Finding time using Impulsive forces and momentum

In summary, the conversation discusses the calculation of the time needed for a rocket to reach its maximum speed of 860 m/s, given its weight of 43,600N and upward force of 120,000N. The solution involves using the equations for momentum and impulse, with the final answer being 312.5 seconds. However, the correct answer is 31.8 seconds, and the conversation also touches on the concepts of weight and mass being inversely related, as well as the definition of mass as resistance to linear acceleration.
  • #1
WarOwk

Homework Statement


A rocket, weighing 43,600N, has an engine that provides an upward force of 120,000N. It reaches a maximum speed of 860 m/s. For how much time must the engine burn during launch in order to reach this speed?

Homework Equations


p=m*Δv
i=F*t

The Attempt at a Solution


I started by using 120,000N as the Force. I used 43,600 as the mass and 860 as the velocity. I multiplied 43,600 by 860 to get 37,496,00. I then divided the momentum(37,496,000) by the force(120,000) to solve for time. The solution I got was 312.5 seconds. Our teacher has already given us the answer to the problem which is 31.8 seconds, but there is an upcoming test and I would like to know how he got this answer.
 
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  • #2
Clue: 312.5/31.8 = 9.8
 
  • #3
George Jones said:
Clue: 312.5/31.8 = 9.8
I think I figured it out. Momentum is measured in kg*m/s, but the mass is measured in Newtons. So, you divide the 312.5 by 9.8 to get it in correct units. When I divided the 43,600 N first I got the same answer. The quote helped. Thank You!
 
  • #4
WarOwk said:
but the mass is measured in Newtons.

Weight is measured in Newtons; mass is measured in kilograms.
 
  • #5
I think I understand it now, weight is the gravitational force of an object. It's equation is (9.8N/kg * mass), and mass is the weight divided by 9.8 N/kg because it is the amount of stuff on an object. Mass and weight are inversely related to each other.
 
  • #6
WarOwk said:
mass is the weight divided by 9.8 N/kg
Yes, but that is not how mass is defined. "Mass" generally means inertial mass and is defined as resistance to linear acceleration. If a firce F is applied to a mass and the resulting acceleration is a then the mass is F/a.
It turns out that inertial mass is proportional to gravitational mass, but there is no known reason the universe had to be that way.
WarOwk said:
Mass and weight are inversely related to each other.
No, directly proportional, not inversely.
 

1. How does impulsive force affect an object's momentum?

Impulsive force, also known as an impulse, is a change in momentum. It affects an object's momentum by altering its velocity, either increasing or decreasing it depending on the direction of the force. The larger the impulse, the greater the change in momentum.

2. Can impulsive forces be used to calculate time?

Yes, impulsive forces can be used to calculate time. This can be done by using the equation FΔt = mΔv, where F is the impulsive force, Δt is the change in time, m is the mass of the object, and Δv is the change in velocity. By rearranging this equation, we can solve for Δt, which represents the change in time.

3. How is impulsive force related to conservation of momentum?

Impulsive forces are closely related to the law of conservation of momentum, which states that the total momentum of a system remains constant unless acted upon by an external force. In the case of impulsive forces, the change in momentum of one object is equal and opposite to the change in momentum of the other object, resulting in a constant total momentum for the system.

4. Can impulsive forces be used to find the time of impact in collisions?

Yes, impulsive forces can be used to find the time of impact in collisions. In order to do this, we must first determine the impulse and the change in velocity of the objects involved in the collision. By then using the equation FΔt = mΔv, we can solve for Δt, which represents the time of impact.

5. How can impulsive forces be used to manipulate an object's trajectory?

Impulsive forces can be used to manipulate an object's trajectory by changing its momentum and velocity. By applying a specific impulsive force at a certain time, the object's trajectory can be altered. This is commonly seen in sports such as tennis or baseball, where players use impulsive forces to change the direction of the ball's trajectory.

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