What Are the 7 Contact Forces?

The seven commonly taught contact forces are: normal reaction (contact) force, friction, tension, drag (air or water resistance), spring (elastic) force, buoyant force (upthrust), and applied push/pull. These forces arise from direct physical interaction between objects or via a surrounding fluid and are consistent with Newton’s laws of motion.

The Seven Contact Forces at a Glance

Below is a concise list of the seven contact forces you’ll encounter in school physics and many engineering contexts. Each entry names the force and explains what causes it and how it typically acts on objects.

• Normal reaction force: The support force exerted by a surface on an object pressing against it, acting perpendicular to the surface (for example, the floor pushing up on your feet).
• Friction: A force parallel to the contact surface that resists relative motion or attempted motion between two surfaces; includes static and kinetic (sliding) friction.
• Tension: The pulling force transmitted along a rope, cord, cable, or string when it is taut and being pulled at both ends.
• Drag (air/water resistance): A resistive force from a fluid (gas or liquid) that opposes motion through it; “air resistance” is drag in air, “water resistance” is drag in water.
• Spring (elastic) force: The restoring force from a stretched or compressed elastic element (e.g., a spring or elastic band), typically modeled as proportional to extension or compression.
• Buoyant force (upthrust): The upward force exerted by a fluid on an immersed object, equal to the weight of the displaced fluid.
• Applied force (push/pull): Any external push or pull exerted by a person or another object at the point of contact that isn’t otherwise categorized (e.g., a hand pushing a box).

Together, these seven cover the routine ways contact interactions show up in real-world problems—from walking and driving to flying, floating, and lifting with ropes or springs.

How They Act in Everyday Situations

Surfaces and sliding

When a book rests on a table, the normal reaction balances its weight, while friction prevents it from sliding if the table is tilted slightly. If the book begins to slide, kinetic friction opposes its motion along the surface.

Ropes, cables, and springs

A hanging sign is supported by tension in its cables; the tension adjusts to balance forces in the system. In a bathroom scale, springs compress under your weight, and the spring force provides the balancing upward force the scale uses to measure mass.

Moving through fluids

A cyclist feels air resistance that grows rapidly with speed; streamlined helmets and frames reduce drag. A ship floats because the buoyant force from displaced water balances its weight; changes in cargo alter how deeply it sits and the upthrust it experiences.

What Determines Their Size

Each contact force depends on specific properties of the objects and environment. The simplified relationships below are widely used in physics and engineering.

• Normal reaction: On a level surface, N ≈ mg for a stationary object; on an incline of angle θ, N ≈ mg cosθ (ignoring other vertical forces).
• Friction: Static friction fs ≤ μsN (adjusts up to a maximum); kinetic friction fk = μkN (approximately constant during steady sliding). μ depends on materials and surface condition.
• Tension: Set by the forces acting on the rope and attached masses; in an ideal massless rope with no friction over a pulley, tension is uniform throughout.
• Drag: For everyday speeds in air, Fd ≈ 0.5 ρ Cd A v²; at very low speeds/viscous flows, Fd ≈ k v. Here ρ is fluid density, Cd drag coefficient, A reference area, v speed.
• Spring force: Fs = kx, where k is spring constant and x is extension/compression, within the elastic limit (Hooke’s law).
• Buoyant force: FB = ρfluid g Vdisplaced, pointing upward; floating objects displace just enough fluid so FB equals their weight.
• Applied force: Depends on the agent (e.g., a person pushing); it can be measured or inferred from acceleration via F = ma when other forces are known.

While these models are idealized, they offer accurate predictions across many practical situations, especially when speeds are moderate and materials remain within elastic limits.

Contact vs. Non-Contact Forces

Contact forces require physical interaction or a mediating fluid. By contrast, non-contact forces—such as gravity, magnetism, and electrostatic forces—act at a distance without direct contact. In advanced physics, all forces arise from fundamental interactions, but for problem-solving in mechanics, the contact/non-contact classification is a useful, accurate simplification.

Common Misconceptions and Clarifications

The following points address frequent misunderstandings students encounter when learning about contact forces.

• “Normal force always equals weight.” Only true on a level surface with no other vertical forces. On slopes or in elevators, the normal force differs from the object’s weight.
• “Friction always opposes motion of an object.” Friction opposes relative slipping at the contact; it can act in the direction of motion of the object’s center of mass (e.g., static friction propelling a car forward).
• “Tension can push.” Ideal ropes and strings can only pull along their length; they cannot push.
• “Buoyancy depends only on the object’s weight.” Buoyant force depends on the fluid displaced, not directly on the object’s weight; density relative to the fluid determines floating or sinking.

Keeping these clarifications in mind helps avoid common errors in free-body diagrams and calculations.

Summary

The seven contact forces—normal reaction, friction, tension, drag, spring force, buoyant force, and applied push/pull—capture the principal ways objects interact through direct contact or via fluids in everyday mechanics. Understanding what sets their direction and magnitude, and when each model applies, is essential for building correct free-body diagrams and solving motion and equilibrium problems accurately.

What are the types of contact forces Class 7?

Types of contact force:

• Frictional Force: Friction is a force exerted by a surface against the motion of a body across its surface.
• Applied Force: Force which is applied to an object by another object.
• Normal Force: The normal force is also called support force.

Which of the 7 forces are contact forces?

Contact forces require direct physical contact between objects and include applied force, normal force, friction, tension, and spring force. Non-contact forces act over a distance and include gravitational force, electrical force, and magnetic force.

What is a contact force grade 7?

Contact forces happen when objects touch each other. For example, contact forces happen when a person kicks a ball or pulls a wagon. Other examples of contact forces are sandpaper rubbing on a piece of wood, wind blowing against a moving car, and a rubber band stretched around a newspaper.

What are the 7 types of forces?

2.3: Types of Forces

• Gravity.
• Tension.
• Normal (Contact) Force.
• Kinetic Friction.
• Static Friction.
• Air Resistance (Drag)
• Elastic (Spring) Force.