Forces, Work, & Power, and Free Body Diagrams - Part 1 (7a)

What is Force?

• arises from the action or reaction of one body on another.

• is any interaction that changes an object’s motion or shape.

• cannot be directly observed, but its effects can be seen.

What does Newton’s Third Law say about action and reaction forces?

• One force is the action of the man on the wall, and the other is the reaction of the wall on the man.

• Third Law states that for every action, there is an equal and opposite reaction.

Where do contact forces occur?

• Contact forces occur when bodies interact with each other.

• They can also occur internally within a single body, where microscopic particles (like molecules and atoms) contact and interact with forces between themselves.

What is an example of a non-contact force, and what is the SI unit of force?

• Magnetic attraction is a non-contact (remote) force caused by the influence of a magnetic field on a body.

• The SI unit of force is the newton (N).

What is Force?

• Force is a vector — it has magnitude and direction.

• SI Unit: Newton (N)

• 1 Newton (1 N) is the force that, when applied to a mass of 1 kg, produces an acceleration of 1 m/s².

How is a force described in two dimensions?

In two-dimensional problems, a force is described by two scalar quantities:

• Its magnitude (e.g., 5 N)

• Its direction (e.g., θ°)

What is the moment of a force about a point?

The moment of a force about a point is the product of:

The magnitude of the force, and

The perpendicular distance from the point to the line of action of the force.

What causes frictional force, and when can it be negligible?

• The frictional force (F) exists due to the roughness of the contact surface between a body and the ground.

• If the contact surface is smooth or lubricated, friction becomes negligible, leaving only the normal reaction force.

• This occurs in special cases, such as lubricated bearings.

How do you analyse forces between two contacting bodies?

• Separate the two bodies and draw on each a free-body diagram (FBD) showing all external forces acting on that body (including contact/reaction forces).

• The goal is to solve for the unknown reaction forces: those between the two bodies and those between body B and the ground.

• Use the equilibrium equations (ΣF = 0 and ΣM = 0) on each body to find the unknowns.

What are the external forces acting on body A when in contact with body B?

• Weight WAW_AWA​ acting at its centre of mass (downwards).

• A vertical reaction force from body B at the contact point (upwards).

• No horizontal friction force at the contact because no horizontal forces are present.

What is the Principle of Transmissibility?

A force can be moved along its line of action without affecting the static equilibrium of the body on which it acts.

What does Newton’s Third Law of Motion state?

For every action, there is an equal and opposite reaction — both forces act on different objects.

Examples:

• Rocket pushes gases down → gases push rocket up.

• Jump off a boat → you move forward, boat moves back.

• Bird flaps down → air pushes bird up.

What is the Principle of Transmissibility in mechanics?

A force can be moved anywhere along its line of action without changing the body’s static equilibrium or external effect.

What are Statically Equivalent Systems?

Two systems of forces are statically equivalent if they produce the same external effect on a body — that is, the same net force and the same net moment.

What is the Two-Force Principle?

For a body acted on by only two forces to be in equilibrium, the forces must:

• Be equal in magnitude,

• Opposite in direction, and

• Act along the same line of action.

What is the Three-Force Principle?

For a body acted on by only three forces to be in equilibrium, the lines of action of all three forces must intersect at a single point.

What are moments and couples, and what are their units?

• Moments and couples produce rotational effects on a body.

• Measured in Newton-metres (Nm).

What conditions must be met for a body to be in equilibrium?

• The sum of all forces is zero (∑F⃗=0\sum \vec{F} = 0∑F=0) → no translation.

• The sum of all moments is zero (∑M=0\sum M = 0∑M=0) → no rotation.

How are energy, work, and power related?

• Work–Energy Theorem: Work done on a body changes its energy.

• Power: Measures the rate of doing work.

Formulas (optional):

• W=ΔEW = \Delta EW=ΔE (Work = Change in energy)

• P=WtP = \frac{W}{t}P=tW​ (Power = Work ÷ Time)

When is work done, and what is its formula?

Work is done when a force causes displacement in its direction.

What is a Free Body Diagram (FBD) and why is it used?

• Helps analyse forces

• Sets up equilibrium equations for solving unknowns

What do Newton’s Laws of Motion explain?

They describe the relationship between force, mass, and acceleration, and include the action–reaction principle.