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Newton's First Law of Motion and Mass

Newton's First Law of Motion and Mass

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Now that we have a better understanding of how forces can affect the motion of objects do you think there is anything else that affects the motion of objects? If so what are they?


By completing this portion of the lesson you will be able to identify how Newton’s first law of motion is related to the mass of an object, explaining that the more massive an object the more it tends to resist changes in its motion.

That is to say we will look at another quantity that affects the motion of an object, that being an object's mass, or a measure of how much "stuff" something is made up of. The other quantity that affects an objects motion is its speed, however that is not related to the content we are studying here. Later on that will be covered when the concept of momentum is discussed.

Think about this for a second, would it be easier to lift a 200 kg set of weights or a 20 kg set of eights? Which of these two would you rather lift. Why?

If you are like me you may not have eaten enough spinach when you were growing up so I may only be able to apply 200 N of force to the weights. Notice in this case only the 20 kg weight would move upwards. This is because move massive objects tend to resist motion more than less massive objects. We say that the more mass an object has the more inertia it possesses, meaning that it has a greater resistance to changes in its motion. Try your hand at the following example problem to test your understanding.

1). If you were give two boulders where one was real and the other was a stage prop, how would you tell which boulder had the greater inertia?

Figure 5

If you were to push on the boulders the one that moved more easily would possess less mass and thus less inertia.

To combine the ideas of force and mass we can say that an object possessing some mass moving at some constant velocity will keep moving with the same velocity unless acted upon by some non-zero net force. Let's look at some examples to help with this.

Example Problems

1). If a 10 kg ball is moving at 3 m/s, what force applied to the ball will keep it rolling at 3 m/s? Take a minutes and come up with your answer.

Okay so this was one a bit of a trick question. If the surface the ball is rolling on is frictionless and we can ignore air resistance then a 0 N force will keep the ball rolling. In other words we don't need to do anything. If there is friction and air resistance a force equal to and opposite the direction of the forces caused by friction and air resistance would keep the ball rolling at a constant velocity of 3 m/s. So if it turns out that friction and air resistance are applying a 1 N force on the ball we would have to apply a 1 N force in the opposite direction to keep the ball moving at a constant velocity. 

2). What would be harder to stop a 100 kg ball moving at 0.1 m/s or a 50 kg ball moving at 0.1 m/s? Formulate your answer and reasoning and then watch the video below to check your understanding.

Solution to Example Problem 2 & Experiment

Source: Parmanand Jagnandan


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stux. (03/06/2014). Floral Abstract Filigree Curlicue. [Digital Image]. Retrieved from 281141/​

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Source: Parmanand Jagnandan