Tap 220- 3: Inelastic collisions




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TAP 220- 3: Inelastic collisions
A railway wagon travelling at 1.0 m s–1 catches up with and becomes coupled to another wagon travelling at 0.5 m s–1 in the same direction. The faster moving wagon has 1.7 times the mass of the slower one. The slower moving wagon is half-full of liquid in a tank.

1. Immediately after impact, what is the speed of the coupled wagons?

2. As a result of the impact the liquid in the part-full wagon sloshes back and forth violently.

Describe the effect this will have on the movement of the wagons.

3. A 400 kg satellite travelling at 7.00 km s–1 is hit head on by a 1.00 kg meteorite travelling at 12.0 km s–1. The meteorite is embedded in the satellite by the impact. By how much is the satellite slowed down?

4. An American spacecraft of mass 18 tonnes docks with a Russian module of mass

6.6 tonnes, both travelling in the same direction. Assume the relative speed before docking is 0.20 m s–1 and a successful ‘soft’ docking is achieved first time.

Calculate how much faster the combined object is travelling than the original speed of the Russian space vehicle.


Practical advice

This set of questions provides useful momentum ideas.


Answers and worked solutions

1 m1v1 + m2v2 = m3v3 If the slower wagon has mass m.

(1.7m x1) + (m x 0.5) = (1.7m + m) v3

2.2 m = 2.7 m v3 so 2.2 = 2.7v3 and 2.2/2.7 = v3 so v3 = 0.81 (to 2sf) m s-1


2 The centre of mass moves at constant velocity after the impact. During the sloshing period, this means that the wagons do not move steadily. (As the liquid sloshes forwards, the truck will slow down; as it sloshes backwards, the wagon will speed up.)
3 m1v1 - m2v2 = m3v3

(400 x 7.00) – (1.00 x 12.0) = (400 + 1) v3 so 2788 = 401 v3 and v3 = 2788/401 =

= 6.95 km s-1

so the speed has changed from 7.00 km s-1 to 6.95 km s-1, a change of 0.05 km s-1.


4 v1 - v2 = 0.2 so v1 = (0.2 + v2)

m1v1 + m2v2 = m3v3

18(0.2 + v2) + 6.6 v2 = (18 + 6.6) v3

3.6 + 24.6 v2 = 24.6 v3



3.6 = 24.6 (v3 – v2) so (v3 – v2) = 3.6/24.6 = 0.15 m s-1
Alternatively from the viewpoint of the Russian Module


External references

This activity is taken from Advancing Physics Chapter 11, Question 150s


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