# PHY C5: Types of Forces

Here’s what we’ll cover:

• Force & electric force
• Upthrust
• Frictional & viscous forces

Let’s go!

Previously, we’ve learnt about what forces are.
Different forces have different sources, & we’ll look at some today.

In general, these forces are generated by FIELDS of forces.

What is Gravitational Force?
Force that acts on a mass within a gravitational field generated by another mass.

Any object with mass creates a gravitational field around it.
The effect on this field is usually only seen around large masses (e.g. the Earth).

What is a Gravitational Field?
Region in space surrounding objects with mass, where other objects with mass experience a force.

What is Electric Force?
Force that acts on charged particles within an electric field generated by another charged particle.
Any object with charge creates an electric field around it.

What is an Electric Field?
Region in space where lines of force terminate from the positive terminal to the negative terminal.

A positively-charged particle will experience an electric force in the direction of the field with a strength of Eq, where
E = electric field strength
q = charge in Coulombs

Thus,
F = Eq

Q: What is a Field, really?

A:

We’ll look at more fields of forces & explain how they are quite similar to each other in another chapter.

For now, let’s look at other types of force: UPTHRUST, & then FRICTIONAL FORCES.

What is Upthrust?
Upward force exerted by a fluid on a submerged object.
It is equal to the weight of fluid displaced by a body placed within the fluid.

It is dependent on the density of the fluid.
The volume of the fluid displaced = volume of object submerged in fluid.
The mass of this displaced fluid = volume displaced x density of fluid
& since
weight = mass x gravitational acceleration (on Earth, the constant ‘g’)

F = volume x density x gravitational acceleration
F = vρg

Why does a fluid exert upthrust?
The pressure exerted by a fluid is LARGER at deeper depths.

Pressure is exerted from ALL directions onto an object’s surface.
Thus, the pressure exerted on the BOTTOM of the object will be LARGER than the pressure exerted on its top.
This difference in pressure causes a net force to be exerted upwards.

Thus, (if the object has a uniform vertical cross-section) you can also calculate upthrust by

• Finding the pressure difference (Δp) below & above the object
• Finding the cross-sectional area of the object (A)
• Multiply the pressure difference with the surface area (Δp x A)

F = ΔpA

Because:
Δp = F/A

Now let’s look at FRICTIONAL FORCES:

What is a Frictional Force?
Resistive force against the motion of a body when in contact with another body.
It is due to the roughness (unevenness) of the surfaces of both bodies.

Since friction is caused by the contact between two bodies, it increases when the NORMAL REACTION FORCE increases.
Normal Reaction Force (denoted as ‘R’) is the force a body exerts perpendicularly against another object.

You can find out more about R here.

Frictional force is given by:
F = μR

Where
μ= coefficient of friction (a constant)
R = Normal Reaction Force

Another kind of frictional force is viscous force.

What is Viscous Drag Force?
Resistive force against motion of objects in a fluid.

It is due to fluid molecules flowing over a body moving through a fluid at a certain velocity.

For example, air resistance acting upon a flying object.
The air resistance increases as the object increases in speed.
We’ve covered this drag force before, here.

Q: Why does Friction cause heat?
A:

When an object moves vertically downwards through a fluid (like air), it experiences:

• Weight (W): downwards
• Upthrust (U): upwards
• Viscous Drag (D): upwards

All 3 are related by:
W – U – D = ma

Because the RESULTANT of all the forces acting in the vertical component are what causes the resultant acceleration.

If the object is NOT accelerating (i.e. at a constant/terminal velocity),
Then W = U + D

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