P8 Forces in Balance

Question 1

There is a thin sheet of plastic. The sheet is 250 mm wide. Two holes,
each with a radius of 2 mm, have been drilled through the sheet.

Describe how you could use:
• a clamp and stand
• a steel rod 100 mm long and with a radius of I mm
• a weight on a thin piece of string (= a plumb line)
• a ruler
• a pen which will write on the plastic sheet

to find the centre of mass of the plastic sheet.

Answer 1

• clamp (steel) rod (horizontally)
• hang plastic / sheet by rod through (one) hole
• hang plumb line from rod
• mark ends of plumb line on the sheet and
  use the ruler to draw a straight line
  -Repeat with other hole
• Centre of mass is where the lines cross
• Check by balancing at this point

Question 2

As a skateboard starts to recoil, it soon slows down and its kinetic energy
decreases.
Explain why.

Answer 2

work is done against friction transforming kinetic energy into heat

Question 3

In a collision, momentum is not always conserved.

Why?

Answer 3

An external force acts (on the colliding objects)

Question 4

A skateboarder jumps forwards off his skateboard.
The skateboard is stationary at the moment the skateboarder jumps.
(a) The skateboard moves backwards as the skateboarder jumps forwards.
Explain, using the idea of momentum, why the skateboard moves backwards.

Answer 4

• Momentum before jumping = momentum after
  jumping
  -Before jumping momentum of skateboard and
  skateboarder is zero
• After jumping skateboarder has momentum forwards so
  skateboard must have equal momentum backwards