On pulling out the sheet from the magnetic field there is a decrease in magnetic flux occurs (due to a decrease in exposed area in the magnetic field ) which results in the production of induced current (due to production of induced EMF) in such a direction that opposes the decrease in magnetic flux.
Therefore, a considerable force is required to pull out the sheet from the magnetic field.
If there is an increase in the speed of pulling out the sheet from the magnetic field then the resulting induced current will also be larger.
Hence, more force is required with an increase in the speed of pulling out the sheet.
Similarly, on inserting the copper sheet there is an increase in magnetic flux which results in an induced current that flows in such a way to oppose magnetic flux.
Hence, a considerable force is required to insert the metal between the poles. Therefore, considerable force is required to insert the copper sheet between poles.