1. Chapter 5 Newton’s 2 nd Law

2. We know that in order for an object at rest to move it has to have a force on it. For a moving object to stop it has to have a force on it. Most of what we were dealing with last chapter was stuff that needed no forces on it (for example a rock in motion in space needs no force to keep moving). For our next chapter we will be dealing with objects that have forces that make them stop moving or start moving.

3. An object that is accelerating is changing its motion. Any object that has a force on it can be accelerated.

4. Usually when we push on something, our force is not the only force on the object. For an object to be accelerated, it must have a total force on it that is not zero.

5. If two people pushed on an object in opposite directions, each with a force on 10 N, what would be the total force on the object? This is a case where forces on an object do not necessarily mean that the object’s motion will change.

6. If you push on a grocery cart that is already moving with 3 N of force and the ground has a frictional force of 3 N, the cart will move but it will not be able to have its motion changed because the total force on it is zero. The cart will just move at a constant speed.

7. The more mass an object has the more force is needed to accelerate it. Big masses have small accelerations. This is called an inverse relationship.

8. Newton came up with a law that tells us how hard an object must be pushed to produce a certain acceleration. It is Newton’s Second Law. The equation for Newton’s Second Law is: force equals mass times acceleration or F = ma

9. Objects in motion on Earth are affected by friction, whether it is friction between two surfaces or friction due to the air (air resistance).

10. Practice Problems. Do these in your notes. 1.What is the force needed to accelerate a 60 kg object at 4 m/s 2 ? 2.How much acceleration will a 90 N force give a 75 kg object?

11. 3.The weight of an object is 35 N. If it accelerates at 16 m/s 2, how much force was used to push it? 4.The force on an object is 15 N. If the force causes an acceleration of 43 m/s 2, what is the mass of the object? What is the weight of the object in Newtons?