1. Work, Impulse, Momentum and Power
Physics 2
Unit 1

2. 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?

3. 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

4. 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

5. 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?

6. 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

7. 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

8. 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

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

10. 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)

11. 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

12. 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

13. 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

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

15. 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

16. 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