Motion, Forces & Energy

Physics explains how the physical world works. For nursing, physics concepts apply to patient lifting and mobility, blood flow dynamics, ventilator mechanics, and radiation therapy.

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Motion

Speed vs. Velocity vs. Acceleration

Example: A car travels 100 km in 2 hours.

• Speed = 100 km / 2 h = 50 km/h

Example: A car accelerates from 0 to 60 m/s in 10 seconds.

• Acceleration = (60 - 0) / 10 = 6 m/s^2

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Newton's Laws of Motion

First Law: Law of Inertia

"An object at rest stays at rest, and an object in motion stays in motion at constant velocity, unless acted upon by a net external force."

• Inertia is the tendency of an object to resist changes in its state of motion
• The more massive an object, the greater its inertia
• Nursing application: A patient on a stretcher continues to move forward when the stretcher stops suddenly

Second Law: F = ma

"The acceleration of an object is directly proportional to the net force acting on it and inversely proportional to its mass."

• F = ma (Force = mass x acceleration)
• SI unit of force: Newton (N) = 1 kg x m/s^2
• More force → more acceleration; more mass → less acceleration for the same force
• Example: What force is needed to accelerate a 50 kg patient in a wheelchair at 2 m/s^2?
  • F = 50 kg x 2 m/s^2 = 100 N

Third Law: Action-Reaction

"For every action, there is an equal and opposite reaction."

• When you push on a wall, the wall pushes back on you with equal force
• When walking, your foot pushes backward on the ground; the ground pushes your foot forward
• Nursing application: When performing CPR, your hands push down on the chest (action), and the chest pushes back against your hands (reaction)

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Friction

Friction is a force that opposes motion between two surfaces in contact:

Static friction is always greater than kinetic friction — it takes more force to start moving an object than to keep it moving.

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Gravity

• Gravitational acceleration near Earth's surface: g = 9.8 m/s^2
• All objects fall at the same rate in a vacuum (regardless of mass)
• Weight = mass x gravity (W = mg)
  • A 70 kg person weighs: W = 70 x 9.8 = 686 N

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Work, Energy & Power

Work

Work = Force x Distance (W = Fd)

• Measured in Joules (J)
• Work is only done when the force causes displacement in the direction of the force
• If you push against a wall and it doesn't move: no work is done (displacement = 0)

Example: A nurse lifts a 20 kg box 1.5 meters. How much work is done?

• Force = mg = 20 x 9.8 = 196 N
• Work = 196 N x 1.5 m = 294 J

Energy

• Law of Conservation of Energy: Energy cannot be created or destroyed, only transformed from one form to another
• As an object falls: PE decreases → KE increases (total energy remains constant)

Power

Power = Work / Time (P = W/t)

• Measured in Watts (W) = Joules per second
• Power measures how quickly work is done

Example: If 294 J of work is done in 3 seconds:

• Power = 294 / 3 = 98 W

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Simple Machines

Simple machines make work easier by changing the direction or magnitude of force:

Mechanical advantage = Output force / Input force (always ≥ 1 for simple machines)

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Momentum

Momentum = Mass x Velocity (p = mv)

• Measured in kg·m/s
• Greater mass OR greater velocity = greater momentum
• Conservation of Momentum: In a closed system, total momentum before a collision equals total momentum after

Nursing application: Understanding momentum helps explain impact injuries — a heavier, faster-moving object causes greater injury on impact.

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Types of Energy

Energy Transformations in the Body:

• Chemical energy (food) → thermal energy (body heat) + mechanical energy (movement)
• Chemical energy (ATP) → electrical energy (nerve impulses)
• Chemical energy (glucose) → kinetic energy (muscle contraction) + thermal energy (heat)

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Elastic vs. Inelastic Collisions

Nursing relevance: Understanding collision types helps explain impact injuries and protective equipment design (helmets, airbags extend impact time to reduce force).

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Center of Gravity and Stability

Center of gravity (COG) is the point where all weight is concentrated:

• Lower center of gravity → more stable
• Wider base of support → more stable

Nursing applications:

• Patient fall prevention: Patients with high center of gravity (tall, top-heavy) are at greater fall risk
• Body mechanics: When lifting patients, nurses should widen their stance (base of support) and bend at the knees to lower their COG
• Wheelchair safety: Locking wheels and positioning footrests prevents tipping
• Crutch walking: A tripod stance provides the widest base of support