What is the power input at time t=3s for the force of 5N on an 8kg object?

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Homework Help Overview

The discussion revolves around two physics problems involving forces and power. The first problem concerns a force acting on an 8kg object and requires finding velocity and power as functions of time. The second problem involves a car's acceleration and power output, given its mass and performance specifications.

Discussion Character

  • Exploratory, Conceptual clarification, Mathematical reasoning, Problem interpretation

Approaches and Questions Raised

  • Participants explore the relationship between force, mass, and acceleration to derive velocity as a function of time. There are attempts to relate work and power through equations, with some questioning how to calculate kinetic energy at specific time intervals.

Discussion Status

The discussion is active, with participants seeking clarification on how to approach the problems. Some guidance has been offered regarding the relationships between force, acceleration, and power, but there is no explicit consensus on the methods to be used.

Contextual Notes

Participants express uncertainty about the calculations required for kinetic energy and power, particularly in relation to the time-dependent nature of the problems. There are also references to assumptions about constant power and the linearity of energy transfer in the context of the car's performance.

nns91
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Homework Statement



1. A single force of 5N acts in the x direction on a 8kg object. (a) If the object starts from rest at x=0 at time t=-, find its velocity v as a function of time. (b) Write an expression for the power input as a function of time. (c) What is the power input of the force at time t=3s

2.A car manufacturer claims that his car can accelertate from frest to 100 km/h in 8s. The car's mass is 800kg.

(a) Assuming that this performance is achieved at constant power, determine the power developed by the car's engine.
(b) What is the car's speed after 4s (neglect friction and air resistance)

Homework Equations



p=W/t


3. The Attempt at a Solution [/b

1. I could not even do part a of number 1. Can you guys help me ?

2. So since it's constant power P=W/t. W can be calculated by 1/2 mv^2 right ?

How can I do part b ? Do I just use the P from previous calculation and multiply by time 4 s to get W. and set W=1/2 mv^2 ??
 
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1a) F = ma gives you the acceleration, use it to find v(t).
1b) With 1a) you can get an expression for the work performed by the force on the object. Relate the work to the power and express p(t).
1c) Use 1b).

2a) You can express the cars kinetic energy at 100 km/h. This energy has been received linearly in time.
2b) You know the kinetic energy given after 4 s from 2a), relate it to velocity.
 
2b. How can I know kinetic energy at 4s ? does the kinetic energy stay the same as in part a ??
 
nns91 said:
2b. How can I know kinetic energy at 4s ? does the kinetic energy stay the same as in part a ??
Well, it differs in time, but it is given linearly according to the problem description, starting at 0 going up to the maximum value after 8 s. From 2a) you can express energy given per time unit which should give you energy given after 4 s.
 
Can I just calculate the acceleration and find velocity from there ?
 
Can I just calculate the acceleration and find velocity from there ?
 

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