This magnet attracts the compass and the magnetized needle without having to come in contact with them. This is an example of a force acting at a distance.

Which of the following is another example of a force that acts at a distance without actual contact?

1. Wind turning a windmill.

   No. The moving air is pushing against the blades of the windmill. Because a physical object (moving air) is pushing on the windmill, this is a contact force.

2. Sound causing you to hear something.

   No. For you to hear a sound, the vibrations of the object make the air vibrate. That is a contact force because there is direct contact between the vibrating object and the air. The vibrating air is in contact with your ear drum, causing it to vibrate too. That lets you hear the sound. Because a physical object (vibrating air) is moving your ear drum, this is a contact force.

3. Gravity causing a rock to fall.

   Yes. The gravitational attraction between the Earth and the rock does not need physical contact. The rock would fall, even if it was in a total vacuum, with no physical object to push or pull on it. This is a force acting at a distance.

4. Throwing a ball to knock over a target

   No. To throw the ball, your arm pushes against it. Your arm moving the ball is a contact force. The moving ball hits the target to knock it over. The ball is touching the target to knock it over, so this is also a contact force.

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