A child is sliding on a sled at 1.3 m/s to the right. You stop the sled by pushing on it for 0.80 s in a direction opposite to its motion. Part A If the mass of the child and sled is 34 kg, what is the magnitude of the average force you need to apply to stop the sled?

find acceleration first

a = vf - vi / t

a = 0 - 1.5 / 0.5s (vf is zero)

a = - 3 (negative sign indicates that acceleration is decreasing)

so F=ma

F = 35 x - 3

F = - 105 N (here negative sign indicates that u have to apply a force opposite to the boy's direction i. e from the left or towards the right)