Exercises: Gravitational, electrical 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?

Complete the sentence: "Gravity is a force of nature that attracts $\dots$ objects toward the center of the $\dots$, without needing to $\dots$ with them."

When you take off a fleece sweater 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?

Emma is playing with two bar magnets that have their poles labeled, 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 toward the north?

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 paper clip sticks to the refrigerator magnet.
3. c) When taking clothes out of the clothes 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 in? Does the Sun need to touch the planets to attract them?

Sophia brings a magnet close to a thumbtack, and it "jumps" toward the magnet before actually touching it, as shown in the figure.

1. a) What do we call forces that, like this one, act without contact?
2. b) Would a thumbtack made of plastic jump in the same way? Why?

In class, we rub two plastic rods with the same cloth and suspend one of them from a thread. When bringing the second rod close, the suspended one moves away, as shown in the figure.

1. a) Do the rods attract or repel each other?
2. b) What does the result indicate about the sign of their charges?
3. c) What type of force are we observing?

You bring a balloon that you have rubbed with wool close to a thin stream of tap water, and the water bends toward 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.

A $1\ \text{kg}$ kitchen scale 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 Moon, gravity is about $6$ times weaker than on Earth. With your strongest jump, you manage to lift your feet $30\ \text{cm}$ off the ground on Earth.

1. a) What height would you reach on the Moon with the same jump?
2. b) Explain why astronauts moved across the lunar surface by bouncing or taking small hops.

Mateo 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.

The needle of a compass points north, but when a powerful magnet is brought 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 north when there are no magnets nearby?
3. c) Would it be reliable to navigate using a compass inside a box full of magnets?

During a thunderstorm, electrical charges separate inside the clouds and suddenly lightning strikes the ground, as shown in the figure.

1. a) Which type of force of nature causes lightning?
2. b) Many buildings have a lightning rod: a metal point at the top connected to the ground. What is it for?

We test a magnet with objects from a drawer: a paperclip, a glass marble, a steel spoon, an eraser, an iron screw and a cork stopper.

1. a) Predict which ones the magnet will attract.
2. b) What do the attracted objects have in common?
3. c) Why do we say that the magnetic force is "selective" while gravity is not?

Compare the three forces of nature studied in this unit (gravitational, electric, 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.

Imagine that overnight Earth's gravity became twice as strong. Describe and justify three changes you would notice in your daily life, thinking for example of your backpack, the sport you play or how objects fall.

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 backpack there? Why?

You want to test how the strength of a magnet depends on distance. Design an experiment using a magnet, a paperclip and a ruler: explain how you would set it up, what you would measure, what you would compare and what result you expect to obtain.

During a thunderstorm in the countryside, it is advisable to avoid certain places.

1. a) Why is it dangerous to take shelter under a tall, isolated tree?
2. b) Lightning rods are placed at the highest point of buildings and connected to the ground: explain the reason for this position and connection.
3. c) Why do you think it is recommended to get out of the water at the pool or beach when there is a storm?

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?

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.

A spacecraft travels from Earth to Mars.

1. a) What force of nature acts on the spacecraft throughout the entire journey?
2. b) As it moves away from Earth, does the attraction that Earth exerts on the spacecraft increase or decrease?
3. c) When the spacecraft is much closer to Mars than to Earth, which body do you think will attract the spacecraft more strongly? Explain your answers considering the mass of the bodies and the distance.