Conservation of Energy 1/6/2020

Conservation of Energy 1/6/2020

Work and Energy

Open Ended

What is work?

Essential Question

What are work and energy and how are they related?

Objective

Students will discover the relationship between work and energy using focused not taking.

Discussion

Exercising to stay healthy is sometimes called working out.  Job related activities are referred to as work.  How do scientists define the term work?

Work

• Consider a force exerted on an object while the object moves a certain distance.

• There is a net force, so the object is accelerated. (a=F/m) and its velocity changes.

• There is a relationship between the acceleration, velocity and distance. (v_f²=v_i² + 2ad)

• We can replace a with F/m

Work

• Long story short, we end up with the following:

• Fd = 1/2m_iv² - 1/2mv_f²

• The left side of the equation describes an action that was done to the system by the external world.

• A force was applied to the system while the point of contact moved.

Work

• The SI unit for work is called a joule (J).

• One joule is equal to 1 N*m.

• That means one joule of work is done when a force of 1 N acts on a system over a displacement of 1 m.

Work

When a force (F) is applied through a displacement, work (W) is done on the system.

Work

In other words, when a force causes on object to move, work is done on the object.

Work done by a constant force

• If a constant force is applied to a system...

• Work is the product of the force and the magnitude of the system's displacement.

• Thus...

• W=Fd

Constant Force exerted at an angle

• When the force is applied horizontally, Θ is 0.

• Cos (0) = 1

• So, W=Fd(1) or W = Fd

• Our original formula.

Constant Force Exerted at an angle

• What happens when the force is exerted at an angle?

• When this scenario takes place, the angle (Θ) has to be included.

• Our work equation changes slightly

• W=Fd cos(Θ)

Constant Forces at an angle

• What about an orbiting planet?

• The velocity is perpendicular to the force and is constant.

• cos(90) = 0

• So, W=Fd(0), or

• W=0 J

Work done by many forces

• Suppose you are pushing a box on a frictionless surface while your friend is trying to prevent you from moving it.

• What forces are acting on the box?

• For instance, you are exerting a force to the right of 3 N and your friend is exerting a force to the left of -1.5 N.

• If the box moves 1 m to the right, The work done by you would by 3 J and the work done by your friend would be -1.5 J.

• The TOTAL work done would be 1.5 J (3 J - 1.5 J)

Problem Solving Strategies for Work

In your Google Classroom Reference Material there is a sheet that provides problem solving strategies for work problems.

Open Ended

Exit Ticket - Why do we have to take the cosine of theta into account when solving for work?