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WARM UP QUESTION
Al Stein nodded off while driving. His 1500-kg car hit a
series of guardrails while moving at 19.8 m/s. The first
guard rail delivered a resistive impulse of 5700 Ns. The
second guard rail pushed against his car with a force of
79000 N for 0.12 seconds. The third guard rail collision
lowered the car's velocity by 3.2 m/s. Determine the final
velocity of the car.
Final velocity = 6.5m/s
FORCE-TIME GRAPHS
The area under the line of a force-time graph represents
the change of momentum (or the impulse of the force)
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F t = Δp
The velocity of a car of mass 600kg was reduced from 15m/s
by a constant force of 400N which acted for 20s then by a
constant force of 20N for a further 20s.
a) Sketch the force versus time graph for this situation
b) i) Calculate the initial momentum of the vehicle
ii) Use the force versus time graph to determine the total
change in momentum
iii) Show that the final velocity of the vehicle is 1m/s
a) Sketch the force versus time graph for this situation
b) i) 9000 kgm/s
ii) 8400 kgm/s
iii) 9000 – 8400 = pf
Pf = 600kgm/s
P = mv
600 = 600v
V=1m/s
400
20
20 40 Time (seconds)
Force (Newtons)
WORK, ENERGY, POWER
ENERGY REFRESH
- Energy stores
- Gravitational potential (position of objects in a gravitational field)
- Kinetic (moving objects)
- Thermal stores (hot objects)
- Elastic stores (objects compressed or stretched)
- Energy transfer
- By radiation (light)
- Electrically
- Mechanically (sound)
- 1 Joule = 1 Newton Metre
Energy cannot be created or destroyed! Only transferred!
FOR EACH OF THE FOLLOWING…
Indicate which force(s) are doing work on the
object
Calculate the work done
- F (^) app is the only force doing work
- Work = +50J
- F (^) frict is the only force doing work
- Work = -50J
- F (^) app and F (^) frict are doing work
- Work done by F (^) app = +50J
- Work done by F (^) frict = -50J
- No work is being done
- F (^) grav and F (^) tens are doing work
- Work done by F (^) grav = -100J
- Work done by F (^) frict = +100J
Calculate the work done by a 2.0-N force (directed at a 30°
angle to the vertical) to move a 500 gram box a horizontal
distance of 400 cm across a rough floor at a constant speed
of 0.5 m/s.
4.0 J
A tired squirrel (mass of 1 kg) does push-ups by
applying a force to elevate its center-of-mass by 5
cm. Estimate the number of push-ups that a tired
squirrel must do in order to do a approximately 5.
Joules of work.
10 push-ups
