Notes
Work and power
- Work $W = Fd$ in joules (J). No motion means no work.
- Power $\displaystyle P = \frac{W}{t}$ in watts (W), where 1 W = 1 J/s.
Machines
- Six simple machines: lever, pulley, wheel and axle, inclined plane, wedge, screw. Machines change the size or direction of a force but do not reduce the work.
- $\displaystyle \text{MA} = \frac{\text{output force}}{\text{input force}}$. Ramp ideal $\displaystyle \text{MA} = \frac{\text{length}}{\text{height}}$. Efficiency $\displaystyle = \frac{\text{work out}}{\text{work in}} \times 100\%$.
Energy
- $\displaystyle KE = \frac{1}{2}mv^2$ and $PE = mgh$. Energy changes form but is conserved; friction changes some into thermal energy.
Topics in this unit
Each topic has its own short notes and practice.
Practice
$W = Fd = 50 \times 6 = 300$ J.
$\displaystyle P = \frac{1200}{40} = 30$.
$\displaystyle \frac{5}{1} = 5$.
$\displaystyle \frac{1}{2}(2)(16) = 16$ J.
Lost PE becomes KE by conservation of energy.
The bag does not move, so no work is done.
Free response
Write your answer first, then open the scoring guide.
Free-response question
A 100 N force pushes a crate up a ramp 6 m long in 12 s. The crate is raised 2 m.
(a) Calculate the work done by the 100 N force along the ramp.
Show the answer and scoring guide
$W = Fd = 100 \times 6 = 600$ J.
Check off the points you earned:
(b) Calculate the power of the pushing force.
Show the answer and scoring guide
$\displaystyle P = \frac{W}{t} = \frac{600}{12} = 50$ W.
Check off the points you earned:
(c) Find the ideal mechanical advantage of the ramp and explain what it means.
Show the answer and scoring guide
$\displaystyle \text{IMA} = \frac{6}{2} = 3$. The ramp lets a smaller force lift the crate, but the force acts over a longer distance.
Check off the points you earned: