Now that you have explored our page on the physics concept of work, you are ready to learn about power.
By definition, Power is the amount of work that is performed in a certain amount of time.
P = W / t
Since work is measured in the unit of Joules, and time is measured in seconds, Power is measured in Joules/Second, or watts. Most often though, watts are to small of a measurement, therefore we use the measurement of kilowatts (1000 watts.) Machines tend to use horsepower rather than watts. One horsepower is equal to 750 watts.
While a bulky weightlifting may look as if he is more powerful than woman lifting lighter weight, the woman may have more power. If each weightlifter was doing the same amount of work, but in shorter amounts of time, they would have different amounts of power.
Scenario 1:
A man is bench presses 225 lbs (102.27 kg), 10 times in 1 minute in a distance of 0.5 m Another man bench presses 225 lbs, but 10 times in 10 seconds also in a distance of 0.5 m Who has more power?
Answer:
Both men do the same amount of work but in different times. They each do1002.27 J of work. W=(102.27*9.8m/s^2)(0.5m). Man one does 16.75 watts of power. (1002.27 J / 60 seconds.) Man two does 100.23 watts of power. (1002.27 / 10 seconds.) Because Power = work/time, the second man has more power because he does the same amount of work in less time.
Now let us take a look on how friction affects weightlifting.
As stated on our “Basics” page, Friction is the force that opposes the motion of an object. It acts against the movement of an object and is equal in strength to the force applied to the object. When using free weights the concept of Friction really does not come into play because the only thing touching the weight is your hand (which is not supposed to move on the bar) and air. The only friction that is applicable when using free weights is Static Friction. Static friction is the frictional force which acts in opposition to the initial movement of an object (in this case a weight) from its resting position. If the force that you apply is not greater than the value of the static friction, the weight will not move. Static friction opposes the start of motion, thus if you are not strong enough to move the weight, it will not move.
The other type of friction involved when using weightlifting machines is Kinetic Friction. Kinetic Friction is the frictional force that opposes the motion of an object and acts to slow it down when no other outside forces are being applied to propel it. In order to keep your weight moving at a constant velocity (Nobody lifts in jerky movements!) you must apply an equal force but opposite in direction of the force friction.
The force of Static Friction is greater than Kinetic Friction. That is why it is easier to continue pushing an object once it is already moving than it is to initially get the object going. The frictional force can be calculated by multiplying the coefficient of friction (a constant) by the normal force. The normal force is equal in magnitude and opposite in direction to the force of weight.
Ff = uFN
Force friction = coefficient of friction x Force normal
The coefficient of friction is a constant, but is different for every material. You must apply a large force over the force friction to get the object moving. If your force applied is not larger than the force friction, the object will not move.
Friction plays a key role in weightlifting machines. You will notice that sometime weightlifters cannot even pull a bar down that is on a system of pulleys? Are they not strong enough? No. Sometime it is actually the friction between the plastic coated wires and the pulleys. Thus, it is always important to keep your machines lubricated. A lubrication is a substance that reduces friction, heat, and wear as a film between solid surfaces. Failure to do so will create difficulty for the weightlifter!