A pictorial device which analyzes forces present on any body with the use of vectors. Allows easier grasping of problems involving either objects at equilibrium or in motion. Can be used to predict presence of forces.

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56
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8answers
52k views

With Newton's third law, why are things capable of moving?

I've got a rather humiliating question considering newton's third law "If an object A exterts a force on object B, then object B exerts an equal but opposite force on object A" -> $F_1=-F_2$ ...
20
votes
6answers
3k views

Conical train wheels

I've been reading about how the conical shape of train wheels helps trains round turns without a differential. For those who are unfamiliar with the idea, the conical shape allows the wheels to shift ...
12
votes
15answers
11k views

Is Newton's third law always correct?

Newton's third law states that every force has an equal and opposite reaction. But this doesn't seem like the case in the following scenario: For example, a person punches a wall and the wall breaks. ...
12
votes
5answers
1k views

What force causes the ball to move up?

Let's consider this super-simple example. The ball (mass $m$) is moving from position 1 to position 2 at constant horizontal velocity $v$ until the ball reaches the red arrow (in position 1, it has ...
11
votes
5answers
4k views

How does Newton's third law apply to an object interacting with air?

We all know that if I punch the wall with $100\,\mathrm{N}$ force, the wall pushes me back with with $100\,\mathrm{N}$ and I get hurt. But if I punch air with $100\,\mathrm{N}$, does air punches me ...
11
votes
8answers
21k views

Is gravity a force and if so what is its opposite?

For every force there is an equal force in the opposite direction on another body, correct? So when the Suns gravity acts on Earth where is the opposite and equal force? I also have the same ...
10
votes
2answers
2k views

Can a mouse rotating a wheel violate conservation of angular momentum?

I have been thinking on the following problem. Part I. Imagine we have a mouse enclosed in a rigid box. We placed the box on a frictionless surface (ice might be a good example). Can the box start ...
10
votes
6answers
33k views

Why is a book on a table not an example of Newton's third law?

My textbook explains Newton's Third Law like this: If an object A exterts a force on object B, then object B exerts an equal but opposite force on object A It then says: Newton's 3rd law ...
7
votes
5answers
133 views

Astronauts on a tether

Recently, I went to a competition with a science test. One of the questions was as follows: Two astronauts (A and B) are tethered together in space. Simultaneously, Astronaut A pulls on the tether ...
6
votes
5answers
12k views

Why can't a rope be pulled completely straight?

I have found several discussions on how to calculate the sag of rope that is tied off at two points (like a tightrope), and I understand it to a certain extent. What I can't wrap my head around is how ...
6
votes
3answers
944 views

If $F=ma$, how do can we experience both gravity and a normal force even though we are not accelerating

So as I sit in my chair, I experience a gravitational force pushing me into the chair and I'm also experiencing the normal force of the chair pushing back at me so I don't fall. According to Newton's ...
6
votes
3answers
2k views

Why doesn't a wall move when you push it if there's space behind it?

In the first screen you can see that if a person were to push a wall within a typical household the wall would not move while keeping themselves tractioned to the floor. If you push hard and do not ...
6
votes
2answers
121 views

Equations of Motion of a Physical Pendulum [closed]

I would like to formulate the equations of motion of a particular type of physical pendulum, shown in the attached figure. The physical pendulum consists of a wheel rolling on a narrow board. A ...
5
votes
3answers
2k views

Paradox in the two block problem [closed]

UPDATE (regarding duplicate) : This question is not a duplicate of another question. Sure, the situation in both the questions is same and, yes, both questions ultimately provide a methodology to ...
5
votes
3answers
763 views

Is there any tension in a massless spring that connects two free falling bodies in different horizontal planes?

Two bodies A and B of same mass $m$ are attached with a massless spring and are hanging from a ceiling with a massless rope. They are in same vertical plane but not in same horizontal plane. Now the ...
5
votes
3answers
8k views

Tension of rope. Different Answers?

The tension of a rope in a tug of war with unequal forces is equal to the least force on the rope. Am I right? (If group A pulls the wire with 20 newton leftwards and the group B pulls with 30 newton. ...
5
votes
2answers
206 views

Free body diagram with forces of friction

I'm reallizing that I'm very confused on how to make a free body diagram of forces in some situations where there is friction between two objects. I'll make an example where I cannot figure out what's ...
5
votes
1answer
7k views

Why am I wrong about this dynamics question?

I disagree with the answer to this homework question. Here it goes. The coefficient of static friction is 0.60 between the two blocks in figure. The coefficient of kinetic friction between the ...
4
votes
4answers
1k views

What Keeps a Pendulum Moving In a Circular Path?

From the figure, we know that $F_{net} = mg\sin\theta$. Now, this force $\vec{F_{net}}$ is in the direction of the velocity $\vec{v}$ of the bob, both are tangent to the path. Therefore, the net ...
4
votes
4answers
1k views

Why doesn't centrifugal force appear on an FBD for a car on a banked track?

Suppose you wish to find the max speed at which a car can travel on a banked curve of radius R and angle theta to the horizontal and coeff. of static friction mu. I don't understand the relevant ...
4
votes
3answers
6k views

Which force makes a wheel roll down the hill? What causes friction?

A wheel rolling down a hill has two axis of rotation. One is where the center or mass is and the other is the point of contact with the surface which acts as a fulcrum. I was trying to understand ...
4
votes
2answers
890 views

Pulley-rope tension question

I'm learning mechanics and I don't understand the following (simple, I guess) concept. Here's a pulley-rope system: The force of tension, as I understand it, is defined as the force exerted on an ...
3
votes
5answers
864 views

Why can't I push myself in a chair? [duplicate]

If I am sitting in a chair with wheels and someone pushes on the back of my chair with sufficient force it will role along the ground. However, if I push on the back of the chair with the same force ...
3
votes
4answers
403 views

Potential Energy Concept

Imagine a book that we lift it with a force that is exactly equal to the force of gravity so the forces cancel out and the book moves with a constant velocity. Consider the situation after the book ...
3
votes
3answers
7k views

Two people pushing off each other

If there are two people on ice skates named $A$ and $B$, and $A$ pushes on $B$ with a force $F_A$, then both of them will experience a force of magnitude $F_A$ in opposite directions. I'm confused as ...
3
votes
1answer
240 views

Forces acting on two balls in a jar [closed]

Consider the following situation: Two balls are inside a jar with diameter $D$. The balls are uniform in mass and both have the same diameter $d$. Express the forces that acts at the colored dots ...
3
votes
3answers
74 views

Why is the rod moving to and from?

These are tow pulleys, and a plastic plank is placed above them which can slide freely. The plank has uniform density and it touches both the pulleys. Both pulleys are placed on the same plane, and ...
3
votes
1answer
207 views

Force Keeping Water Skier Afloat

A motionless water skier sinks because the upward buoyancy force on the skier is less than the downward force of gravity on the skier. A skier pulled by a boat does not sink into the water. Why?
3
votes
2answers
759 views

Acceleration of a ramp

If we have a ramp and a block on it, and if all the surfaces are frictionless so both the the ramp and block can move. Is the normal force different from the time that ramp is not moving? If yes can ...
3
votes
1answer
72 views

Help understanding work

I have this problem: An object with 800kg mass is lifted up 2.4m by a force $F$. How much work does the Force do on the object (gravity is the only other force acting on the object)? From what ...
3
votes
3answers
108 views

Is static friction the only force causing a car to move (without sliding)?

A car is moving without sliding means that the friction between wheels and the ground is static friction. This is the force causing an object to move forward, therefore, its direction is the same as ...
3
votes
1answer
46 views

Free body diagram when forces are not directly in contact with the object

I was trying to use Newton's second law to describe the motion of the following pendulum: However, I was confused as to how to include the spring in Newton's second law. I was able to set up a ...
3
votes
2answers
192 views

Why are some forces not included in the sum of forces?

I was given the following notes in class: 4.32 A skier of mass $65.0\text{ kg}$ is pulled up a snow-covered slope at constant speed by a tow rope that is parallel to the ground. The ground ...
3
votes
3answers
2k views

Conical vs Simple Pendulum

I don't understand why the Tension $T$ on a conical pendulum and a simple pendulum are different. In a simple pendulum, one would say that the tension of the rope is $T=mg \cos(\theta)$. However, ...
3
votes
5answers
219 views

in Newtons 3rd law, where does the second body get its energy to react?

A very basic question, and apologies if I have overlooked something obvious. In newtons 3rd law, presumably energy is transfered to the second body from the first body (which itself was subject to an ...
3
votes
1answer
780 views

How muscle force work? [closed]

If muscle works with force on joint and joint works with exact force on muscle(newton III law) how can muscle drag much heavier weight than itself?
3
votes
1answer
3k views

Determine the acceleration in a double Atwood machine

Trying to solve the following double Atwood machine: Suppose there is a mass of 12kg hanging on an ideal rope that wraps around an ideal pulley, and that the other end of the rope is attached to ...
3
votes
1answer
238 views

Force on bolt holding up jet engine on plane

I am to solve the following: A jet engine of mass $m$ is fastened to the fuselage of a passenger jet by a bolt. During flight, the plane encounters turbulence, which suddenly imparts an upward ...
3
votes
3answers
294 views

Force Body Diagrams of object sliding down ramp

So I think I am confusing myself about forces. I know that when something is sliding down a ramp that it will have the force of gravity, normal force and force of friction acting on it. But for an ...
3
votes
0answers
167 views

Obtaining point of application of Ground Reaction Force with use of a hyperstatic load-cell array platform

I'm looking for the theory of an experiment that is giving me a hard time to perform. I have an instrument composed of a rigid horizontal square plate rigidly supported under each corner by a ...
2
votes
2answers
52 views

Why is the centripetal force vector on a banked turn horizontal rather than parallel to the incline?

Why isn't the centripetal force parallel to the inclined surface? It seems like if you were to draw a circle with one point being at the center of the curve and one point at the car the circle would ...
2
votes
3answers
2k views

When is tension constant in a rope?

Suppose we have a massless rope with pulling forces applied at each end. In which scenarios is the tension in the rope constant throughout? For example if there is a knot in the rope the tension is ...
2
votes
4answers
241 views

Normal force on vertical circular motion

I have a couple of questions concerning vertical circular motion. The object shown in the diagram moves with constant speed on the inside of a circle. In point A the normal force points to the ...
2
votes
4answers
294 views

Turning on a straight, unbanked, frictionless road

I was learning about circular motion when this question struck me: In real life situations we are able to take a turn along a circular arc with our bike because friction provides us the necessary ...
2
votes
3answers
466 views

Weight distributions [duplicate]

If a man is standing on two weighing machines (scales), with one foot on each, Will both machines show equal weight or his weight will be distributed in two machines?
2
votes
1answer
110 views

Why doesn't a parachutist, according to Newton, “fall” upwards (because drag is larger than gravity)?

Part of the second law states that an object will accelerate in the direction of the unbalanced force. However, I've been thinking about parachuting and skydiving a bit lately, and I know that when ...
2
votes
5answers
2k views

Relation between potential energy and forces

Can you also explain why can't we define potential energy corresponding to a non-conservative internal force? Non-conservative forces are those which don't depend on the initial and final states but ...
2
votes
1answer
432 views

Tension in an Atwoods machine conceptual?

Assuming $T_1$ is the force that acts on box $1$ and $T_2$ is the force that acts on box $2$. Exactly what causes the Tension? Why does $T_1 = T_2$? The problem is we are told to memorize that ...
2
votes
2answers
393 views

What are the forces acting on a Pendulum Tuned Mass Damper?

Upon researching tuned mass dampers, I came across this free body diagram of a pendulum tuned mass damper. However, I don't understand where many of the forces come from. What exactly are the forces ...
2
votes
2answers
130 views

Force balancing - Newtonian mechanics [duplicate]

If a horse pulls the carriage, the carriage should pull the horse with equal force (Newton's third law), then how does the whole system move? I drew the free body diagram but not yet figured what ...