1. 3.2.1 Introduction to Work & Energy
How does it do that?
Introduction to Work & Energy

2. Definitions energy: ABILITY TO DO WORK work: A CHANGE IN TOTAL ENERGY
Unit: JOULE
work: A CHANGE IN TOTAL ENERGY
requires MOTION
Unit: JOULE
Equation

3. No, work is NOT being done ‘on the object’ unless it moves.
Example #1 – No Motion
A student is trying to push a heavy crate across the floor, but it does not move.
Is work being done ‘on the crate’?
No, work is NOT being done ‘on the object’ unless it moves.

4. Example #1 – No Motion
But it feels to the student like energy is being used -- what’s going on?
Energy IS being expended and work IS being done, just not ‘on the object’. Work is being done inside the student’s body.

5. Example #2 – Flat Ground
A box is pushed with a force of 100 newtons across a frictionless surface for a distance of 10 meters.
How much work is done on the box?
100 N
10 m

6. Example #2 – Flat Ground
A box is pushed with a force of 100 newtons across a frictionless surface for a distance of 10 meters.
How much work is done on the box?
100 N
10 m

7. Example #2 – Flat Ground
A box is pushed with a force of 100 newtons across a frictionless surface for a distance of 10 meters.
How much work is done on the box?
W = Fd = ΔET
W = (100N)(10m)
W = 1000 J
100 N
10 m

8. Example #3 – Incline
A box is pulled up a 3.0 meter long plane that is inclined at 20° using a force of 10 newtons.
How much work is done in moving the box?
10 N
3.0 m
20°

9. Force is same direction as motion SO W = Fd applies
Example #3 – Incline
A box is pulled up a 3.0 meter long plane that is inclined at 20° using a force of 10 newtons.
How much work is done in moving the box?
Force is same direction as motion SO W = Fd applies
10 N
3.0 m
20°

10. Example #3 – Incline
A box is pulled up a 3.0 meter long plane that is inclined at 20° using a force of 10 newtons.
How much work is done in moving the box?
W = Fd = ΔET
W = (10N)(3.0m)
W = 30 J
10 N
3.0 m
20°

11. Example #4 – Force on an Angle
A sled is pulled a horizontal distance of 4.0 meters across a frictionless surface using a force of 15 newtons at an angle of 30° above the horizontal.
How much work is done on the sled?
15 N
30°
4.0 m

12. Example #4 – Force on an Angle
A sled is pulled a horizontal distance of 4.0 meters across a frictionless surface using a force of 15 newtons at an angle of 30° above the horizontal.
How much work is done on the sled?
Force is DIFFERENT direction than motion… We need to find the horizontal component of the force
15 N
Horizontal Component
30°
4.0 m

13. Example #4 – Force on an Angle
A sled is pulled a horizontal distance of 4.0 meters across a frictionless surface using a force of 15 newtons at an angle of 30° above the horizontal.
How much work is done on the sled?
(1) Ax = Fx = Fcos θ = (15N) cos(30°) = N
(2) W = Fxd =ΔET
W = (12.99 N)(4.0m)
W = 52 J
15 N
30°
4.0 m

14. End of PRACTICE