Forces

What is force?

• A force is a push or a pull one body exerts on another body.

• Sometimes it is obvious a force has been applied, but other forces are not as noticeable.

• Forces can cause a change in motion.

Balanced forces

• When two or more forces act on an object at the same time, the forces combine to form the net force.

• Forces on an object that are equal in size and opposite in direction are called balanced forces.

BALANCED FORCES

• Balanced forces do not cause acceleration. (any change in speed or direction)

Unbalanced forces

• Unbalanced forces cause a change in motion.

• Unbalanced forces cause an object to start moving, stop moving, or change direction. (an unbalanced force will cause an object to accelerate)

Unbalanced forces

• The forces acting on an object may not always be opposite in direction.

• The net force that acts on this box is found by adding the two forces together.

Force DiagramsIn what direction and with how much

force would the object travel?

60 N 30 N

40 N 40 N

20 N 20 N

100 N

50 N

Types of Forces

• A force can be either balanced or unbalanced.

• Common types of force include gravity, magnetism, friction, air resistance, or centripetal force

Friction

• Suppose you give a skateboard a push with your hand.

• According to Newton’s 1st law of motion, if the net force acting on an object is zero, it will continue to move in a straight line with constant speed.

• Does the skateboard keep moving forward with a constant speed after it leaves you hand?

Friction

• Recall that when an object slows down it is accelerating.

• By Newton’s 2nd law of motion, if the skateboard is accelerating, there must be a net force acting on it.

• The force that stops the skateboard is friction.

Friction• Friction is the force that opposes motion between two

surfaces that are touching each other.• The amount of friction depends on; 1: kinds of surfaces 2: the force pressing the surfaces together

Types of FRICTION

• Sliding friction- dry solid surfaces

• Rolling friction- rolling over a surface

• Fluid friction- object moves through a liquid or gas.

Gravity

• Gravity’s attractive force depends on 1. the distance

between the two objects

2. the mass of the two object

Every object exerts a gravitational force on every other object

Gravity

• Isaac Newton formulated the law of universal gravitation, which he published in 1687.

• His law states that gravitational force=

(constant) x (mass 1)x(mass 2) = G m1m2

(distance)2 d2

• According to the law of universal gravitation, the gravitational force between two masses decreases rapidly as the distance between the masses increases.

GRAVITY

• Free fall- when the only force acting on a falling object is gravity. (9.8m/s/s)

• Do all objects regardless of mass fall at this rate?

• What about projectiles?

GRAVITY

• Air resistance- a type of fluid friction that objects falling through air experience.

• Paper vs.. Book• Weight- The measure of the

force of gravity on an object (scale)

• Mass- is a measure of the amount of matter in that object.

Weight

• Weight and mass are NOT the same.

• Weight is a force and mass is a measure of the amount of matter an object contains.

• Weight and mass are related. Weight increases as mass increases.

Weight on different planets

• The weight of an object usually is the gravitational force between the object and Earth.

• The weight of an object can change, depending on the gravitational force on the object.

Air resistance

• When an object falls towards Earth, it is pulled downward by a force called gravity.

• However, a friction-like force called air resistance opposes the motion of objects that move through air.

• Air resistance causes objects to fall with different accelerations and different speeds.

Air resistance

• The amount of air resistance on an object depends on the speed, size, and shape of the object.

• Air resistance, not the objects mass, is the reason feathers and leaves fall slower then pennies or apples.

What about circular motion???

• Bucket Demonstration

• What is the centripetal force for a satellite?

• Centripetal- “center seeking”

• Centripetal force- any force that causes an object to move in a circle.