Physical World 1 - 11.30am

Test 1b - February 7, 2005

(Last name, first name)

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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. Circle the most relevant answer. In a demonstration a ball is fired vertically into the air from a car that is moving at constant velocity along a horizontal track. When the ball lands, it lands in the car! This occurs because the ball and car have the same

a) horizontal velocity

b) horizontal acceleration

c) vertical velocity

d) vertical acceleration

e) none of the above

2. 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.

3. 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__________________ .

4. What is the acceleration of a car that maintains a constant velocity of 10m/s for 10s?

a = 0m/s2 because it`s velocity is constant.

5. Which of the following statements about the moon is correct?

a) The moon has a constant velocity.

b) There is no net force acting on the moon.

c) The earth exerts a stronger force on the moon than the moon exerts on the earth.

d) The moon has a non-zero acceleration because its velocity is changing.

6. 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).

7. 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.

8. 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 your mass on Earth, i.e. m = W/g = 500/10 = 50kg.

9. 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.

10. The configuration of a demonstration is shown below. The two strings 1 and 2 are identical.

Circle the correct answers. (There may be more than one correct answer for this question.)

a) When the stick is hit quickly string 1 breaks because both the gravitational force of mass M and the force applied to the stick acts on string 1.

b) When the stick is hit slowly string 1 breaks because both the gravitational force of mass M and the force applied to the stick acts on string 1.

c) When the stick is hit quickly string 2 breaks because of the inertia of mass M.

d) When the stick is hit slowly string 2 breaks because of the inertia of mass M

e) The two strings break at random, independent of how quickly or slowly the stick is hit.

11. 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. The car could be going at constant speed around a corner. Under these conditions the velocity is changing because the car`s direction is changing.

12. Why do you lurch forward in a bus that suddenly slows?

Initially, before the bus slows, both the bus and you are traveling at the same velocity. You lurch forward because you continue at this same velocity (Newton 1) after the bus slows, until you grab something to slow you down.

13. In a demonstration using two identical balls Ball 1 was dropped vertically while Ball 2 was shot horizontally with velocity v.

Circle the true statement.

a) Ball 1 hit the ground first because it traveled a shorter distance.

b) Ball 2 hit the ground first because it had a higher velocity.

c) Ball 2 hit the ground first because it has a horizontal velocity.

d) Ball 1 and 2 hit the ground at the same time because they both had the same vertical acceleration.

e) Ball 1 and 2 hit the ground at the same time because they both had the same horizontal acceleration.

14. A car is moving at a constant velocity of 50km/hr, East.

The net force on the car is

a) greater than zero.

b) less than zero.

c) equal to zero.

d) 50N, East.

e) indeterminate because not enough information is given.

15. If the forces that act on a bullet and the recoiling gun from which it is fired are equal in magnitude, why do the bullet and gun have very different accelerations?

a = F(net)/m where F(net) for the bullet and gun are the same. The bullet and gun have quite different masses m and therefore they have quite different accelerations.

16. When you drive a car around a corner at constant speed is your acceleration 0m/s2? If not, explain why not.

No. Your velocity is changing because your direction is changing, therefore you are accelerating.

17. 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.

18. 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.

19. 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.

20. 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.

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Five longer questions. Each question is worth 4 points. (Total points = 20.)

Show all calculations!!

21. In 2.5s a car increases its speed from 60m/s to 65m/s while a bicycle goes from rest to 5m/s.

a) Which undergoes the greater acceleration?

They both have the same acceleration.

b) What is the acceleration of each vehicle?

a = change in velocity/time = (65 – 60)/2.5 = 2m/s2.

22. a) If you stand next to a wall on a frictionless skateboard and push the wall with a force of 80N, how hard does the wall push on you?

F(net) = 80N in the opposite direction.

a = F(net)/m = 80/40 = 2m/s2.

23. A 200kg bear grasping a vertical tree slides down at constant velocity.

a) What is the net force acting on the bear?

F(net) = 0N because the velocity is constant.

b) What is the friction force that acts on the bear?

F(net) = W – F(friction) = mg – F(friction) = 0N because the velocity is constant.

Therefore F(friction) = mg = 200 x 10 = 2000N, up.

24. A plane is flying horizontally at a speed of 10m/s when it drops a food package to some refugees on the ground immediately below. It takes 20s for the package to hit the ground.

a) Will the package hit the refugees immediately below?

No.

b) If not, how far do the refugees have to travel in order to pick up the package?

v = d/t therefore d = vt = 10m/s x 20s = 200m.

c) From what height did the plane drop the package?

d = 0.5gt2 = 0.5 x 10 x 20 x 20 = 2000m.

25. A 100kg parachutist jumps from a plane. An upward force of 200N due to air resistance slows down his descent.

a) What is the weight of the parachutist?

W = mg = 100 x 10 = 1000N

b) What net force is acting on the parachutist?

F(net) = 1000 – 200 = 800N, down.

c) Determine the parachutist`s acceleration.

a = F(net)/m = 800/100 = 8m/s2, down.