Contact vs Non-Contact Forces
Contact and non-contact forces are two fundamental types of forces in physics that describe how objects interact with each other. The main difference between them lies in whether physical contact between the objects is required for the force to act.
| Contact forces | Non-contact forces |
| friction, air resistance, tension and normal force | gravitational, electrostatic and magnetic forces |
Gravitational Force or Weight
All matter has mass. Mass is a measure of the amount of substance in a body. The tendency of a body to resist a change in the state of rest or motion depends on its mass and is known as inertia.
A gravitational field is a region in which a mass experiences a force due to gravitational attraction. This attractive force is known as weight, and is given by
$$W=mg$$
where W is weight (unit: Newton or N),
m is the mass (unit: kg), and
g is the gravitational field strength (unit: N kg-1)
Gravitational field strength, g, is defined as gravitational force per unit mass.
Normal Contact Force
The normal contact force is the perpendicular force exerted by the surface of one object on the surface of another when they are in physical contact. It prevents the objects from passing through each other.
Characteristics of the normal contact force:
- It only exists when an object is in contact with something else.
- It is always perpendicular to the contact surface, and its direction is through the object of interest.
- Unlike weight, the normal force does not always pass through the CG of object.
- Its magnitude can change; it is not always equal to the weight of the object! However, in most typical cases, the magnitude of normal contact force is equal to the weight of the object and when this condition is met, the object is said to be in equilibrium.
The following interactive shows the changes in the magnitude and direction of the normal contact force N as the angle of the slope on which an object rests changes. If the surface is inclined, the normal must also be inclined such that it remains perpendicular (or normal) to the surface. Notice that the weight W does not change.
Friction
Friction is the contact force that acts between objects that opposes or tends to oppose or resist motion. It always act opposite to the direction of motion at the point of contact.
Characteristics of the frictional force:
- When the contact surfaces of both objects are smooth, there is little or no frictional force during motion, and can be ignored.
- When the contact surfaces are rough, frictional force on the body becomes non-negligible.
- It is directed parallel to the surface.
- The magnitude of friction is dependent on the normal force and the type/nature of the contact surfaces and is given by:
Note: the equations for friction are not in syllabus.
Static friction and kinetic friction are two types of frictional forces that resist the relative motion or impending motion between two surfaces in contact. They operate in slightly different ways, and their behavior is important in understanding how objects move or stay at rest in the presence of friction.
- Static Friction:
- Static friction is the frictional force that opposes the initiation of motion between two surfaces that are in contact but not yet moving relative to each other.
- It acts to prevent objects from sliding or moving when a force is applied to them but hasn’t yet overcome the threshold required to set them in motion.
- The static frictional force increases in magnitude as an external force is applied, up to a maximum value known as the maximum static frictional force (Fs max).
- Once the applied force exceeds the maximum static frictional force, the object will start to move, and static friction is no longer relevant.
- The equation that describes static friction is: $F_s = μ_s N$, where $F_s$ is the static frictional force, μs is the coefficient of static friction (a material-specific constant), and N is the normal force perpendicular to the contact surface.
- Kinetic Friction:
- Kinetic friction, also known as dynamic friction or sliding friction, is the frictional force that opposes the motion of two surfaces that are sliding or moving relative to each other.
- It acts on objects that are already in motion and opposes their motion by reducing their speed or preventing acceleration.
- The kinetic frictional force is generally weaker than the maximum static frictional force (Fs max).
- The coefficient of kinetic friction (μk) is used to describe the ratio of the kinetic frictional force to the normal force: $F_k = μ_k N$, where $F_k$ is the kinetic frictional force, μk is the coefficient of kinetic friction, and N is the normal force.
- The coefficient of kinetic friction is typically smaller than the coefficient of static friction for the same materials.
Viscous Force
When a solid moves in a fluid (i.e. liquid or gas), it will experience a resistance in the direction opposite to its motion. This resistance is called the viscous force or drag. It is also known as air resistance when the body moves in air.
The viscous force is found to be dependent on the speed of the object and the surface area of the object. The greater the speed, the larger the viscous force. The greater the surface area, the larger the viscous force.
Viscous force also depends on the nature of fluid involved, such as viscosity. For example, the viscous force is higher in liquid than in air for the same object having the same speed. It is also higher in viscous liquid like oil when compared to water.
The following graph shows the two trajectories with and without air resistance.
Elastic Force
An elastic force is one that resists a change in shape and can be a tension or compression.
If we hang a weight at the lower end of a spring, as the magnitude of the weight increases, the spring becomes longer. The increase in length of spring is called the extension. And the force that is being applied is called the tension.
On the other hand, when a force is being applied such that it causes the spring to be compressed, the decrease in length of the spring is called the compression. And the force that is being applied is called the compressive force. Both tension and compressive forces are elastic forces.
Note: Hooke’s law or the equation for tension/compression are not in syllabus
Hooke’s Law states that within the limit of proportionality, the extension or compression, x, produced is directly proportional to the elastic force F. In equation form, it is given by
$$F=kx$$
where F is the elastic force (unit: N),
k is the spring constant (unit: N m-1), and
x is the extension or compression (unit: m)In the simulation below, you can modify the amount of mass hanging off two springs that are identical by default. A ruler can be used to measure the extension of the spring.