Forces Unit 1 Lesson 3

FORCES• Force = push or pull• Force has size and direction• Force is labeled in Newtons (N). • Forces must act on an object• Forces DO NOT always cause movement

FORCES• When objects “bump” into each other and

have contact = contact force• Ex. Friction- rub your hands together quickly- contact

force of friction

• Force can act at a distance – Ex. Gravity

Multiple FORCES• Net force- more than one force working

together • Add them together when they are working

in the same direction• Subtract them if they are different directions

Multiple FORCES• When net force is zero (0) then the forces

are BALANCED• Balanced forced will not cause movement

or change motion• When the net force is not zero, then the

forces are UNBALANCED• Unbalanced forces change motion (speed,

directions or both) and cause acceleration

Newton's Three Laws of Motion

Sir Isaac Newton• Sir Isaac Newton (1643-1727) an English scientist and mathematician famous for his discovery of the law of gravity also discovered the three laws of motion.

First Law of MotionAn object at rest stays at rest; an object in motion stays in motion at the same speed and direction unless it experiences an unbalanced force.

*This law is called the Law of Inertia.

First Law of Motion Examples

When you hit a golf ball an unbalanced force is put on the ball causing the ball to move.

Two teams are playing tug of war. They are both exerting equal force on the rope in opposite directions. This balanced force results in no change of motion.

If objects in motion tend to stay in motion, why don’t moving objects keep moving

forever?Things don’t keep moving forever because there’s almost always an unbalanced force acting upon it.

A book sliding across a table slows down and stops because of the force of friction.

If you throw a ball upwards it will eventually slow down and fall because of the force of gravity.

In outer space, away from gravity and any sources of friction, a

rocket ship launched with a certain speed and direction would keep

going in that same direction and at that same speed forever.

Second Law of MotionThe acceleration of an object depends on the mass of the object and the amount of force applied. (Force = mass x acceleration)

Pushing an empty shopping cart is much easier than pushing a full cart. A larger force is required to accelerate a full cart compared to an empty one.

Larger ForceSmaller Force

Third Law of MotionWhenever on object exerts a force on a second object, the second object exerts an equal and opposite force on the first.

When you drop a ball and it hits the floor, the ball pushes on the ground. The floor also exerts a reaction force on the ball causing it to bounce in the opposite direction.

Third Law of Motion Examples

Why does it hurt so much when you stub your toe? When your toe exerts a

force on a rock, the rock exerts an equal force back on your toe. The harder you hit your toe

against it, the more force the rock exerts

back on your toe (and the more your toe hurts).

More examples…• Its your turn, using your book, your brain, and the

internet, find real world examples of Newton’s 3 Laws.

• Make a list in your notebook. You need to have 10 examples of each law. *They all need to be DIFFERENT. For example, for law one if you have an example of hitting a golf ball then you should not have another example that is the same such as kicking a soccer ball. *You cannot use the examples from the power point, but you can use examples from the book.