Consider two interacting objects. If object 1 pushes on object 2 with a force F = 10 N for 2 s to the right, then the momentum of object 2 changes by 20 Ns = 20 kgm/s to the right. By Newton's third law object 2 pushes on object 1 with a force F = 10 N for 2 s to the left. The momentum of object 1 changes by 20 Ns = 20 kgm/s to the left. The total momentum of both objects does not change. For this reason we say that the total momentum of the objects is conserved.
Newton's third law implies that the total momentum of a system of interacting objects that are not acted on by outside forces is conserved.
The total momentum in the universe is conserved. The momentum of a single object, however, changes when a net force acts on the object for a finite time interval. Conversely, if no net force acts on an object, its momentum is constant. For a system of objects, a component of the momentum along a chosen direction is constant, if no net outside force with a component in this chosen direction acts on the system.
Collisions
In collisions between two isolated objects Newton's third law implies that momentum is always conserved. In collisions, it is assumed that the colliding objects interact for such a short time, that the impulse due to external forces is negligible. Thus the total momentum of the system just before the collision is the same as the total momentum just after the collision. Collisions in which the kinetic energy is also conserved, i.e. in which the kinetic energy just after the collision equals the kinetic energy just before the collision, are called elastic collision. In these collisions no ordered energy is converted into thermal energy. Collisions in which the kinetic energy is not conserved, i.e. in which some ordered energy is converted into internal energy, are called inelastic collisions. If the two objects stick together after the collision and move with a common velocity v_{f}, then the collision is said to be perfectly inelastic. | |
Note: In collisions between two isolated objects momentum is always conserved. Kinetic energy is only conserved in elastic collisions. We always have m_{1}v_{1i }+ m_{2}v_{2i
}= m_{1}v_{1f }+ m_{2}v_{2f}. |
If two objects collide and one is initially at rest, is it possible for both to be at rest after the collision? Is it possible for one to be at rest after the collision? Explain!
In collisions between two objects momentum is conserved. Since the
initial momentum is not zero, the final momentum is not zero. Both objects
cannot be at rest.
It is possible for one of the objects to be at rest after the collision.
For example, if the masses of the two objects are equal, then after a
head-on elastic collision the object initially at rest is moving and the
object initially moving is at rest.
Problem:
A 10 g bullet is stopped in a block of wood (m = 5 kg). The speed of the bullet-wood combination immediately after the collision is 0.6 m/s. What was the original speed of the bullet?Problem:
Two cars of equal mass and equal speeds collide head on. Do they experience a greater force if the collision is elastic or perfectly inelastic and they stick together?
Problem:
A 90 kg fullback running east with a speed of 5 m/s is tackled by a 95 kg opponent running north with a speed of 3 m/s. If the collision is perfectly inelastic, calculate the speed and the direction of the players just after the tackle.The initial momentum of player 1 is p_{1 }=
(90 kg)(5 m/)s i =
450 kgm/s i.
The initial momentum of player 2 is p_{2 }=
(95 kg)(3m/s)
j = 285 kgm/s j.
Momentum is conserved, the final momentum p of both players is p
= p_{1 }+ p_{2}.
p = (m_{1 }+ m_{2})v.
v = 2.432 m/s
i + 1.54 m/s j.
v^{2 }= 8.29 (m/s)^{2}, v = 2.88 m/s.
The speed of the players after the collision is 2.88 m/s.
tanθ = p_{y}/p_{x }= 285/450 = 0.63, θ = 32.34^{o}.
Their direction of travel makes an angle θ = 32.34^{o} with the
x-axis. (The x-axis is pointing east.)