SPI Physics · Formula sheet
Laws of Motion and Forces formulas
3 formulas, 2 reference tables and 9 common traps for SPI Physics Laws of Motion and Forces, grouped by subtopic.
Momentum and Force
Learn this subtopic in the notesNewton's three laws of motion
Newton's second law
Momentum and force as its rate of change
Momentum and force
Conservation of momentum in collisions and recoil
Conservation of momentum
Common traps
Cancelling action against reaction
Forces cancel only when they act on the same body. Action and reaction act on two different bodies (the gun and the bullet), so each body still moves.
Using bullets per minute
Force is momentum per SECOND. If the rate is given per minute, divide by 60 first: 90 bullets a minute is 1.5 a second.
Bringing the gun's mass into the force
The force to hold a firing gun steady is the momentum given to the bullets each second. The gun's mass is extra information here.
Leaving grams in the momentum
Mass must be in kilograms: a 30 g bullet is 0.03 kg. Using 30 makes the momentum a thousand times too large.
Adding momenta that point opposite ways
Bodies moving towards each other have momenta in opposite directions. Subtract them; adding gives a total that is too large.
Forgetting the direction of the answer
After two bodies stick, they move in the direction of the one that had the larger momentum.
Fundamental Forces and Levers
Learn this subtopic in the notesThe four fundamental forces
| Force | Strength (strong nuclear = 1) | Range | Acts between |
|---|---|---|---|
| Strong nuclear | 1 (strongest) | About 10⁻¹⁵ m, inside a nucleus | Protons and neutrons |
| Electromagnetic | About 10⁻² | Unlimited | Charged particles |
| Weak nuclear | About 10⁻¹³ | Less than 10⁻¹⁶ m | Particles in radioactive (beta) decay |
| Gravitational | About 10⁻³⁹ (weakest) | Unlimited | All bodies with mass |
Weakest to strongest: gravitational, weak nuclear, electromagnetic, strong nuclear.
The three classes of lever
| Class | In the middle | Examples |
|---|---|---|
| First | Fulcrum | See-saw, scissors, pliers, crowbar, beam balance |
| Second | Load | Nutcracker, wheelbarrow, bottle opener, door on hinges |
| Third | Effort | Fire tongs, forceps, fishing rod, the human forearm |
Memory key, in class order 1, 2, 3, the middle point is F, L, E.
Common traps
Thinking gravity is strong because planets feel it
Gravity rules the motion of planets only because they are so massive. Between two particles it is the weakest of all the fundamental forces.
Classing by where the fulcrum is
The class depends on which of the three points is in the MIDDLE. A fulcrum at the end can belong to a second or a third class lever; look at whether the load or the effort is in between.
Calling scissors a second class lever
In scissors the pivot (the fulcrum) sits between the handles (effort) and the blades (load), so scissors are first class.