momentum at the start is equal to the momentum at the end
yeah i know that but u got (2) = 500v_1 + 350v_2)
where 
is the final speed of the cart and 
is the final speed of the coal. The momentum is the same, but the speeds could be different, how do u know here the speed of the coal and cart are the same?
While the coal is in the cart, it has the same speed as the cart. An instant after coal falls out, there are no forces acting on it apart from gravity, so it must have the same speed as the train.
also..
2. Two ice skaters, Dean and Melita, are performing an ice dancing routine, in which Dean( with a mass of 70kg) glides smoothly at a velocity of 2 ms^-1 due east towards a stationary Melita(with a mass of 50kg), holds her around the waist and they move off together. During the whole move, no significant friction is applied by the ice. Where is the centre of mass of the system comprising Dean and Melita 3s before impact.
(the answer in the book is, between Dean and Melita, 2.5m from Dean). How do u get that?
3. A car of mass 1500kg travelling due west at a speed of 20 ms^-1 on an icy road collidies with a truck of mass 2000kg travelling in the opposite direction at the same speed, the vehicles lock together after impact.
a) Which vehicle experiences the greatest change in velocity?
b) which vehicle experiences the greatest change in momentum?
c) which vehicle experiences the greatest force?
2. The formula for centre of mass is
At 3s before impact, the skaters are
apart. At this time, define the position of Dean to be
and the position of Melita to be
, then
(0)+(50)(6)}{70+50} = 2.5)
.
Hence, the centre of mass is 2.5m from Dean.
3.
a) Intuitively, the car with less mass should experience a greater change in velocity. You can confirm this by solving
+2000(-20) = v(1500+2000))
for v and noting the differences.
b) The change in momentum can be derived from part c). Since
and by Newton's third, the forces felt by each car is the same, the momentum change will be the same. You can also prove this from manual calculation.
c) By Newton's third law, both vehicles experience the same force.
4. It breaks the law of conservation of energy. You can derive through conservation of kinetic energy that
for elastic collisions. It is a tedious derivation, but have a go with:
and
ANYWAY, Back to the problem
BEFORE:
be speed of hitting ball,
be speed of stationary ball
AFTER:
be speed of hitting ball,
be speed of stationary ball
But
, so using
,
...[1]
Setting up the standard conservation of momentum equation:
 = m_1v_1 + m_2v_2)
 + m_2v_2)
(from the derived expression)
v_2 -m_1u_1)
v_2)
But
So if the masses of the two balls are equal, then the speed of the ball being hit will be equal to the speed of the hitting ball beforehand. The hitting ball loses all its velocity in order to conserve momentum. This is the basis for
Newton's pendulum. The reason why, in pool, you sometimes see the hitting ball continue, is because it does not directly hit the stationary ball.
Home
Help
Search
Login
