Chapter 12 – Forces and magnets

Consider the following statements and click to reveal the answer.

1. Bubbles float upwards in water. Can you think of any examples of things that float upwards in air?


A helium-filled, fairground balloon and a hot air balloon both float upwards in air.

2. Imagine that the Moon is being swung around the Earth very much like a ball on the end of a rope. What provides the force to enable the Moon to constantly change its direction (i.e. to accelerate)?


The force which is the gravitational attraction between the Earth and Moon is exactly the correct size to enable the Moon to orbit the Earth without flying off into space. (If the force happened to be smaller, the Moon would travel on a wider orbit.)

3. Which of the possible methods of reducing friction are used by an ice skater?


The friction between an ice skate and the ice is reduced by having a smooth surface on the base of the skate, reducing the area of contact with the ice and using the lubricating effect of the melted ice directly under the ice skate.

4. Magnets attract tin cans. Does that mean that tin is magnetic?


No. Tin is not magnetic. Tin cans are attracted because tin cans are actually steel cans coated with a very thin layer of tin to prevent rusting.

5. Draw the following simple situations involving a ball or a brick, and use arrows to show:

  • size of all of the forces acting on each ball or brick;
  • the direction of each force.
  1. A ball resting on a table
  2. A brick resting on a slope
  3. A ball just after it has been dropped from a hand
  4. A ball just after it has been thrown upwards by a hand
  5. A ball floating on water
  6. A brick as it drops through water
  7. A brick resting on the bottom of a container of water


In each drawing the ball or the brick will have a force acting vertically downwards on it which is its weight.

In addition to this force:

  1. The ball will have a force acting upwards on it, equal to its weight, provided by the table (the reaction force).
  2. The brick will have a friction force acting up the slope and a reaction force provided by the slope acting at right angles out of the slope.
  3. The ball will have a force smaller than its weight acting upwards on it. This is a small amount of air resistance.
  4. The ball will also have a small air resistance force acting in the opposite direction to the movement. (Note: there is no force acting in the direction of movement once the ball has left the hand.)
  5. The ball will have an upthrust force acting upwards which is equal to its weight.
  6. The brick will have an upthrust force acting upwards which is smaller than its weight. There will also be a small force acting upwards which is water resistance.
  7. The brick will have the same upthrust force acting upwards as in question 5f) (smaller than its weight). There will also be a reaction force acting upwards from the bottom of the container. The upthrust and reaction will equal the weight of the brick.