Trolley with a box sitting on top

[aspodkfpo]

Homework Statement

n/a

Relevant Equations

n/a

If I have a trolley and an object on top traveling at constant speed, the force on the object in the horizontal direction must be zero.

In this case do we count there being no frictional force, or is it because the kinetic friction backwards and the static friction forwards sums up to zero? Is it right to say that there is this friction existent or is it not?

 

[etotheipi]

Can you draw a diagram? It's a little hard to interpret the situation you're talking about.

But remember that you cannot have static and kinetic friction at one interface at the same time! If there's relative motion at the interface, the friction is kinetic, and if there's not relative motion at the interface, the friction is static. Once you've sorted out which one applies, it can still be zero if that is required to satisfy the dynamics.

 

[aspodkfpo]

Can you draw a diagram? It's a little hard to interpret the situation you're talking about.

But remember that you cannot have static and kinetic friction at one interface at the same time! If there's relative motion at the interface, the friction is kinetic, and if there's not relative motion at the interface, the friction is static. Once you've sorted out which one applies, it can still be zero if that is required to satisfy the dynamics.

    <- Fs         |---|      -> Fk
_____________________________

autoformat is a huge yikes. pretend ___________ is the trolley and |----| is the box.

[Mentor Note -- added code tags to get the font to line up]

 

[etotheipi]

So the object is just sitting on top of the trolley, and the object and trolley are moving with some common speed?

Since there's no relative motion at the interface, any potential friction is static. But since the acceleration, and thus net force, on the object is zero, there will be no static friction force.

I will re-iterate, you never have both types of friction at the time; there's only ever one friction force, and it's of one type or the other! In this case, the friction force is zero.

 

[aspodkfpo]

So the object is just sitting on top of the trolley, and the object and trolley are moving with some common speed?

Since there's no relative motion at the interface, any potential friction is static. But since the acceleration, and thus net force, on the object is zero, there will be no static friction force.

I will re-iterate, you never have both types of friction at the time; there's only ever one friction force, and it's of one type or the other! In this case, the friction force is zero.

Why does this "I will re-iterate, you never have both types of friction at the time; there's only ever one friction force, and it's of one type or the other! In this case, the friction force is zero. hold true?

 

[etotheipi]

Why does this "I will re-iterate, you never have both types of friction at the time; there's only ever one friction force, and it's of one type or the other! In this case, the friction force is zero. hold true?

When an object is in contact with another object along an interface, both objects exert an equal and opposite contact force on each other. The contact force has components perpendicular and parallel to the interface; these are called the normal force and friction force.

If the relative velocity at the interface is non-zero, then the friction force is described by the equation $F = \mu_k N$, where $\mu_k$ is the coefficient of kinetic friction. We say the friction is kinetic. If there is no relative velocity at the interface, then the friction force is described by the equation $F \leq \mu N$, where $\mu$ is the coefficient of static friction (notice the $\leq$ sign; it's important!). We say the friction is static.

They are not different forces; in both cases, they are the friction force (of which there is only one!). It's just that the equation describing the friction force depends on what's going on at the interface. Have a look at this figure, showing the friction force acting on a block when it is stationary vs when it is moving:

 

[berkeman]

Homework Statement:: n/a
Relevant Equations:: n/a

If I have a trolley and an object on top traveling at constant speed, the force on the object in the horizontal direction must be zero.

More precisely, the *sum* of the forces on the object must be zero if there is no acceleration. So the force from the air resistance pushing the box back on the trolley is countered by the static friction force from the trolley acting on the bottom of the box.