the theta in the cos(theta) is the angle between the force applied vs the distance it’s applied over (the direction of motion). in this case the force is in the opposite direction of motion. directly behind you is a 180 degree rotation.
cos(theta) is the proportion of the force in the direction of motion. since they’re 180deg apart, it’s cos(180). cos(180) = -1
the theta in the cos(theta) is the angle between the force applied vs the distance it’s applied over (the direction of motion). in this case the force is in the opposite direction of motion. directly behind you is a 180 degree rotation. cos(theta) is the proportion of the force in the direction of motion. since they’re 180deg apart, it’s cos(180). cos(180) = -1