set.seed(234234)
library(data.table)
poolballs <- data.table(
    number = 1:15,
    weight = rnorm(15, 45.7, 0.8),
    diameter = c(3, 2.9, 3.1) #shows recyling
) 
poolballs
    number   weight diameter
 1:      1 45.59534      3.0
 2:      2 45.15776      2.9
 3:      3 45.81486      3.1
 4:      4 45.30962      3.0
 5:      5 45.55232      2.9
 6:      6 46.17808      3.1
 7:      7 45.80789      3.0
 8:      8 45.80801      2.9
 9:      9 46.34040      3.1
10:     10 45.23618      3.0
11:     11 46.47511      2.9
12:     12 45.47280      3.1
13:     13 45.98953      3.0
14:     14 45.40827      2.9
15:     15 45.24824      3.1

I want the following to show only head in line 2.

poolballs[ , weight2:=2 * weight]
    number   weight diameter  weight2
 1:      1 45.59534      3.0 91.19067
 2:      2 45.15776      2.9 90.31552
 3:      3 45.81486      3.1 91.62973
 4:      4 45.30962      3.0 90.61925
 5:      5 45.55232      2.9 91.10464
 6:      6 46.17808      3.1 92.35617
 7:      7 45.80789      3.0 91.61577
 8:      8 45.80801      2.9 91.61602
 9:      9 46.34040      3.1 92.68080
10:     10 45.23618      3.0 90.47236
11:     11 46.47511      2.9 92.95022
12:     12 45.47280      3.1 90.94560
13:     13 45.98953      3.0 91.97905
14:     14 45.40827      2.9 90.81654
15:     15 45.24824      3.1 90.49648
head(poolballs)
   number   weight diameter  weight2
1:      1 45.59534      3.0 91.19067
2:      2 45.15776      2.9 90.31552
3:      3 45.81486      3.1 91.62973
4:      4 45.30962      3.0 90.61925
5:      5 45.55232      2.9 91.10464
6:      6 46.17808      3.1 92.35617