Work is done when A force is exerted on an object An object has a displacement in the same direction as the force W Fdcos q SI unit for work is Joule J 1 J 1Nm 07376 ID: 706969
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Slide1
Energy
Work and PowerSlide2
In physics, work has a very specific definition.
Work is done when:
A force is exerted on an objectAn object has a displacement in the same direction as the forceW=FdcosqSI unit for work is Joule (J)1 J = 1N*m (=0.7376 ft*lb)
WorkSlide3
A 15 kg block is dragged over a rough, horizontal surface by a 70 N force acting at 20 degrees above the horizontal. The block is displaced 5 m, and the coefficient of kinetic friction is 0.3. Find the work done by:
The 70 N force
The normal forceThe force of gravityThe friction forceExample 1Slide4
We are only interested in the component of Force that acts in the same direction (along the same axis) as the displacement.
By using the equation:
W=Fdcosq, we are finding the scalar product of the two vectors, force and displacement.The scalar product is also called the dot product.
WorkSlide5
The dot product is positive for angles between 0 and 90, negative if the angle is between 90 and 180 and zero if the vectors are perpendicular.
Dot productSlide6
Start with the scalar products of the unit vectors:
Calculating Dot ProductSlide7
Calculating Dot ProductSlide8
A particle moving in the
xy
plane undergoes a displacement d=(2.0i+3.0j)m as a constant force F=(5.0i+2.0j)N acts on the particle. Calculate the work done by the force. Calculate the angle between the force and the displacement.Example 2Slide9
Energy is defined as the ability to do work.
Energy dealing with motion and forces is called mechanical:
Kinetic PotentialEnergySlide10
Kinetic Energy is the energy of motion.
Potential Energy is stored energy. We will primarily use gravitational potential energy.
Kinetic and PotentialSlide11
Energy in a closed system cannot be gained or lost; it is transferred from one type to another.
Non-conservative forces (or dissipative forces) can add or subtract to the mechanical energy of the system.
Conservation of EnergySlide12
If the force changes over time, we need to look at the force in small pieces to get a more accurate picture of the work that is done.
W
As we make the divisions smaller and smaller, our picture of the work being done gets more accurate. Which leads us to…
Varying ForceSlide13
Spring force is linearly related to amount the spring is displaced (stretched or compressed).
The spring force acts in the opposite direction of the displacement.
Spring ForcesSlide14
The spring constant varies from spring to spring. It is dependent on the spring’s material, shape and size.
(Newton/meter)
Spring ConstantSlide15
(where x
0
= 0 m)
Work on/by a SpringSlide16
Power is the rate at which work is done.
…..measured in Watts
Power