The point that moves for the whole crowd

Science · Forces & Motion · ages 19-21

Name ______________________ Date ____________

A 2 kg lump sits at x = 0 and a 4 kg lump at x = 3. Where is the centre of mass?
Answer: ______________
What is the centre of mass of a system?
- The heaviest single particle
- The mass weighted average position
- The fastest moving part
Why can internal forces never move the centre of mass?
- They are always zero
- They pair up equal and opposite, summing to zero
- They act only on light parts
How does the centre of mass simplify total momentum?
- Total momentum always equals zero
- Total momentum equals the heaviest momentum
- Total momentum equals total mass times its velocity
A shell explodes mid flight. What path does its centre of mass keep?
- The original parabola
- A straight vertical drop
- A spiral around the fragments
With no outside forces acting, the centre of mass cruises at constant velocity.
Circle one: True False
A 1 kg lump sits at x = 1 and a 3 kg lump at x = 5. Where is the centre of mass?
Answer: ______________
Rae says the explosion knocks the shell's centre of mass off its arc, since fragments gain speed. What is wrong?
- Fragment speeds do not change at all
- The explosion is internal, so the average path continues
- Gravity switches off during explosions

Answer key

For grown-ups. Fold this page away before handing over the rest.

2 · (0 + 12) over 6 equals 2.
The mass weighted average position · It averages every position with mass as the weight.
They pair up equal and opposite, summing to zero · Each inside shove is answered by an opposite shove.
Total momentum equals total mass times its velocity · One point replaces pages of particle tracking.
The original parabola · The breakup is internal, so the average path is unchanged.
True · Only outside forces can change the total momentum.
4 · (1 + 15) over 4 equals 4.
The explosion is internal, so the average path continues · Fast fragments balance around the same steady average.