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 towards the centre of the $\dots$, without needing to $\dots$ with them."

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?

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 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.

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 rucksack, 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 rucksack there? Why?

Unit challenge. Design your own homemade dynamometer and analyse everyday forces in your environment.

1. Experimental calibration: Hang an elastic rubber band from a fixed support. Suspend different known masses from it using containers with water ($50\text{ ml}$, $100\text{ ml}$, $150\text{ ml}$, and $200\text{ ml}$, knowing that $100\text{ ml} \approx 0.1\text{ kg}$). Measure each elongation $\Delta l$ with a ruler, recording its uncertainty ($\pm 0.1\text{ cm}$), and calculate the corresponding weight with $P = m \times g$ (using $g = 9.8\text{ m/s}^2$).

2. Data processing: Construct a table with your values and plot the graph of $P$ versus $\Delta l$ to verify Hooke's law and deduce the spring constant $k$. Justify why the system reaches force equilibrium when the weight stops.

3. Non-contact forces: Rub a plastic pen vigorously with a wool garment and bring it close to a small piece of suspended paper (or bring a fridge magnet close to a suspended metal paper clip). Observe and reason how this electrostatic or magnetic interaction acts as an agent of change capable of overcoming rest at a distance.

Prepare a brief scientific report that includes the data table, the graph, and your reasoned conclusions.

Evaluation criteria:

• Rigour in home measurements and clear specification of instrumental uncertainty.

• Quality of the plotted graph and correct calculation of the spring constant $k$.

• Physical justification of static equilibrium and the effects of non-contact forces.

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?

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 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.

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 conductor: a metal point at the top connected to the ground. What is it for?

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 conductors 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?

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.

Aoife 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?

Aoife brings a magnet close to a drawing pin, and it "jumps" towards 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 drawing pin made of plastic jump in the same way? Why?

Cian 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 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?

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?

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.

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.

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.

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? Reason your answers considering the mass of the bodies and the distance.