3.3: Work, Energy and Power, F

Question 1

Define: Work done

Answer 1

A transfer of energy (Force x distance)

1 Joule of work done when a force of 1 newton moves an object a distance of 1 metre

Question 2

Name one key assumption made with the equation, work done = Fxd

Answer 2

Assumes direction of force is the same as the direction of movement

Question 3

How would you work out the work done if the force isn’t in the same direction as the movement

Answer 3

W = F x cos(Θ)

Θ = angle between direction of force and motion

Question 4

Define: Power (in terms of motion and forces)

Answer 4

Work done per second(Rate of doing work)

Question 5

List 2 equations you can use to link power and speed

Answer 5

P = E(or Wd)/t and Wd = Fx

Question 6

Define: Close system

Answer 6

All energy transfers are accounted for, energy/matter cannot leave or enter a closed system

Question 7

How do you derive the equation for kinetic energy

Answer 7

-we know change in Ek = work done(F x s)
-F=ma ∴ Ek = m a s
-Using equation of motion(SUVAT)
v^2=u^2 +2as rearranged
a s = 1/2(v^2-u^2) ∴
-Ek = 1/2mv^2 - 1/2mu^2
Consider acceleration from rest ∴ u =0
- ∴ Ek = 1/2mv^2

Question 8

How do you derive the equation for gravitational potential energy

Answer 8

-we know change in Ek = work done(F x h)
-F=ma ∴ Ep = ma h
-a = g ∴
Ep = m g h

Question 9

Define: Priniciple of conservation of energy

Answer 9

Energy cannot be destroyed/created. Energy can be transferred from one form to another but the total amount of energy in a closed system will not change

Question 10

Why can a device never be 100% efficient

Answer 10

Because friction always exists as there is work being done