A car is moving in the uniform speed of 20m/s. If the frictional force is 500N, what will be the power of the engine?
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Answer:
Physical World 1 - 11.30am
Test 1a - February 7, 2005
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Formulas
Speed or velocity = change in distance/change in time, v = d/t [Units: m/s]
Acceleration = change in velocity/change in time, a = (vf - vi)/t [Units: m/s/s or m/s2]
For acceleration under gravity (starting from rest at time t = 0s): g = 10m/s2
v = gt
d = 0.5gt2
Newton's second law of motion
Net force, Fnet = ma [Units: N]
Weight, W = mg [Units: N]
Twenty short answer questions. Each question is worth 4 points. (Total points = 80.)
1. If a car moves with a constant speed, can you say that it also moves with a constant velocity? Give an example to support your answer.
No. For example, the car could be traveling at constant speed around a corner. In this case it`s direction is changing and therefore it`s velocity is changing.
2. A car collides with an insect. If the car exerts a force of 10N on the insect what force does the insect exert on the car?
a) Less than 10N.
b) Greater than 10N.
c) Exactly equal to 10N.
d) Indeterminate.
3. You throw a ball into the air while standing on the Earth.
a) The ball has an acceleration of 10m/s2, down as it rises and 10m/s2, up as it falls.
b) The ball has an acceleration of -10m/s2, down as it rises and 10m/s2, down as it falls.
c) The ball has an acceleration of 10m/s2, down as it rises and 10m/s2, down as it falls.
d) The ball has an acceleration of 10m/s2, up as it rises and 10m/s2, down as it falls.
e) None of these answers.
4. What force do you have to supply in order to pull a wagon along a level sidewalk at a constant speed of 1m/s if the frictional force is 400N?
a) 800 N because you need twice the force to overcome the friction force.
b) 0 N because you are walking at constant speed.
c) 400 N because you have to compensate for the frictional force.
d) Indeterminate because you don`t know the mass of the wagon.
5. If a cheetah runs at 25m/s, how long will it take the cheetah to run a 100m dash? (Show your calculation.)
v = d/t therefore t = d/v = 100m/25m/s = 4s.
6. A painting staging (shown below) that weighs 300N supports two painters, one 250N and the other 300N. The tension in the left rope is 400N. Determine the tension in the right rope if the painting stage is in mechanical equilibrium. (Show your calculations.)
F(net) = 400 + F – 250 – 300 – 300 = 0N because the system is in mechanical equilibrium.
Therefore F = 250 + 300 + 300 – 400 = 450N.
7. Circle the most accurate answer.
An object has a net force of 0N acting upon it.
a) No forces are acting on the object.
b) The object is stationary.
c) The object is stationary or moving at constant velocity. (This is just Newton 1.)
d) The object is accelerating at a non-zero acceleration.
e) The object is moving at a non-zero constant velocity.
8. What is the acceleration of a car that maintains a constant velocity of 10m/s for 10s?
0m/s2 because the car is traveling at constant velocity.
9. Circle the most accurate answer.
An object has zero acceleration. Therefore the object is moving at
a) zero speed.
b) zero velocity.
c) constant speed.
d) constant velocity.
10. If you weigh 500N on Earth, what is your weight in space where g = 0m/s2?
W = mg = m x 0 = 0N.
What is your mass in space?
Same as on Earth, i.e. m = W/g = 500/10 = 50kg.
11. When you drive a car around a corner at constant speed is your acceleration 0m/s2? If not, explain why not.
No, because your direction is changing and therefore your velocity is changing. Hence you are accelerating.
12. Can an automobile with a velocity toward the north have an acceleration toward the south? Explain.
Yes. The automobile is decelerating (i.e. slowing down).
13. You drive 400 miles South to Oklahoma City. The trip takes you 8 hours. Your average velocity is _______50 miles/hr, South_______________________ and your average speed is ____________50 miles/hr__________________ .