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Derive a formula expressing the kinetic energy of an object in terms of its momentum and mass.

Short Answer

Expert verified
The kinetic energy in terms of momentum and mass is \( KE = \frac{p^2}{2m} \).

Step by step solution

01

Understanding the Variables

Kinetic energy, denoted as \( KE \), is the energy that a body possesses due to its motion. Momentum, denoted as \( p \), is the product of an object's mass \( m \) and its velocity \( v \), i.e., \( p = mv \). The formula for kinetic energy in terms of mass and velocity is \( KE = \frac{1}{2}mv^2 \).
02

Relate Momentum to Velocity

Since momentum is given by \( p = mv \), you can solve for velocity \( v \) by dividing both sides by mass \( m \): \( v = \frac{p}{m} \). This expression allows us to substitute for velocity in the kinetic energy equation.
03

Substitute Velocity in Kinetic Energy Formula

Substitute \( v = \frac{p}{m} \) into the kinetic energy formula \( KE = \frac{1}{2}mv^2 \) to get:\[ KE = \frac{1}{2}m\left(\frac{p}{m}\right)^2 \] which simplifies to \[ KE = \frac{1}{2}m\frac{p^2}{m^2} \].
04

Simplify the Expression

The expression \( \frac{1}{2}m\frac{p^2}{m^2} \) simplifies as follows:\[ KE = \frac{1}{2} \cdot \frac{p^2}{m} \]. Thus, the kinetic energy in terms of momentum and mass becomes:\[ KE = \frac{p^2}{2m} \].

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Key Concepts

These are the key concepts you need to understand to accurately answer the question.

Understanding Momentum
Momentum is a crucial concept in physics as it describes the motion of an object. It is defined as the product of an object's mass and its velocity. In simple terms, momentum tells us how difficult it is to stop a moving object. This is because more massive objects or those moving faster are harder to halt.

Momentum is represented by the symbol \( p \) and is calculated using the formula:
  • \( p = m imes v \)
Where \( m \) is mass and \( v \) is velocity. Because momentum depends on both mass and velocity, it is a vector quantity. This means it has both a magnitude (how much momentum) and a direction (the direction of motion).

In physics problems, knowing the momentum helps us understand and predict how objects will behave in different situations. Whether it’s a ball rolling down a hill or a car in motion, momentum is key to understanding motion.
Role of Mass in Physics
Mass is a fundamental property of matter, reflecting the amount of material present in an object. In relation to energy and momentum, mass plays an essential role. Why is mass so critical? Because it directly influences an object’s behavior when forces are applied.

In the context of kinetic energy and momentum, mass has two main roles:
  • It serves as a multiplier in the calculation of momentum \( p = m \times v \).
  • It also contributes to kinetic energy \( KE = \frac{1}{2} m v^2 \).
Thus, an increase in mass will lead to an increase in both momentum and kinetic energy if the velocity remains constant.

Mass is a scalar quantity, which means it does not have a direction, only magnitude. In many physics problems, mass helps determine how much force is needed to change an object's motion. Heavier objects require more force to move or stop, which is why understanding mass is essential for calculating momentum and kinetic energy in physics.
Importance of Velocity
Velocity is crucial in understanding how objects move. It measures the rate at which an object's position changes over time. Unlike speed, velocity provides information about the direction of motion, making it a vector quantity. For instance, knowing the velocity of a car not only tells us how fast it's going, but also in what direction.

Velocity is not only important for understanding motion, but also for calculating both momentum and kinetic energy. Its equation is:
  • \( v = \frac{p}{m} \)
where \( p \) is momentum and \( m \) is mass. This relationship shows how momentum and mass influence velocity. If mass is increased while momentum remains constant, velocity will decrease.Bringing it together, velocity is a pivotal element when dissecting kinetic energy \( KE \) as it exponentially influences it, shown in the equation \( KE = \frac{1}{2} m v^2 \). This is why even slight increases in velocity can lead to significant increases in kinetic energy, illustrating the dynamic nature of motion and energy.

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Most popular questions from this chapter

A mass \(m\) moving at velocity \(v\) collides with a stationary target having the same mass \(m\). Find the maximum amount of energy that can be released as heat and sound.

Suppose a system consisting of pointlike particles has a total kinetic energy \(K_{c m}\) measured in the center-of-mass frame of reference. Since they are pointlike, they cannot have any energy due to internal motion. (a) Prove that in a different frame of reference, moving with velocity \(\mathbf{u}\) relative to the center-of-mass frame, the total kinetic energy equals \(K_{c m}+M|\mathbf{u}|^{2} / 2\), where \(M\) is the total mass. [Hint: You can save yourself a lot of writing if you express the total kinetic energy using the dot product.] (b) Use this to prove that if energy is conserved in one frame of reference, then it is conserved in every frame of reference. The total energy equals the total kinetic energy plus the sum of the potential energies due to the particles' interactions with each other, which we assume depends only on the distance between particles. [For a simpler numerical example, see problem 13 on p. \(312 .]\)

A bullet of mass \(m\) strikes a block of mass \(M\) which is hanging by a string of length \(L\) from the ceiling. It is observed that, after the sticky collision, the maximum angle that the string makes with the vertical is \(\theta .\) This setup is called a ballistic pendulum, and it can be used to measure the speed of the bullet. (a) What vertical height does the block reach? (b) What was the speed of the block just after the collision? \(\sqrt{ }\) (c) What was the speed of the bullet just before it struck the block?

The force acting on an object is \(F=A t^{2} .\) The object is at rest at time \(t=0 .\) What is its momentum at \(t=T ?\)

A bullet leaves the barrel of a gun with a kinetic energy of 90 J. The gun barrel is 50 cm long. The gun has a mass of 4 kg, the bullet 10 g. (a) Find the bullet’s final velocity. (b) Find the bullet’s final momentum. (c) Find the momentum of the recoiling gun. (d) Find the kinetic energy of the recoiling gun, and explain why the recoiling gun does not kill the shooter.

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