To practice tactics box 9.1 calculating the work done by a constant force. recall that the work w done by a constant force f⃗ at an angle θ to the displacement δr⃗ is w=f⃗ ⋅ δr⃗ =fδrcosθ. the vector magnitudes f and δr are always positive, so the sign of w is determined entirely by the angle θ between the force and the displacement.

Why is rain likely when warm, moisture-laden air meets cold air? a) the lighter warm air will rise and cool down, causing condensation and rain. b) the cold air moves faster and pushes the warm air away, causing condensation and rain. c) the moisture in the warm air condenses on contact with the cold air, causing rain to fall. d) the cold air mixes with the warm air, reducing its temperature causing moisture to condense.