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Workshop Tutorials for Technological and Applied Physics
MR8T: Equilibrium
A. Qualitative Questions:
1. Levers are used in many different types of
machinery and in the human body. For example,
hydraulic arms such as that shown opposite are
used in heavy lifting machinery. There are three
classes of lever, as shown in the diagram below.
Class one levers have the pivot point or fulcrum
between the load and the applied force. Second
class levers have the load between the fulcrum and
the applied force, and third class levers have the
applied force between the load and the fulcrum.
a. Give an example of the use of each type of lever. Use diagrams to show where the load, fulcrum and
applied force are.
b. What sort of lever is a wheelbarrow? What about a pair of tweezers?
applied force
load
load
fulcrum
fulcrum
fulcrum
applied force
Class 1
load
Class 2
applied force
Class 3
2. Almost any tool you can think of uses a lever to apply a torque. A wrench is an obvious example, but
even a hammer acts like a part of a lever, with the elbow of the person swinging the hammer as the
fulcrum. Well designed tools can make a job much easier, and safer.
a. Why is it easier to undo a tight screw with a thick handled screwdriver?
b. Sometimes mechanics put a bit of pipe over the handle of a wrench to give them more torque. Explain
why this is very effective, but can be dangerous.
Car workshop manuals list torque settings for the bolts on the engine head, and advise that you use a
torque wrench when replacing the engine head.
c. Why do they list torques rather than forces?
B. Activity Questions:
1. Centre of mass
Examine the various displays to get a feel for the centre of mass and stability.
Stand with your back against the wall and try to touch your toes. What happens, and why?
The Workshop Tutorial Project –MR8T: Equilibrium
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2. Your own centre of mass
Use the two bathroom scales and the plank to find your own centre of mass.
Where is your centre of mass when the readings on the two scales are the same?
On a human the centre of mass is usually around 55% of the height, up from the feet, when the arms are
at the sides of the body, a little lower on average for females than males.
3.
Tools
On display are tools that use 'torque' in their design and application.
Identify their pivots, axes of rotation and direction of forces.
C. Quantitative Questions:
1. Brent is going to build a barbeque for Rebecca for her birthday. He’s borrowed his mum’s Volvo
station wagon and set off down to the local brick and pavers shop. The Volvo has a mass of 1700 kg, of
which 60% is supported by the front wheels and 40% by the back wheels. The car has a wheel base (the
distance between the front and rear axles) of 3.5 m. The manual for the car says that it has a load capacity
of 500 kg, so Brent makes sure that he only buys 500 kg of bricks.
a. Brent starts loading bricks into the back of the car and notices that the front is rising. Why is this
happening?
He keeps loading bricks until there are 300 kg of bricks in the back, with their centre of mass about 70 cm
behind the back axle.
b. How much total weight now rests on the front wheels?
c. How much rests on the back wheels?
He continue loading bricks into the back and notice that the car is getting very low at the back, in fact the
underside of the car is virtually touching the back wheels
d. Was the manual necessarily wrong about the load carrying capacity?
e. What should he do with the bricks to solve this problem?
2. Brent has locked the keys in the house and is trying to get in. He walks around the house and
discovers that there is an upstairs window open. The window is 5 m above ground level and the wall is
made of smooth bricks and is effectively frictionless. Brent has a ladder in his shed which he sets up so
the top rests on the window sill. It is 7 m long and weighs 25 kg. The coefficient of friction between the
ground and the ladder is 0.45.
a. Draw a diagram of Brent climbing a ladder
towards the window. Label the forces acting on
the ladder.
b. Will he be able to climb to the top of the
ladder (ie feet on the top rung) without it
sliding? Assume Brent has a mass of 70kg.
c. How could Rebecca help? (Apart from
having a spare set of keys).
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The Workshop Tutorial Project –MR8T: Equilibrium