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If an object is traveling in a straight direction at a constant speed it has no acceleration, right?

So if $F=ma$ and $a =0$ then force should equal 0.

Does that mean a car traveling at a constant speed (no accelerating) has no force?

Does it only have force when it hits an object (and thus decelerates quickly?)

Qmechanic
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Anchower
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  • You're constantly working against air resistance, when driving. Therefore some force is acting on the car. If you're driving at constant speed, the acceleration from the engine balances with the air resistance (and some other contributions). – engineer May 09 '15 at 17:49

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That is partially correct. You are missing a critical part of Newton's 2nd law: $F_{net} = m a$. It's not just any one force that equals the mass times the acceleration. It's the "net force" - the vector sum of all the forces. So a car that is traveling at a constant speed may have many forces on it (gravity, the normal force, friction, air resistance, etc), but they all add up to zero - they balance each other out, so that $F_{net} = 0$.

If the car then hits an object (in your example) and therefore decelerates, then the forces are no longer balanced, they no longer add up to zero, and the car does change its speed - it experiences acceleration.

Brionius
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  • thank you for answering. I understand the idea of net forces acting upon the car. What about the car itself. Does it have force without acceleration? – Anchower May 09 '15 at 19:53
  • What if there was an object in a vacuum traveling at a constant speed (no acceleration). Does the object have No force? – Anchower May 09 '15 at 19:56
  • My answer addresses that - "a car that is traveling at a constant speed may have many forces on it (gravity, the normal force, friction, air resistance, etc), but they all add up to zero" – Brionius May 09 '15 at 19:56
  • Whether or not it's in a vacuum, there may be many forces on an object. The acceleration is only determined by the net force. – Brionius May 09 '15 at 19:59