Block A (70 N) and block B (21 N) are connected by a massless cord and are at rest. Given ìs = 0.8. What is friction A ?
What is 0.8? The coefficient of friction? Static or kinetic?
If they are at rest on a horizontal plane, and the cord is unstretched, friction is zero. In some situations (incline or stretched cord) there can be static friction sufficient to maintain equilibrium.
You have not fully defined the problem.
56N
To calculate the friction force acting on block A, we first need to determine the tension in the cord.
Since blocks A and B are connected by a cord, they will have the same acceleration and we can use Newton's second law for each block separately.
For block A:
Sum of forces on A = mass of A × acceleration of A
The forces acting on block A are the tension in the cord (T) and the friction force (f).
Sum of forces on A = T - f
Using the value of mass of A = 70 N and acceleration of A = 0 (as they are at rest), the equation becomes:
70 N × 0 = T - f
0 = T - f
Now we can analyze the forces acting on block B:
Sum of forces on B = mass of B × acceleration of B
The forces acting on block B are the tension in the cord (T) and the weight of B (21 N).
Sum of forces on B = T - 21 N
Since both blocks have the same acceleration, we can equate the tension in the cord for blocks A and B:
T = T
From the two equations, we can now solve for T:
0 = T - f
T - 21 N = T
Subtracting the second equation from the first, we have:
0 - (-21 N) = T - T + f
21 N = f
Therefore, the friction force acting on block A is 21 N.
To determine the friction force acting on block A, we need to consider the forces involved in the system.
First, let's draw a free-body diagram for block A:
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| Block A |
| |
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The forces acting on block A are:
1. Force due to gravity (weight = mass × acceleration due to gravity):
FgA = m × g
2. Normal force (equal to the weight, since the block is at rest):
FN = FgA
3. Tension force from the connecting cord (equal to the force in block B since the cord is massless):
FT = Fb = 21 N
4. Friction force (opposes motion, directed opposite to the tension force):
Now that we've identified the forces acting on block A, let's calculate their magnitudes.
1. Force due to gravity:
FgA = m × g
= 70 N (since given mass is not provided, we can assume it cancels out with the gravitational acceleration)
2. Normal force:
FN = FgA
= 70 N
3. Tension force:
FT = Fb
= 21 N
4. Friction force:
Friction force depends on the coefficient of friction (μ) and the normal force:
Friction force (Ff) = μ × FN
Given that the coefficient of friction (μ) is 0.8 and the normal force (FN) is 70 N, we can now calculate the friction force:
Friction force (FfA) = 0.8 × 70 N
= 56 N
Therefore, the friction force acting on block A is 56 N.