The Nature of ForceSection 10.1

Page 374

Objectives for 10.1

Describe what a force is.Know that a force is described with both

direction and magnitude.Know how to combine multiple forces that

are acting on an object.Explain how balanced and unbalanced

forces affect an object’s velocity.

What is a Force?• A Force is a push or a pull

– An object exerts a force on another object when it pushes it or pulls it.

• Forces are vector quantities (just like acceleration and velocity): described by their magnitude and their direction

5 Newtons at 20o

Combining Forces• When two forces act in the same direction or in

the opposite direction, they add together.– When forces are acting in opposite directions, you

must pay attention to the direction of each force

– When two forces combine in opposite directions, they combine by subtracting (or adding a negative).

– If one force is greater than the other force, the overall force is in the direction of the greater force.

– The overall force on an object after all of the forces are added together is called the net force.

10 N -6 N

4 N

Combining Equal-but-opposite Forces

Unbalanced vs. Balanced Forces

• Forces can be either balanced or unbalanced.

• Unbalanced Forces cause an object to accelerate or begin moving. *Think about the car and ramps experiment, were the forces unbalanced?

• Balanced Forces prevent any change in motion. The forces acting in opposite directions are equal. *At what point would we reach balanced forces in the cars and ramps experiments?

Unbalanced vs. Balanced Forces

Unbalanced vs. Balanced Forces

Force Diagrams

Weight: force due to gravity

Normal Force

The Normal Force balances Weight giving a zero net force on a stationary object.

Force Diagrams

Weight

Normal Force

An Outside Force

Friction: a force that opposes motion

What is the net force on this object?

Is this object accelerating? If so, in what direction?

Force Demonstrations

On a piece of paper, put your name and number it 1-7, with lots of space in between each number (7-10 Lines).

As we go through each demonstration, draw a representative diagram and the forces that are acting upon the objects.

1. Book Cart and 1 person:

2. Book Cart and 2 people pushing in the same direction:

3. 2 people pushing in opposite directions on the book cart:

4. 1 person pushing on the wall:

5. 1 person picking up a book:

6. Tug of War:

7. Car going down ramp:• Unbalanced force • Balanced force