Work, Impulse, Momentum and Power Physics 2 Unit 1

Work Definition: the transfer of energy to an object by the application of a force Formula: W = F x d (work equals force times distance) Units: Units for work are Joules Work is only done when a force causes a change in position or displacement As a weightlifter holds a barbell over his head, is he doing any work? Why or why not?

Example problem 1 Imagine a father playing with his daughter by lifting her repeatedly in the air. How much work does he do with each lift assuming he lifter her 2 meters and exerts an average force of 190N? W = F x d W = 190N x 2m W= 380J

Example Problem 2 The brakes on a bicycle apply 125N of frictional force to the wheels as the bicycle travels 14m. How much work have the brakes done on the bicycle? W = F x d W = 125N x 14m W = 1750J

Power Definition: power is a quantity that measures the rate at which work is done Formula: P = W / t (Power equals work divided by time) Units: Watts As time decreases while applying the same amount of work, what happens to the power output?

Horsepower Horsepower is referred to the average power output of a draft horse 1hp = 746 watts With that much power, a horse could raise a load of 746 apples, weighing 1lb each, by 1 meter every second

Example Problem 1 While rowing across the lake during a race, John does 3960J of work on the oars in 60 seconds. What is his power output in watts? P = W / t P = 3960J / 60 sec P = 66 watts

Example Problem 2 Using a jack, a mechanic does 5350J of work to lift a car 0.5m in 50 seconds. What is the mechanic’s power output? P = W / t P = 5350J / 50sec P = 107 watts

Kilowatts Often times power is expressed in kilowatts 1kW is 1000 Watts

Impulse Definition: Impulse is the amount of force exerted on an object per unit of time Formula: I = F x t (Impulse equals force multiplied by time) Units: Ns (Newton seconds)

Example Problem 1 Calculate the impulse of a car crash that exerts a force of 250N over a time of 0.8 seconds. I = F x t I = 250N x 0.8sec I = 200 Ns

Example Problem 2 Calculate the impulse of a collision of a ball and baseball bat with a force of 100N over a contact time of 1.25 seconds. I = F x t I = 100 x 1.25 sec I = 125Ns

Momentum Definition: The product of the mass times the velocity of an object Formula: p = m x v (momentum equals mass multiplied times velocity) Units: kg m/s (kilograms meters per second) The greater the mass, the greater the momentum, the greater the velocity the greater the momentum

Which has more momentum? Which has more momentum, a skateboarder skating in a skatepark or a semi truck stopped at a rest stop? Why?

Example Problem 1 Calculate the momentum of a 75kg speed skater moving forward at 16m/s P = m x v P = 75kg x 16m/s P = 1200 kgm/s

Example Problem 2 Calculate the momentum of a 6kg bowling ball moving at 10m/s down the alley towards the pins. P = m x v P = 6kg x 10m/s P = 60kgm/s