Skip to main content
Physics

Return to Answer

Mod Moved Comments To Chat
subscript notation on the $F_r$ force. Without subscript it looks like a product.
Source Link
Bill N
Bill N
  • 15.5k
  • 3
  • 39
  • 60

First of all, you don't need to write $_{max}$ or talk about limits when it is kinetic friction. Kinetic friction has a fixed formula. That formula is:

$$Fr=\mu N$$$$F_r=\mu N$$

and not $Fr=\mu R$$F_r=\mu R$. Don't use the reaction force $R$, only the normal force $N$.

Now, this friction is what the ball affects the surface with. It is not the reaction force, which affects the surface, it is the friction which does.

The ball affects the surface with friction, and the surface holds back with the same force in the ball, yes. This is Newton's 3rd law.

this appears to contradict Newton's Third Law, which clearly states that $Fr$$F_r$ should equal $30 \cos(30)$.

This is not correct, as I stated just above. The $30 \cos(30)$ is the reaction force's horizontal component $R_x$, but that is not the force that affects the surface. This $R_x$ causes friction, you could say, but they are not equal - Newton's 2nd law says that:

$$R_x-Fr=ma\quad\Leftrightarrow\quad Fr=R_x-ma$$$$R_x-F_r=ma\quad\Leftrightarrow\quad F_r=R_x-ma$$

The $R_x$ is clearly larger than the friction $Fr$$F_r$; the rest of $R_x$ Is used for acceleration.

First of all, you don't need to write $_{max}$ or talk about limits when it is kinetic friction. Kinetic friction has a fixed formula. That formula is:

$$Fr=\mu N$$

and not $Fr=\mu R$. Don't use the reaction force $R$, only the normal force $N$.

Now, this friction is what the ball affects the surface with. It is not the reaction force, which affects the surface, it is the friction which does.

The ball affects the surface with friction, and the surface holds back with the same force in the ball, yes. This is Newton's 3rd law.

this appears to contradict Newton's Third Law, which clearly states that $Fr$ should equal $30 \cos(30)$.

This is not correct, as I stated just above. The $30 \cos(30)$ is the reaction force's horizontal component $R_x$, but that is not the force that affects the surface. This $R_x$ causes friction, you could say, but they are not equal - Newton's 2nd law says that:

$$R_x-Fr=ma\quad\Leftrightarrow\quad Fr=R_x-ma$$

The $R_x$ is clearly larger than the friction $Fr$; the rest of $R_x$ Is used for acceleration.

First of all, you don't need to write $_{max}$ or talk about limits when it is kinetic friction. Kinetic friction has a fixed formula. That formula is:

$$F_r=\mu N$$

and not $F_r=\mu R$. Don't use the reaction force $R$, only the normal force $N$.

Now, this friction is what the ball affects the surface with. It is not the reaction force, which affects the surface, it is the friction which does.

The ball affects the surface with friction, and the surface holds back with the same force in the ball, yes. This is Newton's 3rd law.

this appears to contradict Newton's Third Law, which clearly states that $F_r$ should equal $30 \cos(30)$.

This is not correct, as I stated just above. The $30 \cos(30)$ is the reaction force's horizontal component $R_x$, but that is not the force that affects the surface. This $R_x$ causes friction, you could say, but they are not equal - Newton's 2nd law says that:

$$R_x-F_r=ma\quad\Leftrightarrow\quad F_r=R_x-ma$$

The $R_x$ is clearly larger than the friction $F_r$; the rest of $R_x$ Is used for acceleration.

Source Link
Steeven
Steeven
  • 52.3k
  • 15
  • 105
  • 199

First of all, you don't need to write $_{max}$ or talk about limits when it is kinetic friction. Kinetic friction has a fixed formula. That formula is:

$$Fr=\mu N$$

and not $Fr=\mu R$. Don't use the reaction force $R$, only the normal force $N$.

Now, this friction is what the ball affects the surface with. It is not the reaction force, which affects the surface, it is the friction which does.

The ball affects the surface with friction, and the surface holds back with the same force in the ball, yes. This is Newton's 3rd law.

this appears to contradict Newton's Third Law, which clearly states that $Fr$ should equal $30 \cos(30)$.

This is not correct, as I stated just above. The $30 \cos(30)$ is the reaction force's horizontal component $R_x$, but that is not the force that affects the surface. This $R_x$ causes friction, you could say, but they are not equal - Newton's 2nd law says that:

$$R_x-Fr=ma\quad\Leftrightarrow\quad Fr=R_x-ma$$

The $R_x$ is clearly larger than the friction $Fr$; the rest of $R_x$ Is used for acceleration.