Physical Science / Unit 7: Motion and Forces
7.3 Newton's First and Second Laws
Notes
- First law (inertia): An object at rest stays at rest and an object in motion keeps moving at constant velocity unless acted on by a net (unbalanced) force.
- Inertia is the tendency to resist changes in motion. More mass means more inertia.
- Second law: $F = ma$, where force is in newtons (N), mass in kg, and acceleration in $\text{m/s}^2$. 1 N $= 1 \text{ kg·m/s}^2$.
- Example: A 10 kg cart accelerates at 3 $\text{m/s}^2$. $F = 10 \times 3 = 30$ N.
- Balanced forces cancel and cause no change in motion.
Practice
A 5 kg object is pushed with a net force of 20 N. What is its acceleration?
$\displaystyle a = \frac{F}{m} = \frac{20}{5} = 4$.
Passengers lurch forward when a bus brakes suddenly. This is due to:
The passengers' bodies tend to keep moving forward.
What net force accelerates a 1,200 kg car at 2 $\text{m/s}^2$?
$F = ma = 1200 \times 2 = 2400$.
Ready for more? Take the Unit 7 test.