Question 1

When someone throws a football, what should the launch angle be (ignoring air resistance) so that it travels a maximum distance?

Accepted Answer

The answer of When someone throws a football, what should...

Question 2

Suppose your instructor staggers into class after an all - night physics textbook sampling party and states Newton's second law as "If an object experiences a force then it will accelerate. If there are no forces acting on it then it will stand still." Please give three examples (from class, real life or made up) which expose mistakes on his part and briefly explain how they show his inaccuracies.

Accepted Answer

The answer of Suppose your instructor staggers into class after...

Question 3

You throw brick straight up. Ignoring air resistance, after it leaves your hand it will experience
A) only the downward force of its weight, both before and after it reaches its maximum height.
B) both an upward force, and the downward force of its weight after it rises. The upward force will gradually diminish to zero at the ball's maximum height, after which the ball will experience only the
Downward force of its weight.
C) an upward force as it rises. This upward force will gradually diminish to zero at the ball's maximum height, after which the ball will experience only the downward force of its weight.
D) the upward force of its weight as it rises. Once the ball reaches its maximum height, it will begin to experience the downward force of its weight.

Accepted Answer

The correct answer is A because once the brick is released, the only force acting on it is the gravitational force pulling it downward, regardless of whether it is ascending or descending.

Question 4

If you have a mass of 80 kg and are in a planet where the acceleration of gravity is 5 m/s², your weight on the planet is A) 400 N B) 4000 N C) 16 N D) 80 N

Accepted Answer

The answer of If you have a mass of 80...

Question 5

A gymnast jumps upward with an initial speed of 10 m/s. She is in the air for a total time of
A) 1 second
B) 5 seconds
C) 2 seconds
D) 10 seconds

Accepted Answer

The total time a projectile (in this case, the gymnast) is in the air can be calculated using the formula $t = \frac{2v\sin(\theta)}{g}$, where $v$ is the initial velocity, $\theta$ is the launch angle, and $g$ is the acceleration due to gravity. Assuming the gymnast jumps vertically ($\theta = 90^\circ$, so $\sin(90^\circ) = 1$), and using $g = 9.8 \, \text{m/s}^2$, the time in air is $t = \frac{2 \times 10}{9.8} \approx 2$ seconds.

Question 6

The maximum height above the ground for a the gymnast jumping straight upward with an initial speed of 10 m/s is
A) 5 m
B) 10 m
C) 2 m
D) 15 m

Accepted Answer

The maximum height (h) can be calculated using the formula $h = \frac{v^2}{2g}$, where $v$ is the initial velocity (10 m/s) and $g$ is the acceleration due to gravity (approximately 9.8 m/s$^2$). Plugging in the values, $h = \frac{10^2}{2 \times 9.8} = \frac{100}{19.6} = 5.1$ meters, which rounds to 5 meters, making option A correct.

Question 7

A car starts from rest and accelerates at 4 m/s². How much time will it take the car to reach a speed of 20 m/s? A) 5 s B) 10 s C) 4 s D) 8 s

Accepted Answer

The time it takes for the car to reach a speed of 20 m/s can be calculated using the formula $t = \frac{v}{a}$, where $v = 20 \, m/s$ is the final velocity and $a = 4 \, m/s^2$ is the acceleration. Plugging in the values, $t = \frac{20}{4} = 5 \, s$.

Question 8

Suppose you are a football player and you kick a ball for a field goal. Including air resistance, the ball's horizontal velocity
A) changes throughout the path due to gravitational acceleration
B) is zero at the top of the path
C) decreases throughout the path
D) increases throughout the path

Accepted Answer

Including air resistance, the horizontal velocity of the ball decreases throughout its path due to the drag force acting opposite to the direction of motion. Gravitational acceleration affects the vertical component of the ball's velocity, not the horizontal.

Question 9

Suppose an object is traveling at terminal velocity, so it is falling through air but not accelerating. How does the force of air resistance compare to its weight?
A) The two forces are equal and opposite.
B) The two forces are equal.
C) Air resistance is greater than the ball's weight.
D) Air resistance is less than the ball's weight.

Accepted Answer

When an object is falling at terminal velocity, it means that the acceleration is zero because the force of air resistance is equal in magnitude and opposite in direction to the weight of the object, resulting in a net force of zero.

Question 10

Suppose you drop an object out of the window of a building and wait for it to hit the ground. How does the halfway point in distance compare to the halfway point in time? Please explain.

Accepted Answer

The answer of Suppose you drop an object out of...

Question 11

Suppose a 2 kg object is falling through the air and accelerating downwards at 8 m/s². Taking g = 10 m/s² What is the magnitude and direction of the force of air resistance on it? A) 2N downward B) 2N upward C) 1N downward D) 1N upward

Accepted Answer

The net force acting on the object can be calculated using Newton's second law, F = ma, where m is the mass (2 kg) and a is the acceleration (8 m/s^2). The gravitational force acting on the object is its weight, which is mg = 2kg * 10 m/s^2 = 20N downward. If the net acceleration is 8 m/s^2 downward, it means the net force is 2kg * 8 m/s^2 = 16N downward. To achieve a net force of 16N downward, considering the gravitational force is 20N downward, the force of air resistance must be 4N upward (20N downward - 4N upward = 16N downward). However, since none of the options match this calculation, it seems there was a mistake in my calculation process based on the provided options. Re-evaluating with the correct approach: The difference in force due to acceleration less than g indicates an upward force of air resistance. The actual force due to gravity is 20N (2kg * 10m/s^2), and the force needed to achieve the 8m/s^2 acceleration is 16N (2kg * 8m/s^2). The difference, 4N, is the force of air resistance, which opposes gravity, thus it acts upward. Given the options do not match this corrected explanation, it appears there was a misunderstanding in the interpretation of the question or a mistake in the calculation. The correct approach to determine the force of air resistance involves understanding it opposes the direction of gravity and calculating the difference between gravitational force and the net force needed for the given acceleration.

Question 12

Suppose you throw a ball upwards with a certain initial speed from a certain height h above the ground, and you wait for it to go up, stop and come back down to the same (initial) position. Including air resistance, how will its speed hitting the ground on the way back down compare to its launch speed? Please explain.

Accepted Answer

The answer of Suppose you throw a ball upwards with...

Question 13

In our calculations of motion, we have ignored certain aspects of the object and its environment. Please briefly discuss three such aspects, and what kind of differences they make.

Accepted Answer

The answer of In our calculations of motion, we have...

Question 14

Suppose you weigh 986 N. How much force does the Earth exert on you, if any?
A) it cannot be determined
B) 986 N
C) 98.6N
D) none

Accepted Answer

The force that the Earth exerts on you is equal to your weight, which is given as 986 N.

Question 15

You are riding a bicycle into a strong head wind, but manage to keep moving in a straight line at a constant speed. The net force on you is
A) 0
B) in the direction of your motion
C) in the direction tie wind is pushing you (backwards)
D) downward due to your weight

Accepted Answer

When you're moving at a constant speed in a straight line, it means there's no net force acting on you. This is due to Newton's first law of motion, which states that an object will remain at rest or in uniform motion in a straight line unless acted upon by an external force. In this scenario, the force you apply to pedal the bicycle forward exactly balances the force of the headwind pushing against you, resulting in a net force of 0.

Question 16

In order to do a positive amount of work you must
A) exert a force and move in the direction of the force
B) exert a force and just move
C) exert a force or move
D) change an object's position

Accepted Answer

For work to be positive, the direction of the force applied must be in the same direction as the movement of the object. This means that not only must a force be exerted, but the object must also move in the direction of that force.

Question 17

Driving down the road, you hit an insect. How does the force your car exerts on the insect compare to the force the insect exerts on the car?
A) The force the car exerts on the insect is less.
B) The force the insect exerts on the car is less.
C) The two forces are equal in magnitude.
D) The insect exerts no force on the car

Accepted Answer

According to Newton's third law of motion, for every action, there is an equal and opposite reaction. This means the force the car exerts on the insect is equal in magnitude to the force the insect exerts on the car, despite the vastly different outcomes for the car and the insect.

Question 18

Suppose your car is on a 5% grade, meaning that for every 100 m you travel along the road you raise or lower only 5 m in elevation. If your car weighs 1500 kg, what is the component of its weight parallel to the road?
A) 15000 N
B) 1500 N
C) 7500 N
D) 750 N

Accepted Answer

The component of the car's weight parallel to the road can be calculated using the formula: $W_{\text{parallel}} = W \sin(\theta)$, where $W$ is the weight of the car and $\theta$ is the angle of the incline. Since the grade is 5%, this means the tangent of the angle $\theta$ is 0.05 (5/100). For small angles, $\tan(\theta) \approx \sin(\theta)$, so we can use 0.05 as an approximation for $\sin(\theta)$. The weight of the car is $1500 \, \text{kg} \times 9.8 \, \text{m/s}^2 = 14700 \, \text{N}$. Therefore, $W_{\text{parallel}} = 14700 \, \text{N} \times 0.05 = 735 \, \text{N}$, which rounds to 750 N, making D the correct answer.

Question 19

Suppose you are helping take groceries inside your house and you carry a heavy bag from the car to the house, keeping the bag at a constant level and walking at a constant speed. During your trip the work you do on the bag is
A) positive
B) 0
C) negative
D) equal to its potential energy

Accepted Answer

When you carry a heavy bag at a constant level and speed, you are not doing work on the bag in the physics sense, because work is defined as force applied over a distance in the direction of the force. Since the force you apply is upward to counteract gravity, and you move horizontally, the work done on the bag is 0.

Question 20

Suppose you push horizontally on the wall of a building. For you to do work on the wall, which of the following need to happen?
A) the wall moves away from you
B) the wall lifts up
C) the wall moves sideways
D) b and c

Accepted Answer

For work to be done on an object in a physics context, there must be displacement in the direction of the force applied. Since the question specifies pushing horizontally, the wall must move in the direction of the push (away from you) for work to be done.

Quiz 1: The Laws of Motion, Part 1

When someone throws a football, what should the launch angle be (ignoring air resistance) so that it travels a maximum distance?

You throw brick straight up. Ignoring air resistance, after it leaves your hand it will experience

If you have a mass of 80 kg and are in a planet where the acceleration of gravity is 5 m/s2, your weight on the planet is

A gymnast jumps upward with an initial speed of 10 m/s. She is in the air for a total time of

The maximum height above the ground for a the gymnast jumping straight upward with an initial speed of 10 m/s is

A car starts from rest and accelerates at 4 m/s2. How much time will it take the car to reach a speed of 20 m/s?

Suppose you are a football player and you kick a ball for a field goal. Including air resistance, the ball's horizontal velocity

Suppose an object is traveling at terminal velocity, so it is falling through air but not accelerating. How does the force of air resistance compare to its weight?

Suppose you drop an object out of the window of a building and wait for it to hit the ground. How does the halfway point in distance compare to the halfway point in time? Please explain.

Suppose a 2 kg object is falling through the air and accelerating downwards at 8 m/s2. Taking g = 10 m/s2 What is the magnitude and direction of the force of air resistance on it?

In our calculations of motion, we have ignored certain aspects of the object and its environment. Please briefly discuss three such aspects, and what kind of differences they make.

Suppose you weigh 986 N. How much force does the Earth exert on you, if any?

You are riding a bicycle into a strong head wind, but manage to keep moving in a straight line at a constant speed. The net force on you is

In order to do a positive amount of work you must

Driving down the road, you hit an insect. How does the force your car exerts on the insect compare to the force the insect exerts on the car?

Suppose your car is on a 5% grade, meaning that for every 100 m you travel along the road you raise or lower only 5 m in elevation. If your car weighs 1500 kg, what is the component of its weight parallel to the road?

Suppose you are helping take groceries inside your house and you carry a heavy bag from the car to the house, keeping the bag at a constant level and walking at a constant speed. During your trip the work you do on the bag is

Suppose you push horizontally on the wall of a building. For you to do work on the wall, which of the following need to happen?

If you have a mass of 80 kg and are in a planet where the acceleration of gravity is 5 m/s², your weight on the planet is

A car starts from rest and accelerates at 4 m/s². How much time will it take the car to reach a speed of 20 m/s?

Suppose a 2 kg object is falling through the air and accelerating downwards at 8 m/s². Taking g = 10 m/s² What is the magnitude and direction of the force of air resistance on it?