# Forces, Motion, and Gravity

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Forces, Motion, and Gravity
Section 3.1 Forces, Motion, and Gravity

What do you know? How do you think force and motion are related?
Why is gravity called a force? When you throw a ball, what happens to it and why?

Know Your Strength Explain the difference in the amount of strength you use to lift a bag of groceries, to hold an apple, and to push a heavy box of books. Which takes more strength? What causes you to need more effort? How do you know your are using more strength?

Forces Around You Force- any push or pull
An applied force can start, stop or change the direction of an object. To determine the amount of force applied on an object you need to know the mass and acceleration of the object. Forces are all around you. Gravity pulls you down, muscles keep you up. Lots of forces act on objects all the time.

Force and Gravity Amount and direction of most forces can be measured.
Spring scales help measure forces. Newton (N)- SI unit for force One Newton is the amount of force needed to cause a 1-kg mass to accelerate at a rate of 1m/s2. 1N= 1kg x 1m/sec2 Weight of an object depends on the force that pulls the object toward the Earth. Acceleration due to gravity is 9.8 m/s2.

Mass vs. Weight You can determine the mass of an object by dividing its weight by 9.8 m/s2. What is the mass of your book if its weight is 15 N. What about your mass?

Questions Look at Figure 3.2 on page 56:
How do objects with different amount of mass affect the spring of a spring scale? What causes the weight of an object? With what units is weight measured on the spring scale? What does the spring scale show about the force of gravity?

Falling Objects When dropping objects, if no other force except gravity acts on them, should they fall at the same time? If there are other forces acting on them what’s an example? Friction- the force of resistance that occurs when movement takes place between any two objects or substances that make contact. Terminal Velocity- the maximum velocity of a falling object, occurring when the force of friction (upward) equals the force of gravity (downward). All objects reach terminal velocity if given enough time.

Projectile Motion Projectile motion- a curved path an object follows if thrown horizontally. Usually a combination of the downward motion produced by gravity, and the horizontal motion of the thrower.

What have we learned? Give two examples of a force.
Describe how you can use a spring scale to measure the force of gravity on an object. How does friction affect a flat sheet of paper falling through air? How would friction affect a crumpled ball of paper? Make a generalization about friction and falling objects. If you were to throw a ball harder the second time than the first, how would the projectile motion change?