1. Machines - Ch. 7 I. Introduction to Machines (p.180-183) Machines
Force
Work
Mechanical Advantage

2. A. Machines Machine device that makes work easier
changes the size and/or direction of the exerted force

3. B. Force Effort Force (Fe) force applied to the machine “what you do”
Resistance Force (Fr)
force applied by the machine
“what the machine does”

4. C. Work Win = Fe × de Wout = Fr × dr Work Input (Win)
work done on a machine
Win = Fe × de
Work Output (Wout)
work done by a machine
Wout = Fr × dr

5. C. Work Fe × de = Fr × dr Win = Wout Conservation of Energy
can never get more work out than you put in
trade-off between force and distance
Win = Wout
Fe × de = Fr × dr

6. C. Work Win = Wout Win > Wout In an ideal machine...
But in the real world…
some energy is lost as friction
Win > Wout

7. D. Mechanical Advantage
Mechanical Advantage (MA)
number of times a machine increases the effort force
MA > 1 : force is increased
MA < 1 : distance is increased
MA = 1 : only direction is changed

8. D. Mechanical Advantage
A worker applies an effort force of 20 N to open a window with a resistance force of 500 N. What is the crowbar’s MA?
GIVEN:
Fe = 20 N
Fr = 500 N
MA = ?
WORK:
MA = Fr ÷ Fe
MA = (500 N) ÷ (20 N)
MA = 25 MA Fr Fe

9. D. Mechanical Advantage
Find the effort force needed to lift a N rock using a jack with a mechanical advantage of 10.
GIVEN:
Fe = ?
Fr = 2000 N
MA = 10
WORK:
Fe = Fr ÷ MA
Fe = (2000 N) ÷ (10)
Fe = 200 N MA Fr Fe