1. When a parachutist jumps from an a irplane, he eventually reaches a constant speed, called the terminal velocity. This means that
a) the acceleration is equal to $g$
b) the force of a ir resistance is equal to zero
c) the effect of gravity has died down
d) the force of air resistance is equal to the weight of the parachutist
2. When a parachutist jumps from an airplane, he eventually reaches a constant speed, called the terminal velocity. This is an example of
a) Newton's First Law of motion
b) Newton's Second Law of motion
c) Newton's Third Law of motion
d) none of Newton's laws apply to this situation
3. A 777 aircraft has a mass of $300,000 \mathrm{~kg}$. At a certain instant during its landing, its speed is $27.0 \mathrm{~m} / \mathrm{s}$. If the braking force is $450,000 \mathrm{~N}$, what is the speed of the airplane $10.0 \mathrm{~s}$ later?
a) $12.0 \mathrm{~m} / \mathrm{s}$
b) $14.0 \mathrm{~m} / \mathrm{s}$
c) $18.0 \mathrm{~m} / \mathrm{s}$
d) $20.0 \mathrm{~m} / \mathrm{s}$
1. A $50-\mathrm{kg}$ crate is being pulled along a horizontal, smooth surface. The pulling force is $10 \mathrm{~N}$ and is directed $20^{\circ}$ above the horizontal. What is the acceleration of the crate?
a) $0.0684 \mathrm{~m} / \mathrm{s}^{2}$
b) $0.188 \mathrm{~m} / \mathrm{s}^{2}$
c) $0.200 \mathrm{~m} / \mathrm{s}^{2}$
d) $0.0728 \mathrm{~m} / \mathrm{s}^{2}$
5 . An object is being acted upon by three forces and moves with a constant velocity. One force is $60 \mathrm{~N}$ along the $x$ axis, the second is $75 \mathrm{~N}$ along the $y$ axis. What is the magnitude of the third. force?
a) $135 \mathrm{~N}$
b) $15.0 \mathrm{~N}$
c) $48.0 \mathrm{~N}$
(d) $96.0 \mathrm{~N}$
6 . A ball is thrown up into the air. At the highest point in its trajectory, the net force acting on it is
a) equal to its weight
b) greater than its weight
(c) less than its weight, but not zero
d) zero
7. A $40-\mathrm{kg}$ crate is being lowered by means of a rope. Its downward acceleration is $2.0 \mathrm{~m} / \mathrm{s}^{2}$. What is the force exerted by the rope on the crate?
a) $40 \mathrm{~N}$
b) $232 \mathrm{~N}$
(c) $312 \mathrm{~N}$
d) $472 \mathrm{~N}$
Answer
For question 7, the force exerted by the rope on the crate is the difference between the weight of the crate and the force due to its downward acceleration. So, the correct answer is \(\boxed{\text{(c) 312 N}}\).
Step 1 ：For question 1, when a parachutist jumps from an airplane and reaches terminal velocity, the force of air resistance is equal to the weight of the parachutist. So, the correct answer is \(\boxed{\text{(d) the force of air resistance is equal to the weight of the parachutist}}\).
Step 2 ：For question 2, the situation of a parachutist reaching terminal velocity is an example of Newton's First Law of motion, which states that an object in motion stays in motion with the same speed and in the same direction unless acted upon by an unbalanced force. So, the correct answer is \(\boxed{\text{(a) Newton's First Law of motion}}\).
Step 3 ：For question 3, we can use the formula for acceleration, which is force divided by mass, to find the acceleration of the airplane. Then, we can use the formula for final velocity, which is initial velocity plus acceleration times time, to find the speed of the airplane 10.0 s later. So, the correct answer is \(\boxed{\text{(a) 12.0 m/s}}\).
Step 4 ：For question 4, we can use the formula for acceleration, which is force divided by mass, to find the acceleration of the crate. However, since the pulling force is directed 20 degrees above the horizontal, we need to take the horizontal component of the force, which is force times the cosine of the angle. So, the correct answer is \(\boxed{\text{(a) 0.0684 m/s^2}}\).
Step 5 ：For question 5, since the object moves with a constant velocity, the net force acting on it is zero. Therefore, the magnitude of the third force must be such that the vector sum of all three forces is zero. So, the correct answer is \(\boxed{\text{(d) 96.0 N}}\).
Step 6 ：For question 6, at the highest point in its trajectory, the only force acting on the ball is its weight, which is acting downwards. So, the net force acting on it is equal to its weight. So, the correct answer is \(\boxed{\text{(a) equal to its weight}}\).
Step 7 ：For question 7, the force exerted by the rope on the crate is the difference between the weight of the crate and the force due to its downward acceleration. So, the correct answer is \(\boxed{\text{(c) 312 N}}\).
