ENERGY UNIT: WORK
– PowerPoint Note
SPH 3U
Energy can come in many forms including:
– Thermal (energy in the form of moving atoms)
– Electrical (energy possessed by charged particles)
– ________________(energy found in EM waves)
– Nuclear (energy stored in holding the atom together)
– Gravitational (energy stored due to a raised elevation)
Common Energy Categories:
– Kinetic (energy due to motion of objects)
– _______________ (energy stored in compression or stretch)
– Chemical (energy stored in the bonds between atoms molecules or compounds)
– Sound (energy in vibrations)
LAW OF CONSERVATION OF ENERGY:
• Energy cannot be created nor destroyed,
• Energy can only be transformed from one form to another
• Eg. A stove heating a pot of water:
Electrical 🡪 Radiant 🡪 Thermal
WORK
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We know energy is the ability to do work, but what is work?
WORK is the transfer of energy to an object by an applied force over a distance
Symbol for Work is W measured in kg m2/s2 or N•m or J
(J = joules)
Symbol for Energy is E measured in J (joules)
Mechanical work is done on an object when a force displaces an object.
W = F • Δd
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Note that this equation only applies when the ______________________________________________ are
in the _____________ direction.
When the force and displacement are not entirely in the same direction, the component of the force in the
direction of the displacement is used.
SAMPLE PROBLEMS:
1) How much mechanical work will be done pushing a shopping cart 3.5m with a force of 25N in the same
direction as the displacement?
2) A curler applies a force of 15.0N on a curling stone and accelerates the stone from rest to a speed of
8.00m/s in 3.5s. Assuming the ice surface is level and frictionless, how much mechanical work is done on
the stone?
When there is NO displacement… NO work is done!
(Ex. pushing on something that doesn’t move)
3) How much mechanical work is done on a stationary car if a student pushes it with a 300 N force and fails to
displace the car?
Work Done in Different Directions OR on an Angle
The dot product of two vectors gives scalar quantity, in this case, WORK.
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W = F • Δd
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W = F • Δd cos Ɵ
Complete Equation:
As will be shown in the following- example. Work is only done when the net force is in the direction of
the movement of the object. The force applied in the vertical direction accomplishes no work if the object
moves in the horizontal direction.
In addition, if an object experiences displacement but no force, then zero work is done in that direction.
4) Velora wears a 10 kg backpack as she walks forward a velocity of 8.0 m/s for a distance of 12 m.
How much mechanical work is done on her backpack?
5) Calculate the work done by a custodian on a vacuum cleaner if the custodian exerts an applied force of 50.0
N on the vacuum hose and the hose makes a 30° angle with the floor. The vacuum moves 3.0 m to the right
on a flat level surface.
6) A shopper pushes a shopping cart on a horizontal surface with a horizontal applied force of 41.0 N for 11.0 m.
The cart experiences a force of friction of 35.0 N. Calculate the total work done on the cart.
Positive & Negative Work:
Work can also be positive or negative relative to the motion, as shown in the next example.
7) A shopper pushes a shopping cart on a horizontal surface with a horizontal applied force of 41.0 N for 11.0 m.
the cart experiences a force of friction of 35.0 N. Calculate the total mechanical work done on the shopping cart.
A) Work done by the applied force
B) Work done by the force of friction.