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Name: ___________________________________________________________ Date: 9/17/15 Period: _______ NOTES
Notes: Forces
Force- a push or pull (changing motion)
 Force is a vector; it has both size and direction
Types of forces:
1. Contact Force- when one object pushes or pulls another object by touching it
2. Gravity (3.1: pg. D77)- the force of attraction between two masses such as the Earth and you
 Considered a universal force
 Gravity depends on:
i. The mass of the objects – the more mass, the greater the force of gravity.
ii. The distance between the objects – as distance between the objects increases,
the force of gravity decreases.
 The Earth’s gravity is always constant.
i. Weight – the force of gravity acting on an object.
3. Friction (3.2: pg. D85)- force that resists motion between two surfaces that are pressed together
 No material is completely flat/smooth – has bumps and ridges. The bumps on one surface
interact with and stick to bumps on other surfaces.
 Influenced by:
i. Types of surfaces
ii. Motion of the surfaces
iii. Force pressing the surfaces together
 Friction produces heat.
Net Force- the total force that affects an object when multiple forces are combined – depends on size
and direction of individual forces
 Unbalanced forces change motion
 Balanced forces do NOT change motion
Stop and Think:
1. Give an example of a contact force.
2. Why is gravity considered a universal force?
3. Give an example of how friction is influenced by:
a. Types of surfaces: ________________________________________________________________________
b. Motion of surfaces: _______________________________________________________________________
c. Force pressing on surfaces: _______________________________________________________________
4. Why does friction produce heat?
5. What happens to a moving object if all the forces on it are balanced?
Sir Isaac Newton
 scientist in the 1600s
 studied motion and formulated three laws we still use
Newton’s First Law of Motion
Objects at rest remain at rest and objects in motion remain in motion with the same velocity unless acted
upon by an unbalanced force
 Key points of Newton’s First Law
o Objects with no net force acting on them have either constant or zero velocity
o Force is needed to start or change motion
Inertia- the resistance of an object to a change in its speed or direction of motion; it is directly
proportional to the object’s mass
Stop and Think:
6. What is the relationship between force and motion?
7. If you hit a baseball into the air, what forces are acting on it?
Newton’s Second Law States:
____________________________________________________________________________________________________
What does it mean? If you want to give two objects with different masses the same acceleration, you
have to apply more force to the heavier one!
Force = mass x acceleration:
Write the formula:
The standard unit of force is Newtons. Why?
A Newton is defined as the amount of force it takes to accelerate 1kg of mass 1m/s2.
1N=
Mass is also variable in Newton’s Second Law.
Forces can change the direction of motion
Force can make an object:
1.
2.
3.
Stop
Think:
If anand
object
changes direction, it is _____________________________.
8. Look at the drawing of the shopping cart on page D50. What two factors affect the acceleration of
the shopping carts?
9. If the same force is applied to two objects of different mass, which object will have the greater
acceleration?
10. How can an object accelerate if it doesn’t change speed?
11. Calculate:
a. m = 1.2kg a = 2 m/s2 F = __________________
b. m = 15kg a = 3 m/s2 F = __________________
c. m = 75kg a = 5 m/s2 F = __________________
Newton’s Third Law:
For every action there is an equal and opposite reaction
If objects A and B interact:
 The force exerted on object B by object A is equal to the force exerted on object A by object B
 One force is called the action and the other is the reaction
 If you stub your toe on the table, the same force exerted on your toe by the table is exerted on
the table by your toe
>Balanced force- two forces acting on the SAME object
>Action and Reaction- the force is acting on two different objects
***DO NOT GET THESE CONFUSED
Newton’s Three Laws are important because:
 Work together to explain changes in motion
 Useful when calculating how objects move in our everyday life
 Useful in making predictions about an object’s movement
Stop and Think:
12. Give an example of action/reaction forces.
13. Look at the pictures of the skaters on pg. D59. What happens when the girl pushes the boy? Why
does this happen?
14. Look at the pictures of the kangaroo on pg. D60. Explain how each picture describes Newton’s 3
laws.
15. A man pushes on a wall with a force of 50N. What are the size and direction of the force that the wall
exerts on the man?