# Difference between revisions of "Skill Set 06.1"

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Main Page > IB Physics Skill Sets > Skill Set 06.1

### 1. A 5.2 kg object speeds up from 3.1 m/s to 4.2 m/s. What is the change in kinetic energy? (21 J)

This question is just to see if you can apply the basic formulas for energy, and solve for things like velocities and height. The basic IB formulas for this sort of thing are:

• W = Fs$\cos \Theta$
• $\Delta$Ep = mg$\Delta$h
• Ek = 1/2mv2

Missing from the packet is

• Ep = 1/2kx2

So what you need to do is have your final kinetic energy minus the initial kinetic energy.

• Ekf - Eki

Your Final kinetic energy is just $\begin{matrix} \frac{1}{2} \end{matrix}$ (5.2 kg)(4.2 m/s)$^2$ = 45.864 J
And the initial energy is $\begin{matrix} \frac{1}{2} \end{matrix}$(5.2 kg)(3.1 m/s)$^2$ = 24.986 J

Then subtract
45.864 J -24.986 J

The answer is 20.878 J, but that has bad sig figs.

So...
ROUND!
$\Delta$Ek=21 J

### 2. A 1.2 HP motor (1 HP = 745.7 Watts) is used to raise a 1300 kg Land Rover 5.7 m up into a tree. What time will it take? (81 s)

This problem always has to do with calculating power. The data packet gives us

Power = Fv

Which is fairly useless. In general, Power is the rate at which work is done so Power is better explained as:

Power = $\frac{Any\ change\ in\ energy}{\Delta t}$

• W = Fs$\cos \Theta$
• $\Delta$Ep = mg$\Delta$h
• Ek = 1/2mv2

Missing from the packet is

• Ep = 1/2kx2