Forces
Pushes, pulls, friction and gravity, and the three laws Newton used to connect force with motion. The motion lab swings a pendulum on Earth, the Moon or Mars and times it against the formula.
About this course
Force is a vector, so direction matters. Two people pulling a cart the same way with 30 N and 20 N give a total of 50 N, but pulling in opposite directions they give 30 − 20 = 10 N toward the stronger one. When forces at right angles combine, you add them as the sides of a right triangle: 3 N east and 4 N north make 5 N toward the north-east.
The six units are what a force is, Newton's three laws, friction, gravity and weight, the pendulum, and a first look at work and energy. Newton set out the three laws in his Principia of 1687. The pair of forces in the third law acts on different objects, so they never cancel each other. A horse and a cart still move because whether the horse speeds up depends on all the forces on the horse, including the ground pushing on its hooves. Friction hardly depends on the area of contact: a brick slides with about the same friction flat or on its narrow end. A 70 kg person weighs about 687 N on Earth, 260 N on Mars and 113 N on the Moon, with the same mass everywhere.
The motion lab swings a pendulum on Earth, the Moon or Mars and times it against the formula. OpenStax gives the period as T = 2π√(L ÷ g), in which the mass of the bob does not appear, so a heavy bob and a light one of the same length swing in step. A 1.00 m pendulum on Earth has T ≈ 2.006 s. You can run the idea backwards: a 75.000 cm pendulum with a period of 1.7357 s gives g ≈ 9.828 m/s².
Before you start
Forces explain why things move the way Motion describes, so that course is a natural first step, especially its unit on acceleration. You will also use F = ma and a square root for the pendulum, so basic algebra is enough. Nothing else is needed.
When you finish
You will be able to add forces to find a net force, use Newton's second law to get acceleration from force and mass, and work out the push needed to start a crate moving against friction. You can tell mass from weight on Earth, Mars and the Moon, time a pendulum against its formula, and calculate the work done lifting a bag onto a shelf.
- 1What is a force?One newton is about the weight of a small apple held in your hand.
- 2Newton’s three lawsA 1,200 kg car needs a net force of 3,600 N to speed up at 3 m/s².
- 3FrictionA skidding tyre on dry concrete has a friction coefficient of about 0.7; on wet concrete it drops to about 0.5.
- 4Gravity and weightA 70 kg person weighs about 687 N on Earth, 260 N on Mars and 113 N on the Moon, with the same mass everywhere.
- 5The pendulumA pendulum 99.4 cm long takes 2 seconds for a full swing, one second each way.
- 6Work and energy: a first lookLifting a 2 kg bag onto a 1.5 m shelf takes about 29 joules of work, and the bag gives the same 29 J back if it falls.
Sources for this course (10)
- OpenStax College Physics 2e, chapter 4 (Newton's laws)
- 5.1 (Friction, Table 5.1 coefficients)
- 5.2 (Drag)
- 7 (Work and energy)
- 16.4 (The simple pendulum, worked example 75.000 cm and 1.7357 s) (openstax.org/books/college-physics-2e)
- NIST CODATA standard acceleration of gravity 9.80665 m/s² and Newtonian constant of gravitation (physics.nist.gov/cuu/Constants)
- NASA Moon and Mars Fact Sheets (surface gravity 1.62 m/s²; Mars 3.69 to 3.73 m/s²; Earth GM 0.39860 × 10⁶ km³/s², mean radius 6371 km; Earth surface acceleration 9.78 equator, 9.83 pole) (nssdc.gsfc.nasa.gov/planetary/factsheet/)
- Newton, Philosophiae Naturalis Principia Mathematica (1687)
- Galileo, Two New Sciences (1638)
- American Physical Society, "Christiaan Huygens Patents the First Pendulum Clock" (aps.org/apsnews/2017/06/huygens-patents-first-pendulum-clock).
Checked: 2026-10-17