Forces

What is a Force? A push or a pull. When one object pushes or pulls another object, the first object is exerting a force on the second object. Like velocity and acceleration, force is also described by the strength and direction in which it acts. The strength of a force is measured in an SI unit called Newton (N). (named after Isaac Newton) What is a Force?

In any situation the overall force acting on an object is the net force. The net force determines whether an object moves and also in which direction it moves. Force in the same direction add forces. Force in opposite direction is like adding positive and negative numbers. They cancel each other out. Size of the arrows indicates the strength and direction of the force. Combining Forces

Unbalanced vs. Balanced Unbalanced force acting on the object results in a net force which will change the objects motion. Balanced forces equal forces acting on one object in opposite directions. Balanced forces acting on an object will NOT change the objects motion. Unbalanced vs. Balanced

Unbalanced vs. Balanced

Friction

The force that two surfaces exert on each other when they rub against each other. Acts in the direction opposite to the objects motion. Friction opposes motion. Eventually friction will cause an object to come to a stop. Without friction the object would continue to move at a constant speed until acted upon by another force. What is Friction

Determining the Amount of Friction The amount of friction depends on two factors Surface Type Rougher surfaces have greater friction The amount of force pushing surfaces together. Less massive objects exert less force. Changing the amount of the surface (surface area) that touches does not change the amount of friction. Determining the Amount of Friction

When is Friction Helpful? Salt, sand or gravel on icy surfaces Tires on a car Boots or shoes with traction Other examples? When is Friction Helpful?

Static friction-the friction that acts on objects that are not moving Ex: Moving a desk. It takes more force to get it moving (overcome static friction) Sliding friction-when solid surfaces slide over each other Ex: Sliding a box across the floor, the breaks on your bike tire. Rolling friction-when an object rolls over a surface Ex: Skateboards, bikes. Anything with wheels! Fluid friction-when an object moves through a fluid Ex: Biking, surfing, swimming. Types of Friction

Ways to Reduce/ Increase Friction Lubricants Ex: motor oil, wax, grease. Switching to rolling Ball bearings. Smoother surface Plastic as opposed to wood. Increase Make surfaces rougher. Sand on icy roads, batting gloves. Increase amount of force pushing surfaces together. Add weights. Ways to Reduce/ Increase Friction

Friction Video Clips Physics of The Olympics Missy Franklin & Fluid Dynamics Curling Friction Video Clips

Gravity

The force that pulls objects toward each other. What is Gravity?

The Law of Universal Gravitation “All objects in the universe attract each other through gravitational force . The size of the force depends on the masses of the objects and the distance between them.” Sir Isaac Newton (1642-1727) The Law of Universal Gravitation

Factors that Affect Gravity Mass- The more mass an object has the greater it’s gravitational pull. Ex: The sun has a large mass which keeps the planets in orbit Distance- The farther apart two objects are, the lower the gravitational force between them. Factors that Affect Gravity

When the only force acting on a falling object is gravity, the object is said to be in free fall. The acceleration of any object due to gravity is 9.8 m/s/s (ignoring air resistance). Free Fall

Mass vs. Weight Mass is the amount of matter in an object. Measured in grams or kilograms Weight is the measure of the gravitational force exerted on an object. Measured in Newtons Why do we weight less on the moon? Because the moon has less mass than the earth. Mass vs. Weight

Moon Weight

Try it out… Weight = mass X acceleration due to gravity (9.8 m/s/s) How much does a 50 kg person weigh on earth? Weight = mass X 9.8 m/s/s Weight = 50 kg X 9.8 m/s/s Weight = 490 Newtons Where force (weight) is measured in Newtons, acceleration is measured in m/s/s and mass is measured in kg. Try it out…

How many Newtons do you weigh? Convert lbs. to kg (Weight in pounds X 0.4536) Convert kg to Newtons (Weight in kg X 9.8) Ex. 190 lbs X 0.4536 = 86.18 kg 86.18 kg X 9.8 = 844.60 N To get weight on moon just divide by 6. How many Newtons do you weigh?

An object that is thrown is called a projectile. Projectile Motion

Which Will Hit The Ground First?

Air Resistance A type of fluid friction. The greater the surface area the greater the air resistance. The greater the velocity the greater the air resistance. Air Resistance

As velocity increases air resistance increases As velocity increases air resistance increases. When the force of the air resistance equals the force of the gravity on an object, the object will no longer accelerate. It will continue to fall at a constant rate. So, terminal velocity is the point at which a falling object stops accelerating and begins to fall at a constant speed. Terminal Velocity

Exploring Gravity