collision

7) Explain how the speed of a rocket can be increased when travelling hrough space

8) A bullet is fired into a block of wood, the collision is not elastic, explain why
2. a) momentum before = m1u1 + m2u2 (since u2 = 0) therfore momentum = m1u1
= 2 * 3 = 6Kgms^-1
3. (Original post by mackin boi)
a) momentum before = m1u1 + m2u2 (since u2 = 0) therfore momentum = m1u1
= 2 * 3 = 6Kgms^-1
What's u2?? and why would you have ^-1 ?
4. b) after joining the trolleys are taken as one object with mass = 1 + 2 = 3Kg (m3)
therefore by conservation of momentum: momentum before = momentum after
m1u1 = m3v1 (v1 is speed of the 2 together)
v1 = 6/3 = 2ms^-1
5. total momentum before is the product of the mass and velocity of the first trolley cos the first isnt moving, so 6 kg m s^-1

this is conseved, and the mass of the combined trolleys is 3kg, so the momentum of a 3kg trolley is 6 kg m s^-1, so its speed must be 2 m s^-2.

7. by firing the engines, the rocket is effectively pushing fuel in the opposite direction of its own travel. because of newton's third law, the force of the rocket on the fuel being expelled is equal to the magnitude of the force of the fuel on the rocket, and this force on the rocket causes acceleration

8. elastic collision is one in which kinetic energy is conserved. when the bullet impacts the energy it has is transferred into other forms, such as sound and heat in the wood.
6. (Original post by HearTheThunder)
What's u2?? and why would you have ^-1 ?
u2 is the speed of the second trolley (which is 0)
the ^-1 means per second (m/s is the same as ms^-1)

7. (Original post by HearTheThunder)
What's u2?? and why would you have ^-1 ?
I prefer not to think of m1 and u1 and all those letters, it can get too confusing. Just think about it as it were happening right in front of you, like my working, no equations with letters because that just takes more time I think.

But to answer your question, u2 is the velocity of the stationary particle. and the units of momentum is kg m s^-1, or "kilogram metres per second"
8. darn u mik1 for ure mathematical ability and speed typing
9. (Original post by mik1w)
total momentum before is the product of the mass and velocity of the first trolley cos the first isnt moving, so 6 kg m s^-1

this is conseved, and the mass of the combined trolleys is 3kg, so the momentum of a 3kg trolley is 6 kg m s^-1, so its speed must be 2 m s^-2.

7. by firing the engines, the rocket is effectively pushing fuel in the opposite direction of its own travel. because of newton's third law, the force of the rocket on the fuel being expelled is equal to the magnitude of the force of the fuel on the rocket, and this force on the rocket causes acceleration

8. elastic collision is one in which kinetic energy is conserved. when the bullet impacts the energy it has is transferred into other forms, such as sound and heat in the wood.
Oooo yeah i get it now thanks - but now that I understand that the probability that it wont come up in the exam is high

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