Physical World 1 - 11.30am
Test 1 - February 9, 2000
NAME: _____Answers to green test.__
Twenty short answer questions. Each question is worth 3 points. (Total points = 60.)
1. 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__ .
2. What is the acceleration of a car that maintains a constant velocity of 5m/s for 5s?
0m/s2 because the velocity is constant.
3. 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) The two strings break at random, independent of how quickly or slowly the stick is hit.
b) 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.
c) 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.
d) When the stick is hit quickly string 2 breaks because of the inertia of mass M
e) When the stick is hit slowly string 2 breaks because of the inertia of mass M
4. A bird travels North at 3km/h through the air. If it heads directly into a storm with a wind speed of 5km/h what is the bird's velocity relative to the ground below?
-2km/h North or +2km/h South.
5. 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 2 hit the ground first because it has a horizontal velocity.
b) Ball 1 hit the ground first because it traveled a shorter distance.
c) Ball 2 hit the ground first because it had a higher velocity.
d) Ball 1 and 2 hit the ground at the same time because they both had the same horizontal acceleration.
e) Ball 1 and 2 hit the ground at the same time because they both had the same vertical acceleration.
6. Which of the following statements about the moon are correct?
a) The earth exerts a stronger force on the moon than the moon exerts on the earth.
b) The moon has a non-zero acceleration because its velocity is changing.
c) The moon has a constant velocity.
d) There is no net force acting on the moon.
7. When you drive a car around a corner at constant speed is your acceleration 0m/s2? If not, explain why not.
No. Even though your speed is constant your direction is changing. Therefore your velocity is changing and hence your acceleration is NOT 0m/s2.
8. During a collision airbags are used in cars to
a) decrease the impulse.
b) decrease the impact energy.
c) decrease the momentum.
d) increase the interaction time and hence decrease the force.
e) decrease the interaction time and hence decrease the force.
9. Can an automobile with a velocity toward the north have an acceleration toward the south? Explain.
Yes. The automobile is slowing down (i.e. decelerating).
10. In all collisions the ______total______ __________momentum_______ is conserved.
11. 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 and the mass of the wagon is 30kg?
a) 30 N because you need to maintain a constant momentum.
b) 800 N because you need twice the force to overcome the friction force.
c) 0 N because you are walking at constant speed.
d) 400 N because you have to compensate for the frictional force.
12. If a cheetah runs at 20m/s, how long will it take the cheetah to run a 100m dash? (Show your calculation.)
t = d/v = 100/20 = 5 s
13. What is the impact speed when a car moving at 10m/s hits another car head on which is traveling at 10m/s in the opposite direction?
20 m/s
14. Circle the answer which is certainly true.
An object has a net force of 0N acting upon it.
a) The object is accelerating at a non-zero acceleration.
b) The object is moving at a non-zero constant velocity.
c) No forces are acting on the object.
d) The object is stationary.
e) The object is stationary or moving at constant velocity.
15. If you weigh 400N on Earth, what is your mass in space?
On Earth m=W/g=400/10=40kg. Mass in space is also m=40kg.
What is your weight in space?
Weight in space W=mg=0N as g=0m/s2.
16. 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, you could be going around a corner at constant speed. Your velocity is then not constant.
17. 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) Exactly equal to 10N.
b) indeterminate.
c) Less than 10N.
d) Greater than 10N.
18. 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, down as it falls.
b) The ball has an acceleration of 10m/s2, down as it rises and 10m/s2, up as it falls.
c) The ball has an acceleration of 10m/s2, up as it rises and 10m/s2, down as it falls.
d) The ball has an acceleration of 10m/s2, down as it rises and 10m/s2, down as it falls.
e) None of these answers.
19. What does the word conserve mean in physics?
To keep constant.
20. Circle the answer which is certainly true.
An object has zero acceleration. Therefore the object is moving at
a) zero velocity.
b) zero speed.
c) constant velocity.
d) constant speed.
**************************************************************************************
Ten longer questions. Each question is worth 4 points. (Total points = 40.)
Show all calculations!!
21. Consider a vehicle that is traveling in a straight line. It changes its speed from 5 m/s to
-10 m/s in 1s.
What does the negative sign on the -10m/s mean?
It means its going in the opposite direction.
What is the acceleration of this vehicle?
a = (vf-vi)/t = (-10m/s - 5m/s)/1s = -15 m/s2
22. A 200kg bear grasping a vertical tree slides down at constant velocity.
What is the net force acting on the bear?
Fnet = 0 N because the velocity is constant.
What is the friction force that acts on the bear?
Fnet = W - Ffriction = 0 N
Therefore Ffriction = W = mg = 200 x 10 = 2000 N.
23. If an explosion between a 2kg mass and a 4kg mass causes the 4kg mass to move at 1m/s, North how fast and in what direction does the 2kg mass move?
First mass m1 = 2kg. We need to find the velocity v1 of mass m1.
Second mass m2 = 4kg, v2 = 1m/s, North.
Momentum before explosion, p(before) = 0 kg m/s, as both masses are not moving before the explosion.
Momentum after explosion, p(after) = m1v1 + m2v2
p(before) = p(after) from momentum conservation, namely, m1v1 + m2v2 = 0
Therefore v1 = -m2v2/m1 = -(4x1)/2 = -2 m/s, North = +2 m/s, South
24. A plane is flying horizontally at a speed of 30m/s when it drops a food package to some refugees on the ground immediately below. It takes 10s for the package to hit the ground. Will the package hit the refugees immediately below?
No
If not, how far do the refugees have to travel in order to pick up the package?
d = vt = 30 x 10 = 300m
From what height did the plane drop the package?
d = 0.5 g t2 = 0.5 x 10 x (10)2 = 500 m
25. In the diagram below we show the position of a vehicle at 1s time intervals.
Determine the vehicle's speed after 1s.
v = d/t = 10/1 = 10m/s
Determine the vehicle's speed after 2s.
v = d/t = (23 - 10)/1 = 13m/s
Determine the vehicle's acceleration.
a = (vf-vi)/t = (13m/s - 10m/s)/1s = 3m/s2
If the vehicle's mass is 1000kg what is the net force acting on this vehicle during this time period.
Fnet = ma = 1000 x 3 = 3000N
26. A car weighing 1000kg is traveling North at 20m/s. If this car collides with a stationary car weighing 3000kg and the two vehicles stick together, how fast do the two vehicles move after the collision?
First car m1=1000 kg, v1=20 m/s North.
Second car m2=3000 kg, v2=0 m/s.
Total momentum before collision, p(before) = m1v1 + m2v2 = 1000x20 + 3000x0 = 20,000 kg m/s
Total momentum after collision, p(after) = (m1 + m2)v
p(before) = p(after) by conservation of momentum, namely, (m1 + m2)v = 20,000 kg m/s
Hence v = 20,000/(m1 + m2) = 20,000/(1000 + 3000) = 5 m/s
27. A baseball batter hits a ball high in the air. A member of the other team, who is 30m away, catches the ball 2s later.
(i) What is the horizontal speed of the ball?
v = d/t = 30/2 = 15 m/s
(ii) How high did the ball go?
It takes 1s for the ball to rise and 1s for the ball to fall.
d = 0.5 g t2 = 0.5 x 10 x (1)2 = 5 m.
28. A 70kg parachutist jumps from a plane and finds that his downward acceleration is 3m/s2.
What net force is acting on the parachutist?
Fnet = ma = 70kg x 3m/s2 = 210 N
What is the weight of the parachutist?
W = mg = 70kg x 10m/s2 = 700 N
Determine the upward force created by the air resistance of the parachute.
Fnet = W - Fparachute
Fparachute = W - Fnet = 700 - 210 = 490 N
29. What is the instantaneous velocity of a freely falling object 5s after it is released from a position of rest?
vf = gt = 10 x 5 = 50 m/s
What is its average velocity during this 5s interval?
Average velocity = (vi + vf)/2 = (0 + 50)/2 = 25 m/s
How far will it fall during this time?
d = 0.5 g t2 = 0.5 x 10 x (5)2 = 125 m
30. In 5s a car increases its speed from 60m/s to 65m/s while a bicycle goes from rest to 5m/s. Which undergoes the greater acceleration?
They have the same acceleration.
What is the acceleration of each vehicle?
acar = (vf - vi)/t = (65 - 60)/5 = 1 m/s2
abicycle = (vf - vi)/t = (5 - 0)/5 = 1 m/s2