Forces, Safety Technology and Society

Same speed, very different outcome

Drop an egg onto a kitchen floor and it breaks. Drop it from the same height onto a thick pillow and it may survive. In both cases the egg goes from the same speed to zero. The difference is how long the stop takes.

Car-safety engineers use exactly this idea. (Collisions can be a hard topic, especially for anyone who has been affected by one; this lesson focuses on how engineers protect people.) In this lesson you will see the physics behind seat belts, airbags and crumple zones, and then look more broadly at how force technologies affect society and the environment, from winter tires to tidal-energy turbines.

Longer stop, smaller force

To stop a person of mass m moving at speed v, their velocity must change by Δv = −v. If the stop takes time Δt:

a = Δv ÷ Δt, so F = m Δv ÷ Δt

For the same m and Δv, doubling Δt halves the force. That is the whole idea of crash safety:

• Crumple zones crush during the collision, so the car takes longer to stop.
• Seat belts stretch slightly and spread the force over the strong hip and chest bones; they also stop you from hitting the hard interior at full speed (first law: without them you keep moving).
• Airbags inflate in a fraction of a second and cushion the head and chest, lengthening the stop and spreading the force over a larger area. Airbags are designed to work with seat belts.
• Head restraints limit how far the head moves backward in a rear-end collision.

The rigid passenger cell is the opposite: it is designed not to crumple, so that the space around the occupants is protected.

Weighing impacts: four more technologies

Winter tires. Ordinary all-season rubber stiffens in the cold, lowering μ. Winter tires use a rubber compound that stays flexible below about 7 °C and a deep tread with many small slits that grip snow and ice. Higher μ means shorter stopping distances and better steering (look back at the braking calculations in the friction-problems lesson). Societal: fewer collisions and injuries; Québec requires winter tires on passenger vehicles in winter (from December 1 to March 15), and in Ontario insurance companies must offer a discount to drivers who use them. Costs: a second set of tires and rims, storage, and seasonal changeovers, which is harder for low-income drivers. Environmental: more tires to manufacture and eventually recycle.

Lightweight car bodies (plastics, composites, aluminium). Less mass means less force is needed to accelerate (F = ma), so less fuel or battery energy is used. But: some composites are harder to recycle than steel, they cost more to repair, and engineers must make sure the lighter body still manages crash forces safely.

Prosthetics. Running prostheses made of carbon fibre flex and spring back as the runner pushes on the ground (third law). Better materials and fit help people be active and independent. Issues: high cost and unequal access; debates about fairness in some sports competitions.

Tidal-current energy. The Bay of Fundy, between Nova Scotia and New Brunswick, has some of the highest tides in the world. Turbines placed in tidal currents are pushed by the moving water, generating electricity without burning fuel. Issues: the very strong forces of the water can damage equipment, the turbines are expensive, and scientists study their possible effects on fish and marine mammals.

Careers that use the physics of forces

• Mechanical or automotive engineer: designs vehicles, crumple zones, brakes and machines. Usually a 4-year university engineering degree; licensing as a Professional Engineer (P.Eng.) in Ontario through Professional Engineers Ontario.
• Collision reconstructionist: uses skid marks, crush damage and Newton's laws to work out what happened in a crash. Often police officers or engineers with specialized training.
• Kinesiologist: studies human movement and the forces on the body, for injury prevention, rehabilitation and sport. University degree in kinesiology; kinesiology is a regulated health profession in Ontario.
• Prosthetist or orthotist: designs and fits artificial limbs and braces. College or university programs plus clinical training and certification.
• Physiotherapist: helps people recover from injury using exercise and an understanding of forces on joints and muscles. A master's degree after an undergraduate degree.
• Materials engineer or metallurgist: develops stronger, lighter or lower-friction materials.

SPH4U, calculus and the sciences open the door to most of these programs.