Newton's laws of motion
Newton's first law
if the on an object is zero:
- a stationary object stays stationary
- a moving object keeps moving at the in the same direction
So when a car travels at a steady speed, its driving force is balanced by the resistive forces (friction and air resistance).
If the resultant force is not zero, the object's velocity changes: it speeds up, slows down or changes direction.
Higher tier: the tendency of an object to keep doing what it's doing, staying still or moving at a steady velocity, is called .
Key terms
Newton's second law
an object's acceleration is to the resultant force on it, and inversely proportional to its mass. You must recall this equation:
, or F = ma
| Quantity | Symbol | Unit |
|---|---|---|
| Resultant force | F | newtons (N) |
| Mass | m | kilograms (kg) |
| Acceleration | a | m/s² |
Example: a 1000 kg car has a resultant force of 3000 N on it.
- Rearrange: a = F ÷ m
- a = 3000 ÷ 1000
- a = 3 m/s²
Higher tier: measures how hard it is to change an object's velocity. It is the force divided by the acceleration: m = F ÷ a.
Key terms
Newton's third law
whenever two objects interact, the forces they exert on each other are equal in size and opposite in direction.
- When you push on a wall, the wall pushes back on you with the same force.
- A swimmer pushes water backwards; the water pushes the swimmer forwards.
- The Earth pulls you down with gravity; you pull the Earth up with the same size of force.
Common mistake
The two forces in a third-law pair act on different objects and are the same type of force. A book's weight and the table's push on the book are not a third-law pair: they both act on the book.
Key terms
Required practical 7: method
- Tie a string to a trolley on a runway, run it over a pulley at the end of the bench, and hang a mass holder from it. The of the hanging masses is the .
- Set up along the runway, and fix a card of known length to the trolley.
- Release the trolley. The light gates record its at each gate and the time between them, so you can find a = Δv/t.
- To vary the force: move masses one at a time from the trolley to the hanger, so the total mass stays the same.
- add masses to the trolley, keeping the hanging mass the same.
Acceleration against force is a straight line through the origin. Acceleration against mass is a curve that falls as mass increases.
Key terms
Required practical 7: variables, safety and accuracy
- the force (weight of hanging masses).
- the acceleration.
- the total mass of the trolley and hanging masses.
- Independent variable: the mass of the trolley.
- Dependent variable: the acceleration.
- Control variable: the accelerating force.
Hazard: falling masses can hurt feet, so put a box of soft material under the hanger and a stop at the end of the runway.
Improving accuracy: light gates avoid the reaction-time errors of a stopwatch. Tilt the runway very slightly so the trolley just keeps rolling on its own, which cancels out friction.