A student tries to lift a 85lb rock. At the end of 10 seconds the rock hasn't moved. How much work was done on the rock?
If the rock hasn't moved, it means that the net force acting on the rock is zero, and therefore no work is done on the rock.
If it took him 1.4 seconds to lift the rock how much power was needed?
To determine the power needed to lift the rock, we need to know the amount of work done in lifting it and the time it took to do that work.
The work done on an object can be calculated using the formula:
Work = Force x Distance
In this case, the force needed to lift the rock is its weight, which is given as 85 lb. The distance it is lifted can be calculated using the equation of motion:
Distance = 0.5 x Acceleration x Time^2
Assuming the rock is lifted vertically, the acceleration due to gravity is 32 ft/s^2. Plugging in the values, we get:
Distance = 0.5 x 32 ft/s^2 x (1.4 s)^2 = 31.808 ft
Therefore, the work done on the rock is:
Work = Force x Distance = 85 lb x 31.808 ft = 2724.28 lb·ft
To determine power, we can use the formula:
Power = Work / Time
Plugging in the values, we get:
Power = 2724.28 lb·ft / 1.4 s = 1945.914 lb·ft/s
Therefore, approximately 1945.914 lb·ft/s of power was needed to lift the rock.