Week 9 · Lesson 3 Gravitational Forces: contact and field forces, Earth’s gravitational field, mutual attraction, tides, orbits, mass and weight · 3 lessons
1
Forces that touch and forces at a distance
Science Prep 1 Term 1, Unit 2: pp. 47–54
GoalTell contact forces from forces at a distance, and say how gravity pulls things towards the centre of the Earth.
New words
contact force · a force that needs touching, such as frictionfield force · a force that acts at a distance through a fieldgravitational field · the space around the Earth in which gravity pulls
Explanation
Some forces need touching: a collision, the springiness of a stretched rubber band (elasticity) and friction are contact forces. Other forces act without touching, through a field: gravity, electric force and magnetic force are field forces. Gravity pulls every object towards the centre of the Earth. The Earth’s gravitational field is the space where this pull acts, and its field lines point towards the Earth’s centre. That is why a ball dropped in Cairo and a ball dropped on the other side of the world both fall down to the ground, which means towards the centre. A person sliding down a sand dune in Wadi El-Rayan is pulled down the slope by gravity.
Contact forces
Field forces (at a distance)
collision
gravity
friction
electric force
elasticity
magnetic force
Worked examples
Kicking a football is a contact force: the foot touches the ball. A magnet pulling a paper clip across a table, without touching it, is a field force.
object released→gravity pulls it→falls towards the Earth’s centre
A ball is dropped in Egypt and another is dropped on the other side of the world. Both fall towards the ground. Both are pulled towards the centre of the Earth, so “down” is a different direction in each place.
Layla says a rubber band pulling back when stretched is a field force, because she cannot see what pulls. But the pull comes from the band touching the hand and the weights, so it is a contact force (elasticity).
Class activity
Dropping test, with an adult. Hold a book and a flat sheet of paper at the same height and let go together. Then scrunch the paper into a ball and drop it with the book again. Gravity pulls both, but the air slows the flat paper more. Write what you saw. Sort ten forces from daily life into contact and field forces.
Practice
1
Name one contact force and one force that acts at a distance.
Contact: friction. At a distance: gravity.
2
Towards which point does gravity pull objects near the Earth?
Towards the centre of the Earth.
3
Is the pull of a magnet on a paper clip a contact force? Explain.
No. It acts at a distance, through the magnetic field.
4
What does a field line of the Earth’s gravitational field point towards?
