Math Problem Statement

A large block with mass is on a frictionless horizontal surface.𝑚1 = 50 𝑘𝑔 Another smaller block with mass is placed on top of the first block as shown.𝑚2 = 20 𝑘𝑔 The coefficients of friction between the two blocks are and . A forceμ𝑠 = 0. 60 μ𝑘 = 0. 30 is applied to the top block at an angle below the horizontal as shown.𝑃 = 200 𝑁 θ = 30◦ Find the acceleration of each block. (Hint: First determine whether or not the top block slides relative to the bottom block).

Solution

To determine the accelerations of the blocks (and whether the top block slides), we'll need to analyze the forces acting on each block and apply Newton's second law of motion. Here's a step-by-step approach:

Step 1: Analyze forces on the top block (m2m_2)

  • The applied force P\mathbf{P} has a horizontal component Px=Pcos(θ)=200cos(30)P_x = P \cos(\theta) = 200 \cos(30^\circ) and a vertical component Py=Psin(θ)=200sin(30)P_y = P \sin(\theta) = 200 \sin(30^\circ).
  • The gravitational force acting on the block is Fgravity,2=m2g=20×9.8NF_{\text{gravity},2} = m_2 g = 20 \times 9.8 \, \text{N}.
  • The normal force N2N_{2} exerted by the bottom block on the top block adjusts for Fgravity,2F_{\text{gravity},2} and the vertical component of PP:
    N2=m2gPy=209.8200sin(30)N_{2} = m_2 g - P_y = 20 \cdot 9.8 - 200 \cdot \sin(30^\circ)

Step 2: Determine if the top block slides

  • Calculate the maximum static friction force:
    fmax=μsN2f_{\text{max}} = \mu_s N_{2}

  • Check if PxP_x exceeds this maximum static friction force: Px=200cos(30)P_x = 200 \cos(30^\circ) If PxfmaxP_x \le f_{\text{max}}, the top block does not slide. Otherwise, it slides.

Step 3a: Top block does not slide

If the top block does not slide, both blocks move together with the same acceleration aa:

  • The friction force on m2m_2 acts to the left and there is no horizontal acceleration on m1m_1.

Using Newton's second law for both blocks together:

  • Total mass =m1+m2=70kg= m_1 + m_2 = 70 \, \text{kg}

  • Total force =Px= P_x

    a=Pxm1+m2=200cos(30)70a = \frac{P_x}{m_1 + m_2} = \frac{200 \cos(30^\circ)}{70}

Step 3b: Top block slides

If the top block slides, it means Px>fmaxP_x > f_{\text{max}}. In this case, the friction is kinetic friction.

The accelerations are separate:

For m2m_2:

  • Kinetic friction force acting backward: fk=μkN2f_k = \mu_k N_2

    a2=Pxfkm2a_2 = \frac{P_x - f_k}{m_2}

For m1m_1:

  • Force due to friction acting forward: fkf_k

    a1=fkm1a_1 = \frac{f_k}{m_1}

Conclusion

Calculations:
Using values, we find:

  • N2=196100=96NN_2 = 196 - 100 = 96 \, \text{N}
  • Maximum static friction fmax=0.6×96=57.6Nf_{\text{max}} = 0.6 \times 96 = 57.6 \, \text{N}
  • Px=200cos(30)=173.2NP_x = 200 \cos(30^\circ) = 173.2 \, \text{N}

Since 173.2>57.6173.2 > 57.6, the block slides.

Thus:

  • Kinetic friction fk=0.3×96=28.8Nf_k = 0.3 \times 96 = 28.8 \, \text{N}
  • a2=173.228.820=7.72m/s2a_2 = \frac{173.2 - 28.8}{20} = 7.72 \, \text{m/s}^2
  • a1=28.850=0.576m/s2a_1 = \frac{28.8}{50} = 0.576 \, \text{m/s}^2

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Math Problem Analysis

Mathematical Concepts

Newton's Laws of Motion
Static and Kinetic Friction
Trigonometry in Physics
Force Analysis

Formulas

P_x = P * cos(θ)
P_y = P * sin(θ)
f_max = μs * N
a = F / m
f_k = μk * N

Theorems

Newton's Second Law of Motion (F = ma)
Concept of Frictional Force

Suitable Grade Level

Grades 11-12