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### Course: Mechanics (Essentials) - Class 11th>Unit 8

Lesson 3: Can you push a car from inside?

# Conservation of momentum calculations

Conservation of momentum can be used to predict the motion of objects after a collision. Let's try two practice problems together. Created by Mahesh Shenoy.

## Want to join the conversation?

• For the second question, would we have gotten the same answer if we chose to have the other car be moving in a negative direction?
• yes
if two bodies are moving in opposite directions
you can choose any one body to be moving in negative direction.
you will get the same answer.
Here if you chose truck to be moving in negative direction, then--
(2kg)(-4m/s) + (1kg)(17m/s) = 3kgV
-8kgm/s + 17m/s = 3kgV
9kgm/s = 3kgV
V = 3m/s (The magnitude of velocity remain the same, only the sign changes)
In the question he assumed left as negative and right as positive direction..
Here we assumed right as negative and left as positive direction.
Since our answer here is positive, the whole thing will move towards the left. (the same answer as in the video)
• how did he calculate v at the end
• Here's the simplified version. I showed the velocity to the right as positive, and the velocity to the left as negative.

Legend -
M = mass of the truck
U = initial velocity of the truck
m = mass of the car
u = initial velocity of the car
V = total final velocity

MU + mu = (M+m)V
(2 * 4) + {1 * (-17)} = 3 * V
= 8 + (-17) = 3 * V
= -9 = 3 * V

V = -3 m/s

This means that the velocity of the two vehicles together will be to the left; the final velocity will be to the left.

Hope this helps!
• I don't understand how he cancelled the 10 kg from both the sides. Could someone help me out?
• 9kg * 10m/s = 10kg * V
(The 'V' given is a variable)

Now consider this statement as linear equation where the 'V' is the variable. Then what you would do is that transfer the 10kg from the RHS to the LHS leaving the 'V' in the RHS. So when the 10kg comes to the LHS it is going to divide (as signs change when we transfer from one side to the other) so 10 divided by 10 will give 0. So he cancelled the 10s just to reduce the amount of space taken.

Hope this Helps!
• And so whenever we have problems with collisions, we can always start with conservation of momentum.