Forces and Motion

Forces and Motion

Chapter 12 Notes

Force

A) Force1) A push or pull that acts on an object.2) A force can cause a resting object to move, or it can accelerate a moving object by changing the object’s speed or direction.

Example 1

Ex.) Wind is a force. A strong wind could change the speed at which a person is moving or even the direction they are walking.

Example 2

Ex.) Putting fruit or vegetables in a spring scale at the store. By placing more fruit on the scale we can greaten the amount of force on the spring. This is also a way to measure force. Weight is a type of force.

Measuring Force

3) Force is measured in Newtons, abbreviated N.A) The newton is named after Sir Isaac Newton. Sir Isaac Newton lived from 1642-1727. He explained how force, mass, and acceleration are related.B) One newton is the force that causes a 1 kg mass to accelerate at a rate of 1 m/s^2. Therefore, 1 newton is equal to 1 kilogram - meter per second squared (1N = 1kgm/s^2).

Representing Force

4) Forces are represented by arrows (vectors). The direction of the arrow represents the direction of the force. And the length of the arrow represents the strength, or magnitude, of the force.

Example 3: Spring Scale

Combining Forces

B) Combining Forces1) You can combine force arrows to show the result of how forces combine. 2) Forces in the same direction can be added together.3) Forces in opposite directions subtract from one another.

Combining Forces

4) After we have combined all of the forces we end up with the net force.5) Balanced forces are forces that combine to create a net force of zero.

6) Unbalanced forces are forces that result when the net force acting on an object is not equal to zero.A) The net force equals the size of the larger force minus or the combination of the size of the smaller force.

Example 4: Adding Forces

Tug of War: Subtracting Forces

Tug of War: Opposite Forces

Friction

C) Friction1) A force that opposes the motion of objects that touch as they move past each other. 2) Without friction every surface would be more slippery than a sheet of ice, your food would slide off your force, walking would be impossible, and cars would slide around out of control.

Static Friction

3) There are four main types of friction: Static friction, sliding friction, rolling friction, and fluid friction.

A) Static friction is the friction force that acts on objects that are not moving. Static friction always acts in the opposite of the applied force.

Ex.) Taking a step or pushing a book.

Sliding Friction

B) Sliding friction is a force that opposes the direction of motion of an object as it slides over a surface.

1.) Sliding friction is less than static friction, therefore it is easier to keep an object moving than it is to start it moving.

Example 5: Static vs. Sliding

Rolling Friction

C) Rolling Friction is the friction force that acts on rolling objects.

1) When a marble rolls across a flat floor, both the object and the floor are bent slightly out of shape. This change in shape at the point of rolling contact is the cause of rolling friction.

2) The force of rolling friction is about 100 to 1,000 times less than the force of static or sliding friction.

Fluid Friction

D) Fluid Friction opposes the motion of an object through a fluid.

1) Fluid friction increases as the speed of the object moving through the fluid increases.Ex 6) Swimming, stirring cake batter2) Fluid friction acting on an object moving through the air is known as air resistance.

Example 7

Ex 7) An airplane flying through the air. Water and a mixture of gases such as air are known as fluids. These fluids create air resistance on the plane.

3) Gravity causes objects to accelerate downward, whereas air resistance acts in the direction opposite to the motion and reduces acceleration.

Example 8

A) When the force of air resistance equals the force of gravity we have reached terminal velocity. At this point the forces acting on the object are balanced. Acceleration is zero and the object continues falling at a constant velocity.

Projectile Motion

D) Projectile Motion1) The motion of a falling object after it is given an initial forward velocity. At this point the only forces acting on the object are air resistance and gravity.

Ex.) Figure A shows that balls of different mass fall at the same rate. Figure B shows a ball that is the result of the force of gravity and the initial horizontal velocity. The two balls fall with the same acceleration and strike the ground at the same time.

Figure A: Figure B: