UNIT-1: Chapter-4
MOMENTUM
Elistic collision
Again, if after the collision the two bodies instead of being attached to each other gets separated from each other and gets separated from each other, then that collision is called a partially elastic collision.
Total kinetic energy is not conserved in this type of collision also.
Now according to the conservation principle of momentum,
total momentum before = total momentum after collision
INELASTIC COLLISION
You just make a collision between two balls made of soft mud. You will see that total kinetic energy does not remain conserved in this collision. This type of collision is also an ideal event. It is an example of inelastic collision. That means, after collision between two bodies if they combine with each other and move as a single body i.e, the relative velocities of the two bodies become zero, then it is called inelastic collision.
CAR SAFETY
The force on a person in a car crash can be very large. Steep roads often have escape lanes filled with deep, soft sand. The soft sand slows heavy lorries that are out of control slowly – by making time, t, for the lorry to stop longer, the force, F, slowing the lorry is smaller and the driver is less likely to suffer serious injury.
Cars are now designed with various safety features that increase the time over which the car’s momentum changes in an accident.
Crumple zones are just one of the safety features now used in modern cars to protect the passengers in an accident. They only work if the passengers are wearing seat belts so that the reduced deceleration applies to their bodies too. Without seat belts the passengers will continue moving forward unit they come into contact with some part of the car or with a passenger in front. If they hit something that does not crumple they will be brought to rest a very short time, which, as we have seen in Example – means a large deceleration and, therefore, a large force acting on them.
WORK EXAMPLE 4.1
The first stage of the type of rocket used in Moon missions provides an unbalanced upward (away from the Earth) force of 30 MN and burns for 2.5 minutes.
a. Calculate the increase in the rocket’s momentum that results.
b. If the rocket has a mass of 3000 tones what is the velocity of the rocket after the first stage has completed its ‘burn’?
using the triangle.
As we have seen above this has been calculated with the mass of the rocket staying the same. In a real launch fuel is used up so the mass of the rocket will decrease and the calculation would be harder.
WORK EXAMPLE 4.2
A railway truck with a mass of 5000 kg rolling at 3 m/s collides with a stationary truck of mass 10 000 kg. The trucks join together. At what speed do they move after the collision?
We shall assume that friction forces are small enough to ignore, so we can apply the law of conservation of momentum:
Momentum before the collision = momentum after the collision.
লেখক:
Assistant Teacher (Senior section)
O and A level Physics Teacher
Bangladesh International School and College, Dhaka.
জাহ্নবী
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