Forces and Magnets: Contact and Non-Contact Forces
Objectives
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Know that forces can be used to make objects move
Measure the forces required to move objects on different surfaces
Describe how some forces need contact but the magnetic force does not
Plenary
Pupils are given mini whiteboards and are read the following series of situations in which a
force is used. Pupils should write down whether it is a push or a pull force.
1.
2.
3.
4.
5.
Old MacDonald uses a horse to heave a plough through his field.
Nikhita’s dog didn’t want a walk, so she had to drag it out of the door by the lead.
As a punishment, Sisyphus had to roll a boulder up a hill every day.
Lewis helped in the garden by tugging weeds out of the soil.
Rosie shoved her brother into the pool when their parents weren’t watching.
1, 2 and 4 are ‘pulls’. 3 and 4 are ‘pushes’.
Contact Forces
Explain that by using a force, an object can be made to move. Ask the pupils to suggest what
the difference is when a bodybuilder pushes a car on a road compared with when a toddler
pushes it. Pupils should identify that the bodybuilder can produce a bigger force and perform
better at the task.
Ask the pupils what would change if the road was icy. Pupils should recognise that ice is
slippery and may see that a slippery surface needs a smaller force to make things move on it.
EXPERIMENT: Set up slanting ramps for each group of pupils with a number of
interchangeable covers (e.g.: linoleum, carpet, artificial grass). Each group should also have a
wooden block and a Newton metre.
Explain that a Newton metre measures the size of a force and demonstrate how to read it.
The pupils should measure what forces are required to drag the wooden block up the ramp,
recording a measurement for each surface. Ask the students to put the surfaces in order
depending on how large a force was needed. By observing the characteristics of the surfaces,
the pupils should recognise that slippery surfaces need smaller forces than rough surfaces.
Pupils must note:
Forces can be used to make objects move.
Pupils may note:
The size of a force can be measured using a Newton metre.
Non-Contact Forces
Ask pupils for examples of an object being made to move without touching anything else.
Show the pupils a magnet and demonstrate moving a pin on a table top while the magnet is beneath
the surface. Name the force as the magnetic force and ask for suggestions for how it is different to
the pushes and pulls described before. They should be able to identify that the magnet is not touching
the pin, yet it has caused the pin to move.
Explain that all forces are contact forces, which need objects to be touching to work, or non-contact
forces, which do not. Pupils will be able to easily identify the magnetic force as non-contact.
Pupils must note:
Contact forces need two objects to be touching. Non-contact forces do not need objects to be
touching. The magnetic force is a non-contact force.
Possible extra-curricular questions
What makes some surfaces harder to drag things on than others?
When a rough surface resists motion, this is because of friction between the object and the
surface. Friction is a force which acts in the direction opposite to the direction of motion.
How do non-contact forces work?
Magnetic, electrostatic and gravitational forces are all non-contact and are caused by a
field. A field is a region of space in which objects are affected by a force. A magnetic field is
caused by a magnetic dipole (a magnet) while a gravitational field is caused by a mass and
an electrostatic field is caused by a charge.