# Can we determine the force an object exerts by its mass and acceleration?

I understand that the objects acceleration is determined by the force exerted on it, and that the force exerted on it is determined by its acceleration.

But, does an object's (named A) acceleration (and mass) tell us anything about how much force the object will exert on another object (named B)?

• Yes, an accelerating object can exert a force on another object but so can a constantly moving object or one at rest. How much energy is transferred depends on the situation. – AdamRedwine Feb 29 '12 at 21:58

## 1 Answer

No, in general it will not. The acceleration and mass tell you only the total (net) force being exerted on object A at that moment. That is equivalent to the total force object A is exerting on all other objects (B, C, etc.) it is interacting with at that moment. However, the mass and acceleration do not tell you anything about the individual forces that object A exerts on, say, just object B.

If you are able to determine that object A is only subject to one force, then you can find that one force because you know it's equal to the total force. But that's the only case in which the mass and acceleration tell you about a specific force.

• "If you are able to determine that object A is only subject to one force, then you can find that one force because you know it's equal to the total force. But that's the only case in which the mass and acceleration tell you about a specific force." Yes, I can find that one force that is being exerted on the object, but does that mean that the object has (or exerts) that amount of force? If yes, can you tell me how that is? – w4j3d Feb 29 '12 at 23:02
• Forces are exerted, not possessed. If you find that one force is being exerted by object B on object A, then you know that object A is exerting the same amount of force in the opposite direction on object B. That's just Newton's third law. – David Z Feb 29 '12 at 23:10
• I understand it's Newton's third law, but I thought that object B doesn't necessarily only exert the force object A is exerting on it. I can intuitively understand it has to be the only force, but how would I know using Newton's Laws? I mean, how did you conclude that the net force object X gets, is equal to the net force it exerts? – w4j3d Mar 1 '12 at 0:03
• That's exactly what Newton's third law says. The force exerted on object X is equal in magnitude and opposite in direction to the force exerted by object X. This applies both to the individual forces and to the net force. – David Z Mar 1 '12 at 1:33