Contact and non contact forces

• Physics
• Lower Secondary (Key Stage 3)

Contact and non contact forces

What are forces?

What is a force?

A force is a push or a pull that acts on an object due to the interaction with another object.

Force is measured in newtons (N).

There are two main categories of forces - 

contact forces

 and 

non-contact forces

.

Three question marks

Did you know?

You cannot see a force but you can see its effect. For example, when you apply a force to push a door open

Contact forces

Contact forces act between two objects that are physically touching.

Here are some examples of contact forces:

Tension

Friction

When two objects slide past each other they experience friction.

Friction is a force that acts between two touching surfaces and prevents or resists them moving against each other.

Air resistance

When an object moves through the air it experiences air resistance. This force acts against the direction of movement.

The faster the object is travelling the greater the air resistance.

Upthrust 

Upthrust is an upwards force that acts on an object when it is in a fluid (a liquid or gas).

Thrust

Thrust is a driving force exerted by an engine to make an object move.

Normal contact force

When an object pushes on a surface like a table, wall or the ground, the surface pushes back on the object with a balancing force.

The normal reaction force always acts at right angles to the surface.

Other examples of friction

Friction can sometimes be useful and sometimes it can be a nuisance.

Identify whether friction is useful or not in these situations.

The brakes on a bike

Inside the hub of the wheel on a bike

In the moving parts of the chain and gears

Which force propels a rocket upwards into the sky?

1. Friction
2. Tension
3. Upthrust
4. Thrust

What is NOT useful when talking about friction?

1. Rough surfaces
2. Nature of materials
3. Weight of objects
4. Smooth surfaces

Which force is responsible for the tension in a stretched rope or cable?

1. Thrust
2. Air resistance
3. Tension
4. Upthrust

What force pushes an object upwards when submerged in a fluid?

1. Upthrust
2. Tension
3. Air resistance
4. Friction

Which of the following forces acts to oppose an object's motion through a fluid?

1. Thrust
2. Tension
3. Friction
4. Air resistance

Which force opposes the motion of an object through air?

1. Air resistance
2. Thrust
3. Upthrust
4. Tension

What force is experienced when an object moves over a surface?

1. Air resistance
2. Friction
3. Thrust
4. Upthrust

Which force pushes or pulls an object upwards in a fluid?

1. Friction
2. Tension
3. Thrust
4. Upthrust

What is not useful when talking about friction?

1. Rough surfaces
2. Smooth surfaces
3. Type of material
4. Weight of the object

Which force keeps a satellite in orbit around the Earth?

1. Magnetic force
2. Gravitational force
3. Nuclear force
4. Electrostatic force

Non-contact forces

Non-contact forces act between two objects that are not physically touching.

Magnetic Force

Magnetic force is experienced by a magnet or a magnetic material, eg iron, when placed in a magnetic field.

This force can pull the two objects together or push them apart.

Electrostatic Force

Electrostatic force is experienced by a charged particle in an electric field.

This force can be either attractive or repulsive.

Gravitational Force

Gravitational force is experienced by a mass when it is sufficiently close to another mass.

Gravity always pulls two objects towards each other. It never pushes them apart.

In the diagram, Earth pulls the satellite and the satellite pulls Earth. The forces are equal in size and opposite in direction.

Measuring Forces

Measuring forces

Weighing an apple using a newton meter.

Image caption,

A force meter, also called a newton meter, is used to measure forces

A force meter, also known as a newton meter, can be used to measure the size of a force.

Force meters have a spring inside them. The weight of the mass causes the spring to stretch.

It is important to ‘zero’ the force meter before you use it. This means adjusting it so that the reading is zero when there is no force acting on it.

If you don’t ‘zero’ the force meter, all your measurements will be inaccurate by the same amount. This is called a 

systematic error

.

What unit is used to measure force?

1. N
2. Joule
3. Pascal
4. Newton

Which is a non-contact force?

1. Gravitational force
2. Magnetic force
3. Frictional force
4. Tension force

What type of force pulls objects together?

1. Applied force
2. Frictional force
3. Gravitational force

What does a Newton measure?

1. Pressure
2. Energy
3. Weight
4. Force

What have we learned about forces?

Contact forces are more impactful than non-contact forces.

I completely agree with this statement.

I partially agree, context matters.

I disagree, non-contact forces are crucial.

I have no strong opinion here.

Understanding Newtons is essential for measuring forces.

Absolutely, it's fundamental to physics.

Somewhat, but not always necessary.

No, other units can suffice.

I’m not sure about either.

Measuring forces enhances our understanding of physics.

Definitely, it clarifies many concepts.

Somewhat, but it's often complex.

Not really, theories can suffice.

I disagree, measurement is overrated.