Aplane flying horizontally above earth’s surface at 100. meters per second drops a crate. the crate strikes the ground 30.0 seconds later. what is the magnitude of the horizontal component of the crate’s velocity just before it strikes the ground? [neglect friction.]

Since the plane is traveling at 100m/s horizontal to the surface of the earth, the crate will always have this horizontal velocity (neglecting air resistance) until it hits the ground.  So the answer is 100m/s.  Note the vertical speed continues to change as the crate accelerates downward toward earth (again neglecting air effects)

This is what a calorimeter is used for simpler ex, to measure calories in food.