# Momentum Physics Gcse Coursework

https://isaacphysics.org/s/jRz6rc (this is the question if you wanted to see it in detail)

I'm completely stuck with a physics question on Isaac Physics. I realise that you aren't allowed to give me the answer, but I would appreciate it if you could tell me what I'm doing wrong in my working and direct me on the right path.

The question is in the link (but if you can't click it or something I pasted it):

Two trolleys are at rest and in contact on a smooth, level surface. A coiled spring in one trolley is released so that they 'explode' apart. The lighter trolley moves off at 50cm/s. (picture on link) Find the minimum energy which was stored in the coiled spring before the release.

I got the speed of the other trolley which was 20 cm/s (no idea why but that doesn't matter lol)

My working for this was:

kinetic energy = 1/2 x mass x speed^2

= 1/2 x 7 x 0.7^2 (because it's 0.7 m/s if you convert)

= 1.715

Isaac Physics says this is wrong, I've tried it with all numbers of sig figs, and I tried not converting from cm/s to m/s and it still said I was wrong, so I would really appreciate some help.Sorry to be so impatient but I'm seriously clueless and I would appreciate anyone's help!!

no no no no no, you have to calculate the kinetic energy of each object separately - don't add their velocities and masses together. Look a quick example:

if we have an object moving at 1 m/s and it has a mass of 1kg then its kinetic energy is 0.5 * 1 * 1^2=0.5J. Now if we have two of these objects we have a total kinetic energy of 2*0.5 = 1J. With your add the velocities and masses together method we have a total energy of 0.5 * 2 * 2^2 = 4J(Original post by**mundosinfin**)

https://isaacphysics.org/s/jRz6rc (this is the question if you wanted to see it in detail)

I'm completely stuck with a physics question on Isaac Physics. I realise that you aren't allowed to give me the answer, but I would appreciate it if you could tell me what I'm doing wrong in my working and direct me on the right path.

The question is in the link (but if you can't click it or something I pasted it):

Two trolleys are at rest and in contact on a smooth, level surface. A coiled spring in one trolley is released so that they 'explode' apart. The lighter trolley moves off at 50cm/s. (picture on link) Find the minimum energy which was stored in the coiled spring before the release.

I got the speed of the other trolley which was 20 cm/s (no idea why but that doesn't matter lol)

My working for this was:

kinetic energy = 1/2 x mass x speed^2

= 1/2 x 7 x 0.7^2 (because it's 0.7 m/s if you convert)

= 1.715

Isaac Physics says this is wrong, I've tried it with all numbers of sig figs, and I tried not converting from cm/s to m/s and it still said I was wrong, so I would really appreciate some help.

## Momentum calculations

Here is a worked example:

Two railway carriages collide and move off together. Carriage A has a mass of 12,000 kg and moves at 5 m/s before the collision. Carriage B has a mass of 8,000 kg and is stationary before the collision. What is the velocity of the two carriages after the collision?

### Step 1

Work out the total momentum before the event (before the collision):

p = m × v

Momentum of carriage A before = 12,000 × 5 = 60,000 kg m/s

Momentum of carriage B before = 8,000 × 0 = 0 kg m/s

**Total momentum before** = 60,000 + 0 = **60,000 kg m/s**

### Step 2

Work out the total momentum after the event (after the collision):

Because momentum is conserved, **total momentum afterwards = 60,000 kg m/s**

### Step 3

Work out the total mass after the event (after the collision):

**Total mass** = mass of carriage A + mass of carriage B = 12,000 + 8,000 = **20,000 kg**

### Step 4

Work out the new velocity:

p = m × v, but we can rearrange this equation so that **v = p ÷ m**

**Velocity** (after the collision) = 60,000 ÷ 20,000 = **3 m/s**

Watch this illustrated podcast on momentum for a summary of how momentum and motion are related:

## Comments