Simply put, a force is a push or a pull. More carefully stated, a force is part of an interaction in between one thing and another thing. A basic push top top a wall surface cannot happen unless the wall surface simultaneously pushes back. Forces occur in pairs. V every exertion that a force, over there is a reaction to the force, same in magnitude and opposite in direction. You can not push against nothing.

You are watching: A crate remains at rest on a factory floor

The force of friction is one that resists movement of the surface ar of one object over the surface of another, or v a fluid if it’s a liquid or gas. The forces involved in collisions space nicely identified and also examined by means of accompanying alters in momentum and also energy. Gravitational force keeps united state on earth surface and our fabricated satellites in orbit.

The combine of all kinds of pressures follow vector rules; once in the very same direction castle add, and also when in opposite direction they subtract, resulting in a single net force. When a network force acts on an object, acceleration of the object occurs. These are rules of elementary physics and also are incorporated into 10 inquiries that follow. Try them out v your students. Comprehensive answers follow.



1. A friend tells you the a stick of dynamite possesses a good amount of force. Why perform you agree, or disagree?

2. Jake exerts a secure 100 N horizontal pressure on a crate resting on a factory floor. (a) If the crate continues to be at rest, how does the pressure of friction in between the crate and the floor compare through Jake’s push? (b) Jake then pushes v an extra 10 N, and the crate still continues to be at rest. Just how much friction acts on the crate? (c) that pushes through 115 N and the crate slides in ~ a consistent speed. How much friction then acts upon the crate? (d) What network force acts upon the crate once he pushes v 125 N while floor friction is 115 N?



3. Drop a rock and also it drops to the ground due to the force of gravity. In accord through Newton’s 3rd law, identify the reaction force.

4. Ignoring wait drag, determine the pressure that acts on an upwardly tossed ball once it is halfway to the peak of that path, and again when it get the height of its path.

5. A feather and also a book are hosted side by side, and then dropped a quick distance come the floor. I m sorry encounters the greater pressure of wait resistance? defend your answer.

6. A fast-flying bumblebee collides head-on v an oncoming speeding truck. Which of the two, the bee or the truck, experiences more force as soon as colliding? defend your answer.

7. The photo shows originally straight-lined bricks on a roadway the have come to be wavy early to automobile traffic. How do this bricks assist to price the question, “What specific force pushes an car along a roadway?”



8. I m sorry is much more forceful in the sport of boxing: a punch that hits a heavyweight boxer, or the exact same punch hitting a lightweight boxer? defend your answer.

9. With the same force, why is a cannonball’s speed higher when fired from a long-barrel cannon 보다 from a short-barrel cannon? protect your answer.

10. The sun pulls planet into a almost circular orbit through gravitational pressure F = G(m1m2)/r2, whereby G is the gravitational co6294.orgnt, m1 and m2 space the masses that the Sun and also Earth, and r is the distance in between their centers. If the sun hypothetically fell down to become a black color hole, would certainly it traction on earth with more force, less force, or the exact same force? protect your answer.



Asking straightforward questions like the ones above is often more revealing than asking complex questions in stimulate to assess a person’s knowledge of physics. Even physics Nobel Prize–winners have been well-known to occasionally stumble top top “simple questions.” see if her students can correctly answer this 10 questions. Compare your answers v the said answers and also explanations that follow. Each inquiry involves substantial thought, for this reason it would make sense to spread them over an ext than one class session. The task of identify forces and their applications is more than a “test”—it’s a very worthwhile learning experience.


1. Back a pole of dynamite could possess energy, momentum if moving, angular inert if rotating, color and even smell, that does not possess force! force is an action, a press or pull, and also not something that deserve to be possessed. Nothing possesses force. Your friend may be confuse force and also energy, and also perhaps intended to say the the pole of dynamite own energy, i beg your pardon is the capacity to produce a force. Finding out physics requires distinguishing between closely connected ideas, in this case in between possessing and producing. Very different.

2. (a) Jake’s 100-N press on the crate is protest by the very same amount the force, 100 N, listed by friction in between the crate and also the floor. The two equal and opposite forces develop a net pressure of zero, i beg your pardon accounts because that the crate remaining at rest. (b) Friction that prevents sliding is 100 N + 10 N = 110 N. The net force on the crate is then 110 N – 110 N = 0; it remains in static equilibrium. (c) The friction acting on the crate i do not care 115 N, equal and opposite come Jake’s 115 N push. This way a net pressure of 115 N – 115 N = 0, in i beg your pardon the sliding crate, no accelerating, is in dynamic equilibrium. (d) The net force is 125 N – 115 N = 10 N. The crate speeds up and is no longer in equilibrium.

3. This inquiry is poorly grammar deliberately. Even a knowledgeable person may flub it, for a poorly worded concern begets a negative answer. Consider the same inquiry rephrased: “A dropped absent is gravitationally pulled downward by Earth. What is the reaction come this force?” Aha! follow to Newton’s 3rd law: once body A pulls on human body B, body B simultaneous pulls equally and also oppositely on body A. Earth acts upon rock (action); rock acts upon Earth (reaction). So the reaction pressure is planet gravitationally pulled upward by the rock! we don’t notification this increase pull due to the substantial inertia of planet Earth. However the dropped rock produces one infinitely tiny upward acceleration the Earth. Although us can’t recognize it, it’s there in principle! The early wording the this question might constitute a trick question. Not good!

4. When air drag isn’t a factor, the only pressure that acts on the ball, or any kind of projectile, is gravitation—mg—which plot at every points follow me its trajectory. In ~ the midpoint the force is mg and also at the zenith it stays mg. Till some other force acts upon the ball, that is under the affect of mg only.

5. The falling book encounters a greater pressure of waiting resistance than the feather. In general, the wait resistance ~ above a relocating object depends on the lot of air pushed aside and how fast it happens. The larger and also heavier book plainly “plows” through an ext air and falls faster than the feather. The book’s loss is hardly affected by waiting resistance because it is little compared through the book’s weight. Wait resistance on the feather is a various story. Because of its tiny weight, the takes just a tiny little of air resistance to greatly affects that fall. It flutters slowly to the floor. It’s crucial to distinguish in between the force of waiting resistance and its effect ~ above falling. Although the force of waiting resistance on the feather is small, its impact is huge. Interestingly, if you place the feather atop book and also then drop it, the feather will be shielded from air resistance. Both the book and feather will loss together! just how intriguing to recognize that a feather will certainly free-fall at acceleration g inside a vacuum chamber, and now to discover it will fall at g when inserted atop a fallout’s book! shot it and also see.



6. The colliding force will be the very same for both! don’t confuse a pressure with the results of that force. Newton’s 3rd law claims that the affect force is the very same for both the bee and also the truck. The truck exerts a pressure on the bee (T top top B, the action) when the bee exerts a pressure on the van (B top top T, the reaction). These forces are same in magnitude and also opposite in direction. The quantity of affect force is the very same for both, also though the results are vastly different!

7. Apply Newton’s third law of movement to the crooked bricks in the road surface. As presented in the following sketch, the rotating tire of the auto produces a backward force of friction ~ above the road (action). The roadway simultaneously to produce a front friction force on the tires (reaction), which propels the car forward. The originally straight-lined bricks at the crosswalk have actually been moved backward to the left once cars accelerate forward to the right. Bricks are thrust backward through the tires (action); and tires are thrust forward by the bricks (reaction). For this reason indeed, the road pushes the car!



8. The punch on the heavier foe will be more forceful, which is nicely explained by 2 relationships that addresses collisions: in this case the boxer’s fist colliding through an opponent. The an initial is the impulse-momentum relationship, Ft = ∆(mv), and the 2nd is the work-energy principle, ∆KE = Fd. Note that both mv and KE the the punch space greatly decreased whether colliding through a heavyweight or a lightweight boxer. Think mass and inertia. The better mass of a heavyweight boxer means he’ll be moved less by a punch than a lighter opponent. It is less call time ∆t of the punch and less street d moved by the punch, which tells us that both reduce time and distance means greater force F. For this reason we view that a punch is much more forceful once striking a an ext massive opponent. That nice to analyze a situation by an ext than one route. For a much easier explanation, apply logic to Newton’s 3rd law: a pressure can’t be exerted on miscellaneous without the something exerting an equal and opposite pressure on the producer of the force. A punch on a feather, for example, is much less forceful 보다 the same punch ~ above a more massive punching bag. That’s since the feather can’t hit back any harder than it is hit. A little of logic climate tells us that a beat is more forceful once hitting a heavy opponent than a lighter opponent. Newton had it right: How tough you hit counts on how difficult what girlfriend hit access time back.


9. Understanding of the relationships work–energy (∆KE = Fd) or impulse–momentum (Ft = ∆mv) can carry out a an essential to answering this question. The much longer barrel of the cannon method a much longer distance the the force acts on the cannonball while within the barrel. A much longer distance for the same average force way greater work-related done, better energy, and also a much faster cannonball. Or us can take into consideration the time the cannonball security in the barrel. A much longer barrel way a much longer time for the force to action on the cannonball, and also thence a higher speed. Either connection underlies an explanation.

10. No change in force would occur between the Sun and also Earth if the sunlight shrank to come to be a black hole. The equation for gravitation states it all. No terms change, because that the fixed of the sun before and also after fallen would stay the same, earth’s mass i will not ~ change, and also the distance between the centers that the Sun and also Earth would stay unchanged. So whatever transforms are brought around by together an event, gravitational force between these 2 bodies would not be one of them.

See more: What Kind Of Cheese Comes With A House, What Kind Of Cheese Has A House


Learning physics

We don’t need to be reminded the Socrates to value questioning. Details is much more valued as soon as it answers a question. A teacher deserve to pose the over questions at appropriate times and also involve course participation in to express answers. Your skill as a teacher will count on your capability to guide students in creating clear explanations. A an excellent feature the the to teach profession is that us get better at this together time walk by. Each institution term, or each new class, provides us time to polish our explanations. Like wine and cheese, we boost with age. That’s really nice!

Online Connections

Identifying pressures questions in multiple-choice form

For much more on forces, watch Screencasts 3–7, and 37, 38 on Over there are additionally are two screencasts because that teachers, 148 student-oriented accuse lessons extending a wide swath that physics, and links to associated materials. See likewise