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 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.

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.

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?

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?

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.