Fundamental Forces

• We have talked about a lot of different kinds of forces.• Gravitational, friction, normal, fluid resistance, tension…

• Are these all actually different in nature?

• Four fundamental forces (we think):• Gravitational

• Electromagnetic

• Strong

• Weak

• All interactions arise through one of these fundamental forces.

• All of the interactions we have dealt with so far were electromagnetic or gravitational.

Chapter 5 SummaryApplying Newton’s Laws

• Statics:

• Dynamics:

• Friction• Kinetic friction:

• Static friction:

• Rolling resistance:

• Fluid resistance: or

• Terminal velocity:

• Circular Motion

• Fundamental forces

Chapter 6 OutlineWork and Kinetic Energy

• Work and energy• Conservation of energy

• Kinetic Energy• Work-energy theorem

• Work with varying forces

• Power

• Fundamental forces

Work and Kinetic Energy

• In everyday conversation, work would be any activity that requires some effort.

• In physics, work is something that is done to change the energy of an object.

• Energy can take many forms.

• Consider the case of applying a force on a body.• As we saw last chapter, the body will accelerate.

• Its kinetic energy, or energy of motion, will increase.

• If we apply the force over a longer distance, we increase the kinetic energy more.

Work and Forces

• If we apply a constant force, , over some distance , in the same direction as the force, the work done is:

• What are the units for work?• Force is measured in newtons,

and distance in meters.

• Work is therefore measured in newton-meters.

• We call this a joule.

Work and Forces Not Aligned with Displacement

• If the force is not applied in the same direction as the displacement, only the component of the force aligned with the displacement contributes to the work done.

• We only consider the component of the force parallel to the displacement multiplied with the magnitude of the displacement.

• This is simply the dot product of the force and the displacement.

Sign of Work

• Work is a scalar. It does not indicate a direction, but its sign is still quite important.

• The sign of the work done on an object depends on the direction of both the force and the displacement.

• If the force and the displacement are in the same direction, the work done by the force on the body is positive.

• It is crucially important to indicate who/what is doing the work and who/what the work is done upon.

Total Work

• In general, there are often more than one force acting on an object.

• The total work done on the object depends on the net force and the displacement of the object.

• Multiple forces/bodies can do different, sometimes opposing, amounts of work on the body.

Work Example

Kinetic Energy

• When a force is applied to a body, it causes an acceleration.

• A force applied over a distance, as we have said is the definition of work.

• This changes the kinetic energy of the object.

• How can we relate work done to change in kinetic energy?

Kinetic Energy

• When a force is applied to a body, it causes an acceleration.

• A force applied over a distance, as we have said is the definition of work.

• This changes the kinetic energy of the object.

• How can we relate work done to change in kinetic energy?

Work-Energy Theorem

• Kinetic energy is:

• The net work done on a body is equal to the change in kinetic energy.

• This is the work-energy theorem.

Work-Energy Example