Contact and non-contact forces

That is how AQA's GCSE Physics specification sorts forces: a force is a push or pull that acts on an object because of its interaction with another object, and forces are either contact or non-contact. Force is a vector quantity, so it has a size, measured in newtons (N), and a direction.

Non-contact forces

A non-contact force acts across a gap, even through empty space. It works through a field: a region around an object where another object feels a force.

Non-contact forces get weaker with distance. Gravity holds the Moon in orbit 384,000 km away, but it pulls on a spacecraft far less strongly there than on the ground. Gravity only ever attracts, while electrostatic and magnetic forces can attract or repel: like charges and like poles push apart, and opposite ones pull together.

Contact forces

Is upthrust a contact force?

Yes. Upthrust comes from the water (or air) pressing on the surface of the object, so the fluid must be touching it. AQA's Higher tier explains why it pushes up: a submerged object feels a greater pressure on its bottom surface than on its top surface, because the bottom is deeper. The difference gives an overall upward force. If the upthrust equals the object's weight, it floats.

Contact and non-contact forces together

Most real objects feel both kinds at once. A book resting on a table has:

• Weight pulling it down: a non-contact force, the Earth's gravity acting on it.
• The normal contact force pushing it up: a contact force from the table.

These two balance each other exactly, giving a resultant force of zero, so the book stays still. A free-body diagram shows each force on an object as an arrow pointing in its direction, with a longer arrow for a bigger force. Try labelling one, then press Mark to check it in our Laws of Motion course.

Worked examples

Example 1: weight, a non-contact force

Weight is calculated with W = mg, where m is mass in kg and g is gravitational field strength in N/kg. AQA gives the value of g in every question. Taking g = 9.8 N/kg, a 60 kg person has a weight of 60 × 9.8 = 588 N. On the Moon, where g is about 1.6 N/kg, the same person weighs 60 × 1.6 = 96 N, but their mass is still 60 kg.

Example 2: a resultant force

A car's engine provides a forward thrust of 800 N, and air resistance and friction add up to 300 N backwards. The resultant force is 800 − 300 = 500 N forwards, so the car speeds up. When the backward forces grow to 800 N, the forces balance and the car travels at a steady speed.

Example 3: classify the forces on a skydiver

• Weight (down): non-contact, gravitational.
• Air resistance (up): contact, from the air rushing past.
• When the parachute opens, the air resistance grows, a contact force, while the weight stays the same.

Forces come in pairs

Newton's third law says that when one object exerts a force on another, the second exerts an equal and opposite force on the first. The two forces are always the same type. The Earth pulls you down with gravity, and you pull the Earth up with exactly the same gravitational force. Your feet push down on the floor, and the floor pushes up on your feet with a contact force. Both forces in a pair act on different objects, which is why they don't cancel out.

A deeper look: what is a contact force really?

Zoom in far enough and nothing actually touches. When you press on a table, the electrons in the atoms of your hand repel the electrons in the table's atoms. So contact forces such as friction and the normal force are electrostatic forces acting over tiny distances. The contact and non-contact labels describe what we see at everyday scales, which is why they are so useful.

Common mistakes

• Calling weight a contact force because the object is touching the ground. Weight is gravity, which acts whether or not anything is touching.
• Forgetting the normal contact force. Something resting on a surface always has one pushing up.
• Calling upthrust non-contact. It comes from fluid pressing on the object's surface.
• Mixing up mass and weight. Mass is in kg and doesn't change. Weight is a force in N that depends on g.
• Saying a third law pair cancels. The two forces act on different objects.

Read next

Physics · MechanicsNet force calculator: resultant force and F = maFind the net (resultant) force on an object, then its acceleration with F = ma, for forces along a line or at angles, with worked examples.