Work: a force acting through a distance
Work = Force X Distance
W = F d
The units of force are Newtons and the units of distance are meters
The answer is in Newton-Meters. These units are referred to as Joules
Work Problems
1. A force of 800 Newtons is needed to push a car across a lot. Two students push the car 40 meters. How much work is done?
2. How much work is done in lifting a 60Kg crate a vertical distance of 10 meters?
3. A 1000 N mountain climber scales a 100 m cliff. How much work is done?
4. You are using 50 N of force to push an empty cart down the school hall. Your friend, who weighs 800 N, jumps on the cart and wants to be taken for a ride. You push your friend down the hall for 20 meters before being caught by the Principal. How much work did you do pushing your friend on the cart?
Power: the rate at which work is done
Power = Work / Time or Power = Force X Distance / Time
P = F d / t
In this equation, the units of work are Joules and the units of time are seconds
The answer is in Joules per second. These units are referred to as Watts
Power Problems
1. A machine produces 4000 Joules of work in 5 seconds. How much power does the machine produce?
2. A box that weighs 1000 Newtons is lifted a distance of 20 meters in 10 seconds. How many kilowatts of power are produced?
Machines make work easier by changing the size or direction of a force.
2 forces are always involved when using a machine
1. Resistance Force - FR -the force applied by a machine
2. Effort Force - FE - the force applied to a machine
Work is done to a machine as well as by a machine
1. Work Output - WO - the work done by a machine
2. Work Input - WI - the work done on a machine
1. Effort Distance - dE - the distance through which the machine moves 2. Resistance Distance - dR - the distance the object moves
When using a machine, distances are not the same
Mechanical Advantage: the number of times a machine multiplies the effort force.
Mechanical Advantage = Resistance Force / Effort Force
MA = FR / FE
MA is equal to 1 - the machine is used to change the direction of the effort force.
MA is less than 1 - the machine is used to increase the distance an object moves or the speed at which it moves.
MA is greater than 1 - the machine is used to increase the effort force.
Efficiency is always expressed as a percentage
Work output is never greater than work input.
Efficiency: a comparison of work output to work input
Efficiency = Work Output / Work Input X 100
Efficiency = WO / WI X 100
Mechanical Efficiency Problems
1. What does friction do to the efficiency of a machine?
2. You use a pair of pliers to crack a pecan. It takes 1200 N of resistance force to crack the pecan, but you only exert 400 N of effort force with your hand on the pliers. What is the mechanical advantage of the pliers?
3. Think of pliers as a pair of first-class levers. The fulcrum about which the levers turn is located at point B. The lever arm represented by the line AB is 6 cm long. The lever arm represented by line BC is 3 cm long. What is the mechanical advantage of this set of pliers?
- adapted from www.middleschoolscience.com