Question

have you ever put a picture on the refrigerator? have you used a key card to unlock a hotel room door, listened to music through speakers, or saved information on a computer? all of these tasks, and many more, are made possible by magnets.
magnets come in different shapes and sizes. the magnets in a computer are thinner than a human hair. the magnets on industrial cranes can be several feet across. but all magnets have the same basic properties. a magnet is something that has a magnetic field—an area that exerts magnetic force. this force can attract metal or other magnets. it can also push other magnets away. the stronger the magnet, the more force the magnetic field exerts.
which of these exerts the most magnetic force?
choose one option
a a compass needle that points north but doesn’t attract objects
b a freezer door that uses magnets to stay shut
c a refrigerator magnet that holds up two pieces of paper
d a junkyard crane that uses magnets to pick up cars

Explanation:

To determine which exerts the most magnetic force, we use the text's idea that "the stronger the magnet, the more force the magnetic field exerts". We analyze each option:

• Option A: A compass needle that doesn't attract objects likely has weak magnetic force.
• Option B: A freezer door magnet keeps the door shut, but its force is for a smaller, lighter - duty task.
• Option C: A refrigerator magnet holding two papers has a relatively small magnetic force as it only needs to hold light papers.
• Option D: A junkyard crane magnet picking up cars needs a very strong magnetic force to lift the heavy weight of cars. So, the junkyard crane magnet exerts the most magnetic force.

Answer:

D. A junkyard crane that uses magnets to pick up cars