Ngā Whāinga Ako 🔗
Copy these learning outcomes and the date into your books!
- Recap forces
- Explain force and pressure in terms of everyday situations. Use $P = F/A$.
Mahi Tuatahi 🔗
- Calculate the net force acting upon these objects:
- What does an unbalanced force do to the motion of an object?
Whakatika Rua 🔗
It causes the object to accelerate (or de-accelerate).
We know this because of the equation
$$ \begin{aligned} force &= mass\times acceleration \cr F &= m \times a \end{aligned} $$
Pātai Toru 🔗
For the followig equation give the name and unit for each variable in the equation.
$$ \begin{aligned} & F = m \times a \end{aligned} $$
Whakatika Toru 🔗
- F stands for force and has units Newtons (N)
- m stands for mass and has units kilograms (kg)
- a stands for acceleration and has units meters per second squared ($ms^{-2}$)
Pātai Whā 🔗
- What do you think of when you think of pressure?
- What affects the pressure exerted by an object?
Pātai Rima 🔗
The most classic case of pressure is sharpening a knife. Why do we sharpen knives? What does sharpening a knife change about the knife?
Pressure / Pēhanga 🔗
The amount of force per square meter
$$ \begin{aligned} pressure &= \frac{force}{area} \cr P &= \frac{F}{A} \end{aligned} $$
- Force (F) is measured in Newtons (N)
- Area (A) is measured in meters squared ($m^{2}$)
$$ \begin{aligned} P &= \frac{F}{A} \cr P &= \frac{Newtons}{m^{2}} \end{aligned} $$
- Therefore, pressure is measured in Newtons per meters squared ($\frac{N}{m^{2}}$ OR $Nm^{-2}$)
- This is also known as a Pascal (Pa): $Pa = \frac{N}{m^{2}} = Nm^{-2}$
- You may use whichever unit you prefer.
Calculating Pressure 🔗
$$ \begin{aligned} P = \frac{F}{A} \end{aligned} $$
- Calculate the pressure created by a force of $3N$ with an area of $0.5m^{2}$
- Calculate the area used by a force of $3N$ with pressure $7Pa$
- Calculate the force created by a pressure of $5Nm^{-2}$ with an area of $0.125m^{2}$
Whakatika Tahi 🔗
$$ \begin{aligned} F&=3N, A=0.5m^{-2}, P=? \cr P &= \frac{F}{A} \cr P &= \frac{3}{0.5} \cr P &= 6Nm^{-2} = 6Pa \end{aligned} $$
Whakatika Rua 🔗
$$ \begin{aligned} F&=3N, P=7Pa, A=? \cr P &= \frac{F}{A} \cr A &= \frac{F}{P} \cr A &= \frac{3}{7} \cr A &= 0.43m^{-2} \end{aligned} $$
Whakatika Toru 🔗
$$ \begin{aligned} P&=5Nm^{-2}, A=0.125m^{-2}, F=? \cr P &= \frac{F}{A} \cr F &= PA \cr F &= 5 \times 0.125 \cr F &= 0.625N \end{aligned} $$
Pressure Whakamātau 🔗
Open Google Classroom and find the Pressure Whakamātau document.
Mahi Tuatahi (2018 Exam) 🔗
Jacob bikes down The Flying Nun. He and his bike have a mass of $82kg$ and he accelerates at $0.8ms^{-2}$.
- Calculate the net force acting upon Jacob and his bike to cause this acceleration
- Draw a force diagram showing the forces acting upon Jacob as he accelerates
- Describe the size and direction of the forces compared to each other (horizontal and vertical)
Exam Question (2018) 🔗
Giovanni is running a 100m sprint. Each of his feet have a surface area of $200cm^{2}$ ($0.0200m^{2}$), which sink into the track. Together, his feet exert a pressure of $13000Pa$. Calculate Giovanni’s weight.
Whakatika 🔗
Because weight is a force, we know that we are looking for $F$.
$$ \begin{aligned} P &= 13000Pa \cr A &= 0.02m^{2} \times 2 = 0.04m^{2} \cr F &= P \times A \cr F &= 13000 \times 0.04 \cr F &= 520N \end{aligned} $$