KIN 3514 Exam 2: Work, Power, and Energy

What is work?

The product of force and displacement in the direction of the force.

When is work performed?

When a force causes an object to move (displacement occurs).

What is the formula for work?

W = F · d (in the direction of the force)

What is positive work?

Work done on an object when force and displacement are in the same direction.

What is negative work?

Work done by an object when force and displacement are in opposite directions.

What does the sign of work indicate?

The direction of energy transfer.

If you push an object forward and it moves forward, what type of work is done?

Positive work.

If a force opposes motion (like friction), what type of work is done?

Negative work.

Can you do work without movement (e.g., holding a heavy object still)?

No mechanical work is done because there is no displacement.

What is zero work?

Work = 0 when there is no displacement.

Example of zero work?

Holding a barbell still during a bench press.

Example of negative work in bench press?

Lowering the barbell.

Why is lowering the bar negative work?

Force (upward) opposes displacement (downward).

Example of positive work in bench press?

Lifting the barbell upward.

Why is lifting the bar positive work?

Force and displacement are both upward.

What is an isometric contraction?

A contraction where the muscle length does not change.

Is displacement present in isometric contraction?

No.

Is mechanical work performed in isometric contraction?

No

What is a concentric contraction?

A contraction where the muscle shortens.

In what direction does muscle force act during a concentric contraction?

In the same direction as the motion.

What type of work is performed in concentric contraction?

Positive work.

What is an eccentric contraction?

A contraction where the muscle lengthens while contracting.

In what direction does muscle force act during an eccentric contraction?

Opposite to the direction of motion.

What type of work is performed in eccentric contraction?

Negative work.

When a force is applied but no movement occurs, what is the work?

Zero work.

What type of muscle contraction results in zero work?

Isometric contraction.

What type of work occurs when a pitcher throws a ball?

Positive work.

What type of work occurs when a first baseman catches a ball?

Negative work.

If force and displacement are in the same direction, what type of work is done?

Positive work.

If force and displacement are in opposite directions, what type of work is done?

Negative work.

What is energy (in mechanics)?

The ability to do work.

What is the relationship between energy and work?

ΔE = W

What are the two main types of mechanical energy?

Kinetic energy (KE) and potential energy (PE).

What is kinetic energy?

Energy an object has due to its motion.

What objects have kinetic energy?

Any object that is moving.

What factors affect kinetic energy?

Mass and velocity.

What is the formula for kinetic energy?

KE = ½mv²

What are the units of kinetic energy?

Joules (J) = kg·m²/s²

What does greater kinetic energy mean?

Greater ability to do work.

What is potential energy?

Stored energy due to position or deformation.

What is gravitational potential energy?

Energy due to an object's height above the ground.

What is the formula for gravitational potential energy?

PE = mgh

How does height affect PE?

Higher height → greater potential energy.

What is strain energy?

Energy stored when an object is stretched or compressed.

What is another name for strain energy?

Elastic potential energy.

What is the formula for strain energy?

SE = ½k(Δx)²

What does each variable represent?

k = stiffness (spring constant) Δx = change in length (deformation)

What affects strain energy?

Stiffness of the material and amount of deformation.

What increases potential energy?

Greater height or greater deformation.

What increases kinetic energy?

Greater mass and especially greater velocity.

What is the work-energy theorem?

Net work done on a system equals the change in kinetic energy. (Wₙₑₜ = ΔKE)

What causes a change in kinetic energy?

Work done by external forces.

What is required for a large change in kinetic energy?

A large force applied over a large displacement.

Why do athletes increase displacement in movements (e.g., shot put)?

To maximize work and increase kinetic energy.

What is the law of conservation of energy?

Energy cannot be created or destroyed, only transformed.

When is mechanical energy conserved?

When only gravity acts (no friction or other external forces).

What is total mechanical energy?

KE + PE

What happens to total energy in an isolated system?

It remains constant.

Where is kinetic energy maximum in a pendulum?

At the lowest point.

Where is potential energy maximum in a pendulum?

At the highest points.

What energy conversions occur in a pendulum?

PE → KE → PE

Why does a real pendulum eventually stop?

Friction removes energy from the system.

What type of energy is present at the start of the jump?

Gravitational potential energy (PE).

What happens to energy as the jumper falls?

PE → KE

When is kinetic energy maximum?

Just before the rope starts to stretch.

What happens when the rope stretches?

KE → strain (elastic) energy (SE).

When is strain energy maximum?

At maximum rope stretch.

What happens as the jumper moves upward again?

SE → KE → PE

What type of energy is present at the top again?

Gravitational potential energy.

What does power measure?

How quickly work is performed.

Why is power important for athletes?

It determines how quickly they can generate force (important in explosive movements).

Examples of power-based sports?

Weightlifting, sprinting, jumping.

How does power change at different speeds in powerlifting?

Power is low at very slow speeds and also low at very high speeds.

When is power maximized?

At moderate speeds (balance of force and velocity).

When is mechanical power greatest?

When both force and velocity are at moderate levels.