Physics Review - CarlosSlays

A car travels along the x axis with increasing speed. We don't know if to the left or the right. Which of the graphs in Fig. 2-34 most closely represents the motion of the car?

(a) Increasing speed means that the slope must be getting steeper over time. In graphs (b) and (e), the slope remains constant, so these are cars moving at constant speed. In graph (c), as time increases x decreases. However, the rate at which it decreases is also decreasing. This is a car slowing down. In graph (d), the car is moving away from the origin, but again it is slowing down. The only graph in which the slope is increasing with time is graph (a).

A bullet fired from a rifle begins to fall (a) as soon as it leaves the barrel (b) after air friction reduces its speed. (c) not at all if air resistance is ignored.

(a) As soon as it leaves the barrel The bullet falls due to the influence of gravity, not due to air resistance. Therefore, (b) and (c) are incorrect. Inside the rifle the barrel prevents the bullet from falling, so the bullet does not begin to fall until it leaves the barrel.

You drop a rock off a bridge. When the rock has fallen 4m, you drop a second rock. As the two rocks continue to fall, what happens to their velocities? (a) Both increase at the same rate. (b) The velocity of the first rock increases faster than the velocity of the second. (c) The velocity of the second rock increases faster than the velocity of the first. (d) Both velocities stay constant.

(a) Both increase at the same rate. Since the distance between the rocks increases with time, a common misconception is that the velocities are increasing at different rates. However, both rocks fall under the influence of gravity, so their velocities increase at the same rate.

A car travels 10 m/s east. Another car travels 10 m/s north. The relative speed of the first car with respect to the second is (a) less than 20 m/s (b) exactly 20 m/s. (c) more than 20 m/s.

(a) Less than 20 m/s. The maximum relative speed between the two cars occurs when the cars travel in opposite directions. This maximum speed would be the sum of their speeds relative to the ground or 20 m/s. Since the two cars are traveling perpendicular to each other (not in opposite directions), their relative speed must be less than the maximum relative speed.

Which of the following should be part of solving any problem in physics? Select all that apply: (a) Read the problem carefully. (b) Draw a picture of the situation. (c) Write down the variables that are given. (d) Thick about which physics principles to apply. (e) Determine which equations can be used to apply the correct physics principles. (f) Check the units when you have completed your calculation. (g) Consider whether your answer is reasonable.

(a) Read the problem carefully. (b) Draw a picture of the situation. (c) Write down the variables that are given. (d) Thick about which physics principles to apply. (e) Determine which equations can be used to apply the correct physics principles. (f) Check the units when you have completed your calculation. (g) Consider whether your answer is reasonable. All these actions should be a part of solving any problem in physics.

The magnitude of a component of a vector must be (a) less than or equal to the magnitude of the vector (b) equal to the magnitude of the vector (c) greater than or equal to the magnitude of the vector. (d) less than, equal to, or greater than the magnitude of the vector.

(a) less than or equal to the magnitude of the vector. The components of a vector make up the two legs of a right triangle when the vector is the hypotenuse. The legs of a right triangle cannot be longer than the hypotenuse, therefore (c) and (d) cannot be correct answers. Only when the vector is parallel to the component is the magnitude of the vector equal to the magnitude of the component, as in (b). For all other vectors, the magnitude of the component is less than the magnitude of the vector.

You are in the middle of a large field. You walk in a straight 100 m, then turn left and walk 100 m more in a straight line before stopping. When you stop, you are 100 m from your starting point. By how many degrees did you turn? (a) 90 (b) 120 (c) 30. (d) 180° (e) This is impossible. You cannot walk 200 m and be only 100 m away from where you started

(b) 120 degrees If you turned 90°, as in (a), your path would be that of a right triangle. The distance back would be the hypotenuse of that triangle, which would be longer than 100 m. If you turned by only 30°, as in (c), your path would form an obtuse triangle; the distance back would have to be greater than if you had turned 90°, and therefore it too would be greater than 100 m. If you turned 180 as in (d), you would end up back at your starting point, not 100 m away. Three equal distances of 100 m would form an equilateral triangle, so (b) is the correct answer.

Which statements are not valid for a projectile? Take up as positive (a) The projectile has the same x velocity at any point on its path. (b) The acceleration of the projectile is positive and decreasing when the projectile is moving upwards zero at the top, and increasingly negative as the projectile descends (c) The acceleration of the projectile is a constant negative value. (d) The y component of the velocity of the projectile is zero at the highest point of the projectile's path. (e) The velocity at the highest point is zero.

(b) The acceleration of the projectile is positive when the projectile is moving upwards, zero at the top, and increasingly negative as the projectile descends. (e) The velocity at the highest point is zero. In projectile motion the acceleration is vertical, so the x velocity is constant throughout the motion, so (a) is valid. The acceleration is that of gravity, which, when up is positive, is a constant negative value, so (b) is not valid and (c) is valid. At the highest point in the trajectory the vertical velocity is changing from a positive to a negative value. At this point the y component of velocity is zero, so (d) is valid. However, the x component of the velocity is constant, but not necessarily zero, so (e) is not valid.

A hunter is aiming horizontally at a monkey who is sitting in a tree. The monkey is so terrified when it sees the gun that it falls off the tree. At that very instant, the hunter pulls the trigger. What will happen? (a) The bullet will miss the monkey because the monkey falls down while the bullet speeds straight forward (b) The bullet will hit the monkey because both the monkey and the bullet are falling downward at the same rate due to gravity (c) The bullet will miss the monkey because although both the monkey and the bullet are falling downward due to gravity, the monkey is falling faster (d) It depends on how far the hunter is from the monkey

(b) The bullet will hit the monkey because both the monkey and the bullet are falling downward at the same rate due to gravity. Both the monkey and bullet fall at the same rate due to gravity. If the gun was pointed directly at the monkey and gravity did not act upon either the monkey or bullet, the bullet would hit the monkey. Since both start falling at the same time and fall at the same rate, the bullet will still hit the monkey.

One ball is dropped vertically from a window. At the same instant, a second ball is thrown horizontally from the same window. Which ball has the greater speed at ground level? (a) The dropped ball. (b) The thrown ball. (c) Neither-they both have the same speed on impact (d) It depends on how hard the ball was thrown.

(b) The thrown ball If we ignore air friction, the horizontal and vertical components of the velocity are independent of each other. The vertical components of the two balls will be equal when the balls reach ground level. The ball thrown horizontally will have a horizontal component of velocity in addition to the vertical component. Therefore, it will have the greater speed.

A ball is thrown downward at a speed of 20 m/s. Choosing the +y axis pointing up and neglecting air resistance, which equation(s) could be used to solve for other variables? The acceleration due to gravity is g= 9.8 m/s^2 downward. (a) v=(20 m/s) - gt (b) y= yo + (-20 m/s)t - (1/2)gt^2 (c) v^2 = (20 m/s)^2 - 2g(y-yo) (d) (20 m/s) = (v+vo)/2 (e) All of the above.

(b) y= yo + (-20 m/s)t - (1/2)gt^2 (c) v^2 = (20 m/s)^2 - 2g(y-yo) Each of the given equations is based on Eqs. 2-11a-d. Answer (a) has the acceleration replaced properly with -g, but the initial velocity is downward and as such should be negative. Answer (d) is incorrect because the initial velocity has been inserted for the average velocity. Answers (b) and (c) have the correct signs for each variable and the known values are inserted properly.

You are adding vectors of length 20 and 40 units Which of the following choices is a possible resultant magnitude? (a) 0. (b) 18. (c) 37. (d) 64. (e) 100

(c) 37 The shortest possible resultant will be 20 units, which occurs when the vectors point in opposite directions. Since 0 units and 18 units are less than 20 units, (a) and (b) cannot be correct answers. The largest possible result will be 60 units, which occurs when the vectors point in the same direction. Since 64 units and 100 units are greater than 60 units, (d) and (e) cannot be correct answers. Answer (c) is the only choice that falls between the minimum and maximum vector lengths.

You are riding in an enclosed train car moving at 90 km/h. If you throw a baseball straight up, where will the baseball land? (a) In front of you. (b) Behind you. (c) In your hand. (d) Can't decide from the given information.I

(c) In your hand Both you and the ball have the same constant horizontal velocity. Therefore, in the time it takes the ball to travel up to its highest point and return to ground level, your hand and the ball have traveled the same horizontal distance, and the ball will land back in your hand.

You drive 4km at 30 km/h and then another 4km at 50 km/h. What is your average speed for the whole 8-km trip?

(c) Less than 40 km/h. Since the distances are the same, a common error is to assume that the average speed will be halfway between the two speeds, or 40 km/h. However, it takes the car much longer to travel the 4 km at 30 km/h than at 50 km/h. Since more time is spent at 30 km/h, the average speed will be closer to 30 km/h than to 50 km/h.

A ball is dropped from the top of a tall building. At the same instant, a second ball is thrown upward from the ground level. When the two balls pass one another one on the way up, the other on the way down, compare the magnitudes of their acceleration: (a) The acceleration of the dropped ball is greater. (b) The acceleration of the ball thrown upward is greater. (c) The acceleration of both balls is the same. (d) The acceleration changes during the motion, so you cannot predict the exact value when the two balls pass each other. (e) The accelerations are in opposite directions.

(c) The acceleration of both balls is the same. A common misconception is that the acceleration of an object in free fall depends upon the motion of the object. If there is no air resistance, the accelerations for the two balls have the same magnitude and direction throughout both of their flights.

baseball is hit high and far. Which of the following state- ments is true? At the highest point. (a) the magnitude of the acceleration is zero. (b) the magnitude of the velocity is zero (c) the magnitude of the velocity is the slowest. (d) more than one of the above is true. (e) none of the above are true.

(c) the magnitude of the velocity is the slowest. The baseball is in projectile motion during the entire flight. In projectile motion the acceleration is downward and constant; it is never zero. Therefore, (a) is incorrect. Since the ball was hit high and far, it must have had an initial horizontal component of velocity. For projectile motion the horizontal component of velocity is constant, so at the highest point the magnitude of the velocity cannot be zero, and thus (b) is incorrect. However, at the highest point, the vertical component of velocity is zero, so the magnitude of the velocity has a minimum at the highest point. So (c) is the correct answer.

A ball is thrown straight up. What are the velocity and acceleration of the ball at the highest point in its path? (a) v=0, a=0. (b) v=0, a=9.8 m/s^2 up. (c) v=0, a=9.8 m/s^2 down. (d) v=9.8 m/s up, a=0. (e) v=9.8 m/s down, a=0

(c) v=0, a=9.8 m/s^2 down Students commonly confuse the concepts of velocity and acceleration in free-fall motion. At the highest point in the trajectory, the velocity is changing from positive (upward) to negative (downward) and therefore passes through zero. This changing velocity is due to a constant downward acceleration.

In which of the following cases does a car have a negative velocity and a positive acceleration? A car that is traveling in the (a) -x direction at a constant 20 m/s (b) -x direction increasing in speed. (c) +x direction increasing in speed (d) -x direction decreasing in speed. (e) +x direction decreasing in speed

(d) -x direction It is a common misconception that a positive acceleration always increases the speed, as in (b) and (c). However, when the velocity and acceleration are in opposite directions, the speed will decrease.

At time t=0 an object is traveling to the right along the +x axis at a speed of 10.0 m/s with acceleration -2.0 m/s^2. Which statement is true? (a) The object will slow down, eventually coming to a complete stop. (b) The object cannot have a negative acceleration and be moving to the right. (c) The object will continue to move to the right, slowing down but never coming to a complete stop. (d) The object will slow down, momentarily stopping, then pick up speed moving to the left.

(d) The object will slow down, momentarily stopping, then pick up speed moving to the left. Since the velocity and acceleration are in opposite directions, the object will slow to a stop. However, since the acceleration remains constant, it will stop only momentarily before moving toward the left.

Which of the three kicks in Fig. 3-32 is in the air for the longest time? They all reach the same maximum height h. Ignore air resistance (a) (b) (c) or (d) all the time.

(d) They are all the same (Picture)Both the time of flight and the maximum height are determined by the vertical component of the initial velocity. Since all three kicks reach the same maximum height, they must also have the same time of flight. The horizontal components of the initial velocity are different, which accounts for them traveling different distances.

A baseball player hits a ball that soars high into the air. After the ball has left the bat, and while it is traveling upward (at point P in Fig. 3-31), what is the direction of acceleration? Ignore air resistance

b) (picture) Assuming that we ignore air resistance, the ball is in free fall after it leaves the bat. If the answer were (a), the ball would continue to accelerate forward and would not return to the ground. If the answer were (c), the ball would slow to a stop and return backward toward the bat.