Excursions in Physics

First Hour Exam

14 May 1999

Statistics:

High score: 97.5 %

Mean: 79.4%

Low: 57.5%

PHY 3050C

Excursions in Physics

"Excursions Live!"

First Hour Exam

May 14, 1999

Enter all your answers in the "scantron sheet" or the "bubble sheet". Turn in only that sheet. Anything you write on this exam will not be seen or used or considered or graded. Be sure your name is on the "bubble sheet" you hand in. Be sure your name is bubbled-in. Be sure your answers are recorded correctly.

For every question, also consider the following as a possible answer:

E) none of the above

1. Kinematics is a description of motion. Motion was first well understood

A) by Aristotle in ancient Greece

B) by Ptolemy in Egypt

C) by Galileo in Italy

D) by Joule in England

2. To measure the time needed to investigate motion,

A) Aristotle used the pendulum clock which had just been invented

B) Ptolemy used a sundial

C) Galileo invented his own water clocks

D) Newton invented the pendulum clock

3. What is the average speed of a motorcycle that travels 60 m in 4 s?

A) 15 m/s; v = x /t = 60 m/ 4 s = (60/4)(m/s) = 15 m/s

B) 12 m/s

C) 10 m/s

D) 9.8 m/s

4. What is the average speed of a car that travels 100 km in 5 h?

A) 500 km/h

B) 25 km/h

C) 20 km/h; v = x /t = 100 km/ 5 h = (100/5)(km/h) = 20 km/h

D) 9.8 km/h

5. Consider a train that has an acceleration of 2 m/s². Initially, at time t = 0, it has a velocity of v_i = 10 m/s. What is its speed at t = 4 s?

A) 43 m/s

B) 32 m/s

C) 24 m/s

D) 18 m/s; v = v_i + a t = (10 m/s) + (2 m/s²)(4 s) = 10 m/s + 8 m/s = 18 m/s

6. Consider a car that starts at rest (v_i = 0) and accelerates at 2 m/s² for 3 seconds. At that time, t = 3 s, how fast is it going?

A) 36 m/s

B) 24 m/s

C) 12 m/s

D) 9 m/s

E) 6 m/s; v = v_i + a t = ( 0 ) + (2 m/s²)(3 s) = 6 m/s

7. Consider a car that starts at rest (v_i = 0) and accelerates at 3 m/s² for 4 seconds. At that time, t = 4 s, how far has it gone?

A) 36 m

B) 24 m; x = v_i t + (¹/₂) a t² = (¹/₂) a t² = (¹/₂) (3 m/s²) (4 s)² = (¹/₂) (3) (16) m = 24 m

C) 12 m

D) 9 m

8. Consider a ball that is thrown upward at the edge of a canyon with an initial velocity of 20  m/s. Three seconds later, at t = 3 s, what is its velocity?

A) 30 m/s

B) 15 m/s

C) - 10 m/s; v = v_i + a t = ( + 20 m/s ) + ( - 10 m/s²)(3 s) = ( 20 - 30 ) m/s = - 10 m/s

D) - 30 m/s

9. Consider a ball that is thrown straight upward at the edge of a canyon with an initial velocity of 20 m/s. Three seconds later, where is it located? Take its initial position, at the edge of the canyon, to be the origin; that is, yi = 0.

A) 30 m

B) 15 m; y = v_i t + (¹/₂) a t² = (+ 20 m/s)(3 s) + (¹/₂) ( - 10 m/s²) (3 s)²

y = [ + 60 + (¹/₂) ( - 10 ) (9) ] m = [ + 60 - 45 ] m = 15 m

C) &endash; 10 m

D) &endash; 30 m

10. Consider a ball that is thrown horizontally from the edge of a building with an initial velocity of 20 m/s. The building is 5 m above the driveway below. How far from the building does the ball strike the driveway?

A) 5 m

B) 10 m

C) 20 m

D) 40 m

Before we can answer how far (distance), we must first answer how long (time).

How long does it take the ball to fall the vertical distance of 5 m?

y = (¹/₂) a t²

- 5 m = (¹/₂) ( - 10 m/s²) t²

1 = (1/s²) t²

1 s² = t²

t = 1 s

Now, how far does it travel horizontally in this time of 1 s?

v_x = v_xi = constant

x = v_x t = (20 m/s) (1 s) = 20 m

11. When a ball or stone or other object is thrown or hit or fired, and air resistance can be neglected, the resulting motion is known as projectile motion The path of an object in projectile motion is

A) a straight line

B) a hyperbola

C) a parabola

D) a quadrant of a circle

12. Projectile motion is a combination of

A) horizontal motion with constant, non-zero acceleration and vertical motion with constant velocity

B) horizontal motion with constant non-zero acceleration and vertical motion with constant, non-zero acceleration

C) horizontal motion with constant velocity and vertical motion with constant, non-zero acceleration

D) horizontal motion with constant velocity and vertical motion with constant velocity

13. Consider a ball thrown from a level surface with an initial upward velocity of 20 m/s and an initial horizontal velocity of 10 m/s. How long is the ball in the air?

A) 1.0 s

B) 2.0 s

C) 4.0 s

D) 5.0 s

There are certainly several different -- but equivalent -- ways to solve this.

How long does it take the ball to get to the top where its vertical velocity is zero?

v_y = v_yi + a_y t

0 = (20 m/s) + ( - 10 m/s²) t

0 = 20 - 10 t

10 t = 20

t = ( 20 / 10 ) s

t = 2 s

If it takes 2 s to get to the top it also requires the same amount of time to get back down, so the total time is

t_tot = 4 s

14. Consider a ball thrown from a level surface with an initial upward velocity of 20 m/s and an initial horizontal velocity of 10 m/s. Where does it land? That is, measured from its initial position, where does it come back to and strike the level surface?

A) 10 m

B) 20 m

C) 40 m

D) 80 m

In the previous question, we have already determined that the ball will be in the air for a total time of 4 seconds. During this time of 4 s, how far does it travel horizontally?

v_x = v_xi = constant

x = v_x t = (10 m/s) (4 s) = 40 m

15. "Kinematics" is a description of motion. "Dynamics"is

A) an alternate description of motion in terms of Earth, air, fire, and water.

B) an explanation of the cause and effect of motion.

C) another name for the dynamic use of the Conservation of Energy.

D) the Conservation of Momentum.

16. Newton's First Law of Motion states that, in the absence of a net force, an object in motion will

A) eventually come to rest.

B) continue in motion but slow down until it stops.

C) continue in motion with the same speed along the same straight line.

D) continue along the same straight line with a decrease in speed

16. Mass is a measure of

A) the volume of an object

B) the size of an object multiplied by its speed

C) the velocity of an object multiplied by its mass

D) how difficult it is to change the motion of an object

18. The weight of an object is

A) another name for the mass of an object

B) the sum of all the forces on an object (the net force)

C) the force of gravity on an object

D) always less than the mass, even in a vacuum

19. The net force on a 10-kg object, at rest, is

A) 98 N

B) 49 N

C) 10.0 N

D) zero; F = m a

F_net = m a

a = 0 for an object at rest

Therefore, F_net = 0

20. The net force on a 10-kg object, in free fall, is

A) 98 N; F_net = w = m g = (10 kg) (9.8 m/s²) = 98 kg-m/s² = 98 N

B) 49 N

C) 10.0 N

D) zero

21. Harry the Painter has a weight of 500 N. When he is suspended as shown in the sketch here, what is the tension in the rope?

A) 1000 N

B) 500 N

C) 250 N; The tension in the rope pulls UP on Harry twice so the tension in the rope is on-half his DOWNward weight.

D) 125 N

22. Little Nellie Newton has a mass of 40 kg. What is the tension in the cord as Nellie suspends herself as shown above?

A) 800 N

B) 400 N

C) 200 N

D) 100 N

First, we need Nellie's weight.

w = m g = (40 kg)(10 m/s²) = 400 N

This weight is balanced by the force in the rope pulling up TWICE on her.

Therefore, the force in the rope is only 200 N

F_rope = T = 200 N

23. A force of 24 N acts on an object whose mass is 3 kg. This causes the object to accelerate at

A) 2 m/s²

B) 3 m/s²

C) 8 m/s² ; F = m a; 24 N = (3 kg) (a) = (3 kg) ( 8 m/s²); a = 8 m/s²

D) 10 m/s²

24. A car, with mass of 1,200 kg, accelerates at 3 m/s². The net force exerted on the car must be

A) 300 N

B) 400 N

C) 600 N

D) 1,200 N

E) 3,600 N; F = m a = (1,200 kg) (3 m/s²) = 3,600 kg-m/s² = 3,600 N

25. The weight of a 1,000-kg car is

A) 500 N

B) 1,000 N

C) 2,000 N

D) 10,000 N; w = m g = (1,000 kg) (10 m/s²) = 10,000 kg-m/s² = 10,000 N

26. A fireman, whose weight is 500 N, slides down a pole with an acceleration of 3 m/s². The forces that act on him are his weight pulling him down and the force of friction pulling up on him to slow him down. The force of friction must be

A) 90 N

B) 150 N

C) 350 N

D) 500 N

w = m g

500 N = m (10 m/s²)

m = 50 kg

F = m a

F_net = m a = (50 kg) (3 m/s²) = 150 kg-m/s² = 150 N

F_net = w - F_frict = 500 N - F_frict = 150 N = F_net

F_frict = 350 N

27. Suppose you are driving along in an open car and throw a ball straight up into the air. While the ball is still in the air you step on the accelerator. Where does the ball land relative to the car?

A) behind the car; The car has sped up and is ahead of the ball.

B) back into the car

C) in front of the car

28. Newton used the term "motion" to mean what we now call "momentum". "Momentum" is

A) mass times acceleration

B) force times distance

C) mass times velocity

D) weight multiplied by time

29. Impulse is

A) always conserved.

B) conserved only in "elastic collisions".

C) conserved in "inelastic collisions".

D) equal to the change in momentum.

30. Impulse is

A) force multiplied by distance.

B) force divided by distance.

C) force multiplied by time.

D) force divided by time.

31. Which of the following has the largest momentum relative to Earth?

A) a tightrope walker crossing Niagara Falls.

B) a pickup truck speeding along a highway.

C) a Mack truck sitting in the parking lot.

D) the Science building on campus.

32. A moving object on which no forces are acting will continue to move with constant

A) acceleration

B) impulse

C) momentum

D) all of these

33. Conservation of momentum is directly related to

A) Newton's First Law of Motion

B) Newton's Second Law of Motion

C) Newton's Third Law of Motion

D) International shortages of momentum

34. A rifle recoils from firing a bullet. The speed of the rifle's recoil is small because the

A) force against the rifle is smaller than against the bullet.

B) momentum of the rifle is smaller than that of the bullet.

C) rifle has more kinetic energy than the bullet.

D) momentum of the rifle is larger than that of the bullet.

E) mass of the rifle is larger than that of the bullet.

35. Two objects, A and B, have the same size and shape, but A is twice as heavy as B. When they are dropped simultaneously from a tower, they reach the ground at the same time, but A has a greater

A) speed (same!)

B) acceleration (same!)

C) momentum; p = m v; If the two v's are the same, the momentum will be larger for the object with the larger mass.

D) all of the above

36. A car traveling along the highway needs a certain amount of force exerted on it to stop. Greater stopping force will be required when the car has

A) more mass.

B) more momentum.

C) less stopping distance.

D) all of the above

37. A 5 kg ball has a momentum of 20 kg m/s. What is the ball's speed?

A) 4 m/s; p = m v; 20 kg m/s = (5 kg) (v); v = 4 m/s

B) 8 m/s

C) 40 m/s

D) 80 m/s

38. If two objects collide and do not stick together, their total momentum after the collision is

A) less than

B) the same as; Momentum is always conserved.

C) greater than

their total momentum before the collision.

39. Impulse is the change in

A) displacement.

B) velocity.

C) acceleration.

D) momentum.

40. Momentum is conserved in any collision. In an "elastic collision" another quantity is conserved. That other conserved quantity is the

A) velocity, v

B) acceleration, a

C) angular velocity,

D) work, W

E) Kinetic Energy