Gravitational, Electric, and Magnetic Phenomena

You drop your sandwich and it lands on the floor. What force brought it down there? Did anything need to touch it or push it down? What are forces that act without contact called?

A $1\ \text{kg}$ kitchen weight travels to the Moon.

1. a) Does it still have a mass of $1\ \text{kg}$ there?
2. b) Does it weigh the same on the Moon as on Earth?
3. c) Explain in your own words the difference between mass and weight.

On the surface of Mars, gravity is about $3$ times weaker than on Earth.

1. a) If you weigh $450\ \text{N}$ on Earth, how much would you weigh on Mars?
2. b) Would your mass have changed? Justify your answer.
3. c) Would it be easier or harder to lift your rucksack there? Why?

When you take off a fleece jumper in winter you hear crackling sounds and notice that your hair stands on end. What type of force produces these effects: gravitational, electrical or magnetic? Does it act by contact or at a distance?

You bring a balloon that you have rubbed with wool close to a thin stream of tap water, and the water bends towards the balloon, as shown in the figure.

1. a) What type of force deflects the water?
2. b) Why do we say that it acts at a distance?
3. c) Would the stream bend if the balloon had not been rubbed? Justify your answer.

After rubbing a balloon with a dry cloth, the balloon attracts your hair even though the hair is not charged.

1. a) Explain how the balloon can attract hair without touching it.
2. b) The trick stops working after a while: where do you think the charges on the balloon go?
3. c) Predict whether the experiment will work better on a dry day or a humid day, and give reasons for your answer.

Marta is playing with two bar magnets that have their poles labelled, as shown in the figure.

What happens if she brings two north poles together? What if she brings a north pole and a south pole together? Write the general rule for magnetic poles.

What is a compass and what has it been used for over the centuries? Why does its needle always point approximately towards the north?

The needle of a compass points to the north, but when bringing a powerful magnet close, it deflects from its position.

1. a) Why does the needle deflect?
2. b) What does this tell us about why the compass points to the north when there are no magnets nearby?
3. c) Would it be reliable to navigate using a compass inside a box full of magnets?

A hiker has lost their compass, but they have a sewing needle, a small magnet and a cork stopper, and they can get a glass of water.

1. a) Propose a method to make an improvised compass using these objects.
2. b) Explain why the needle will end up pointing approximately north.
3. c) What would happen to all the compasses in the world if the Earth stopped behaving like a magnet?

Classify these phenomena according to the force of nature that produces them (gravitational, electrical or magnetic):

1. a) A pear falls off a tree.
2. b) A paperclip sticks to the fridge magnet.
3. c) When taking clothes out of the tumble dryer, some socks stick to a t-shirt.
4. d) The Moon orbits the Earth without escaping.

The planets of the solar system orbit the Sun without leaving their orbits. What force holds them back? Does the Sun need to touch the planets to attract them?

Teo moves a paperclip on top of the table by sliding a magnet under the tabletop, without touching the paperclip at any point, as shown in the figure.

1. a) What type of force acts on the paperclip?
2. b) Name another force of nature that also acts without contact.
3. c) State one similarity and one difference between the magnetic force and the gravitational force.

Compare the three forces of nature studied in this unit (gravitational, electrical and magnetic) by answering these questions for each one: does it attract all objects or only some?, can it also repel?, does it need contact? Finish by giving an everyday example of each.