Power, velocity and acceleration help ! Watch

HAtestemur
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Hi guys, I have tried answering this question. My methods are all correct I believe, but my answer does not match the textbook.

A car of mass 1.2 tonnes is driven along a level road against a
total constant resistance to motion of 350 N. With the engine
of the car working at a constant rate of 9.5 kW,

find

(i) the acceleration of the car when its velocity is 10 m/s;

(ii) the maximum velocity of the car.
The answers are:
(i) acceleration of car = 0.5 m/s2
(ii) maximum velocity of car = 27.143 m/s
for part i, I did

Power = Force x Velocity
Force = Mass x Acceleration
so P = Mass x Acceleration x Velocity
so Acceleration = P / mv
so a = 9500 / (1200 x 10)
therefore a = 0.79 metres per second per second but the answer is 0.5 metres per second per second.

What did I do wrong ?

and for the second part, I couldn't do.
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Stonebridge
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(Original post by HAtestemur)
Hi guys, I have tried answering this question. My methods are all correct I believe, but my answer does not match the textbook.

A car of mass 1.2 tonnes is driven along a level road against a
total constant resistance to motion of 350 N. With the engine
of the car working at a constant rate of 9.5 kW,

find

(i) the acceleration of the car when its velocity is 10 m/s;

(ii) the maximum velocity of the car.
The answers are:
(i) acceleration of car = 0.5 m/s2
(ii) maximum velocity of car = 27.143 m/s
for part i, I did

Power = Force x Velocity
Force = Mass x Acceleration
so P = Mass x Acceleration x Velocity
so Acceleration = P / mv
so a = 9500 / (1200 x 10)
therefore a = 0.79 metres per second per second but the answer is 0.5 metres per second per second.

What did I do wrong ?

and for the second part, I couldn't do.
You haven't taken into account the 350N resistive force.
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HAtestemur
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(Original post by Stonebridge)
You haven't taken into account the 350N resistive force.
what do you mean?

How do I take that in to account when there are no other given forces?
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Stonebridge
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(Original post by HAtestemur)
what do you mean?

How do I take that in to account when there are no other given forces?
I mean
The question you have answered is "what is the acceleration of the car when its velocity is 10m/s assuming no resistive forces."

Using P=Fv for the engine at that speed and power gives you the force developed by the engine. The acceleration of the car will be given by F=ma where F is the resultant force.
This is the force developed by the engine less the resistive force.
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HAtestemur
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Okay, I calculated that and got 0.5 as the answer.

will I resolve the next part using P=Fv, where P is the Power given, and F is the resultant force ?
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Stonebridge
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(Original post by HAtestemur)
Okay, I calculated that and got 0.5 as the answer.

will I resolve the next part using P=Fv, where P is the Power given, and F is the resultant force ?
It's the same principle.
The car will accelerate up to a maximum speed determined by the 350N resistive force.
What will the force developed by the engine be if the car has reached this speed? (Acceleration is zero. Speed constant.)

When you think you know the answer to that, use P=Fv to find v
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HAtestemur
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(Original post by Stonebridge)
It's the same principle.
The car will accelerate up to a maximum speed determined by the 350N resistive force.
What will the force developed by the engine be if the car has reached this speed? (Acceleration is zero. Speed constant.)

When you think you know the answer to that, use P=Fv to find v

Okay, thank you. I got the right answer for both questions
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