Problem 41 - College Physics

Question

You are working for a shipping company. Your job is to stand at the bottom of an 8.0 -m-long ramp that is inclined at \(37^{\circ}\) above the horizontal. You grab packages off a conveyor belt and propel them up the ramp. The coefficient of kinetic friction between the packages and the ramp is \(\mu_{\mathrm{k}}=0.30 .\) (a) What speed do you need to give a package at the bottom of the ramp so that it has zero speed at the top of the ramp? (b) Your coworker is supposed to grab the packages as they arrive at the top of the ramp, but she misses one and it slides back down. What is its speed when it returns to you?

Step-by-Step Solution

Answer

(a) The initial speed is 6.5 m/s. (b) The return speed is 4.2 m/s.

Step 1: Identify the known variables

We have a ramp of length 8.0 m inclined at an angle of \( 37^{\circ} \). The coefficient of kinetic friction \( \mu_k \) is 0.30. We need to find the initial speed needed and the speed when the package returns.

Step 2: Analyze energy changes for part (a)

We will use the principle of conservation of energy where the initial kinetic energy and work done against friction equal the potential energy at the top. Formula: \[ K_i - W_{friction} = U_f \] where \( K_i = \frac{1}{2} m v^2 \), \( W_{friction} = f_k d \) and \( U_f = mgh \).

Step 3: Find expressions for energy components

Calculate the work done by friction: \[ f_k = \mu_k \cdot m \cdot g \cdot \cos(37^{\circ}) \] Work done by friction: \[ W_{friction} = f_k \cdot d \] Potential energy at the top: \[ U_f = m \cdot g \cdot h \] where \( h = d \cdot \sin(37^{\circ}) \).

Step 4: Solve for initial velocity for (a)

Setting up the energy equation: \[ \frac{1}{2}mv^2 = mgh + \mu_k \cdot m \cdot g \cdot \cos(37^{\circ}) \cdot d \] Cancel \( m \) and solve for \( v \): \[ v^2 = 2gh + 2\mu_k \cdot g \cdot \cos(37^{\circ}) \cdot d \] Substitute known values to find \( v \).

Step 5: Calculate initial speed for (a)

Substitute \( g = 9.8 \, \text{m/s}^2 \), \( h = 8 \cdot \sin(37^{\circ}) \), and \( d = 8 \) m:\[ v^2 = 2 \times 9.8 \times 8 \times \sin(37^{\circ}) + 2 \times 0.30 \times 9.8 \times 8 \times \cos(37^{\circ}) \] Calculate to find \( v = 6.5 \, \text{m/s} \).

Step 6: Analyze energy changes for part (b)

The package starts from rest at the top of the ramp and slides back down. Use energy conservation again: Potential energy at top equals kinetic energy at the bottom plus work done by friction: \[ mgh = \frac{1}{2} mv^2 + \mu_k m g d \cos(37^{\circ}) \]

Step 7: Solve for speed at bottom for (b)

Cancel \( m \) and rearrange to find \( v \):\[ \frac{1}{2}v^2 = gh - \mu_k \cdot g \cdot d \cdot \cos(37^{\circ}) \] Substitute known values to find \( v \).

Step 8: Calculate final speed for (b)

Substitute \( g = 9.8 \, m/s^2 \) and given values:\[ \frac{1}{2}v^2 = 9.8 \times 8 \times \sin(37^{\circ}) - 0.30 \times 9.8 \times 8 \times \cos(37^{\circ}) \] Calculate to find \( v = 4.2 \, \text{m/s} \).

Key Concepts

Conservation of Energy

The principle of conservation of energy is essential for solving problems where energy is transformed from one type to another. When dealing with physics problems like propelling a package up an inclined plane, this principle states that the total mechanical energy of the package is conserved if we ignore any external energy input or loss, except gravity and friction.
Energy can exist in several forms such as kinetic energy (energy of motion) and potential energy (energy stored due to position). For our inclined plane problem, this means that the kinetic energy you impart to the package at the bottom of the ramp converts into potential energy as it reaches the top. There is also some energy lost due to friction, which needs to be considered.

• Initial Kinetic Energy: The energy given to the package as it starts its upward journey. Expressed as \( \frac{1}{2}mv^2 \).
• Work done by Friction: Energy lost as the package slides up, calculated using the frictional force \( f_k \) and the distance \( d \).
• Potential Energy at the Top: The energy the package has when it reaches the height, expressed as \( mgh \).

In our problem, you'll use the conservation of energy principle to find the speed required to just reach the top with zero speed and also the speed when it returns to the bottom.

Inclined Plane

An inclined plane is a flat surface tilted at an angle to the horizontal. It allows objects to be moved with less effort compared to lifting them vertically. When dealing with inclined planes in physics problems, it's important to analyze how gravity and other forces interact with the object on the slope.
The ramp in our problem is inclined at a specific angle, which means the gravitational force acting on the package needs to be broken into components:

• Parallel component: The component of gravity that acts down the slope, trying to accelerate the package down. It is given by \( mg\sin(\theta) \).
• Perpendicular component: The component acting into the slope, balancing the normal force and given by \( mg\cos(\theta). \)

These components are crucial for calculating both the work done against friction and the changes in energy. By understanding how these forces work, you can solve for the initial speed required for the package to reach the top.

Friction

Friction is a force that opposes the motion of an object. It acts in the opposite direction to the object's movement. In our inclined plane problem, kinetic friction plays a critical role. It needs to be overcome for the package to ascend the ramp.
Kinetic friction depends on two main factors:

• Coefficient of Kinetic Friction \( \mu_k \): A dimensionless number that represents the friction between the surfaces. Given as 0.30 in the exercise.
• Normal Force \( N \): The perpendicular force between the package and the slope, calculated using \( mg\cos(\theta) \).

The force of friction \( f_k \) is computed as \( \mu_k \cdot N \). This force is then used to calculate the work done by friction \( W_{friction} = f_k \cdot d \), which must be considered in energy calculations to find the initial and final speeds in both parts of the problem.

Physics Problem Solving

Solving physics problems involves a systematic approach to dissecting and analyzing the situation. Here’s a simple approach that applies to the incline plane problem:

• Identify Known Variables: Determine what's given in the problem, such as ramp length, angle, coefficient of friction, and gravity.
• Establish Equations: Use relevant physics equations, like the conservation of energy, to set up relationships between known and unknown quantities.
• Handle Units Carefully: Ensure all units are consistent, typically in meters, kilograms, and seconds.
• Substitute and Solve: Insert the known values and solve the equations for the requested unknown variables, step by step, scrupulously checking your work.

By methodically breaking down each facet of the problem, such as identifying energy transfer or friction effects, you can reliably reach the correct solution. Remember, practice in breaking down complex problems into simpler parts can greatly enhance your problem-solving skills.