Notes
Describing motion
- Speed $\displaystyle v = \frac{d}{t}$. Velocity includes direction. Acceleration $\displaystyle a = \frac{v_f - v_i}{t}$ in $\text{m/s}^2$; changing direction counts as accelerating.
Newton's laws
- First law: no net force means no change in motion (inertia).
- Second law: $F = ma$ (force in newtons).
- Third law: forces come in equal and opposite pairs acting on different objects.
Friction, gravity and momentum
- Friction opposes motion between surfaces. Weight $W = mg$ with $g = 9.8 \text{ m/s}^2$ on Earth. Mass does not change with location.
- Momentum $p = mv$ is conserved in collisions in a closed system.
Topics in this unit
Each topic has its own short notes and practice.
Practice
$\displaystyle v = \frac{240}{4} = 60$.
$F = ma = 4 \times 6 = 24$.
Constant velocity means no net force (first law).
$\displaystyle a = \frac{25 - 5}{4} = 5$.
$W = mg = 10 \times 9.8 = 98$.
Momentum before $= 12$ kg·m/s, total mass 6 kg, so $v = 2$ m/s.
Free response
Write your answer first, then open the scoring guide.
Free-response question
A 50 kg student on a skateboard (total mass 60 kg) pushes off a wall and speeds up from rest to 3 m/s in 2 s.
(a) Calculate the acceleration.
Show the answer and scoring guide
$\displaystyle a = \frac{3 - 0}{2} = 1.5 \text{ m/s}^2$.
Check off the points you earned:
(b) Calculate the net force needed to produce this acceleration.
Show the answer and scoring guide
$F = ma = 60 \times 1.5 = 90$ N.
Check off the points you earned:
(c) Explain how Newton's third law applies when the student pushes off the wall.
Show the answer and scoring guide
The student pushes on the wall and the wall pushes back on the student with an equal and opposite force. This reaction force acts on the student and causes the motion.
Check off the points you earned: