A horse is urged to pull a wagon. The horse refused to try, citing Newton’s third law as a defense: the pull of the horse on the wagon is equal but opposite to the pull of the wagon on the horse. “If I can never exert a greater force on the wagon than it exerts on me, how can I ever start the wagon moving?” asks the horse. How would you reply?

A $5.5$-kg block is initially at rest on a frictionless horizontal surface. It is pulled with a constant horizontal force of $3.8$ N.
$(a)$ What is its acceleration? $(b)$ How long must it be pulled before its speed is $5.2$ m/s? $(c)$ How far does it move in this time?

Answer the following question.

What is the primary role of the citric acid cycle in the production of ATP?

A large rock $\it{falls}$ on your toe. Which of the concepts is most important in determining how much it hurts?
$(A)$ The mass of the rock. $(B)$ The weight of the rock. $(C)$ Both the mass and the weight of the rock are impor- tant. $(D)$ Either the mass or the weight, as the two are related by a single multiplicative constant $g$.

A piano is rolling down a frictionless slope at an ever increas- ing speed. The piano tuner sees it, runs up to it and pushes on it, slowing it down to a constant speed. The magnitude of the force on the man by the piano is $F_{MP}$ ; the magnitude of the force on the piano by the man is $F_{PM}$. If we compare these forces, we find $(A)$ $F_{PM}>F_{MP}$ always. $(B)$ $F_{PM}>F_{MP}$ while the piano slows down but $F_{PM}=$ $F_{MP}$ when the piano is moving at constant speed. $(C)$ $F_{PM}=F_{MP}$ always. $(D)$ $F_{PM}=F_{MP}$ while the piano slows down but $F_{PM}<$ $F_{MP}$ when the piano is moving at constant speed.

Two blocks, with masses $m_1=4.6$ kg and $m_2=3.8$ kg, are connected by a light spring on a horizontal frictionless table. At a certain instant, when $m_2$ has an acceleration $a_2=$ $2.6$ m/s$^2$, $(a)$ what is the force on $m_2$ and $(b)$ what is the accel- eration of $m_1?$

Suppose a body that is acted on by exactly two forces is ac- celerated. Does it then follow that $(a)$ the body cannot move with constant speed; $(b)$ the velocity can never be zero; $(c)$ the sum of the two forces cannot be zero; $(d)$ the two forces must act in the same line?

(a) Neglecting gravitational forces, what force would be re- quired to accelerate a $1200$-metric-ton spaceship from rest to one-tenth the speed of light in $3$ days? In $2$ months? (One metric ton $=1000$ kg.) (b) Assuming that the engines are shut down when this speed is reached, what would be the time required to complete a $5$-light-month journey for each of these two cases? (Use $1$ month $=30$ days.)

Two blocks are in contact on a frictionless table. A horizontal force is applied to one block, as shown in earlier Fig. $(a)$ If $m_1=2.3$ kg, $m_2=1.2$ kg, and $F=3.2$ N, find the force of contact between the two blocks. $(b)$ Show that if the same force $F$ is applied to $m_2$ rather than to $m_1$ , the force of contact between the blocks is $2.1$ N, which is not the same value de- rived in $(a)$. Explain.

The auditory tubes open into the​ ______

A) larynx.

B) laryngopharynx.

C) nasal cavity.

D) nasopharynx.

E) oropharynx.

What is the relation$—$if any$—$between the force acting on an object and the direction in which the object is moving?

Differentiate between the vocal folds and the vestibular folds.

A rock is on an inclined surface. The rock is originally at rest, but starts to slide down the incline. The magnitude of the force on the surface due to the rock is $F_{SR}$ , and the magnitude of the force on the rock due to the surface is $F_{RS}$. If we com- pare these forces, we find
$(A)$ $F_{SR}<F_{RS}$. always. $(B)$ $F_{SR}=F_{RS}$ when the rock is at rest but then $F_{SR}>F_{RS}$. $(C)$ $F_{SR}=F_{RS}$ always. $(D)$ $F_{SR}>F_{RS}$ always.

Of the objects listed in earlier Table, which might be considered as a particle for the motion described? For those that behave like particles, can you describe a type of motion in which they could $\it{not}$ be considered as particles?