Friction, and Gravity

Copy and Answer 9/7/16 A hiker’s velocity begins at 1.8 m/s uphill and changes to 1.5 m/s uphill. How do you know that the hiker has a negative acceleration?

4 Types of Friction The 4 types of friction are: Sliding friction (strongest) Rolling friction Fluid friction (weakest). Static friction

Friction Friction is a force that two surfaces exert on each other when they rub against each other. Without friction or other unbalanced forces, an object will not stop until it hits another object. Friction acts in the direction opposite to motion.

Causes of Friction Smooth surface has less friction than rough surface. Strength of force of friction depends on types of surfaces involved and how hard the surfaces push together. i.e. rubbing hands together

4 Types of Friction Static Friction – Friction that acts on objects that are not moving i.e. moving a piece of furniture across room Must use extra force to start friction of stationary objects.

4 Types of Friction Sliding Friction – when two solid surfaces slide over each other. When an object is pulled at a constant velocity across a level surface, the pulling force equals the sliding friction.

4 Types of Friction Rolling friction – when an object rolls across a surface. Rolling friction is less than sliding friction Ball bearings

4 Types of Friction Fluid friction – when a solid object moves through a fluid. Less than sliding friction Parts of machines bathed in oil

What type of friction?

What type of friction?

What type of friction?

2 Factors of Friction 1-Type of surface. 2-How hard the two surfaces push together.

Which type of hockey has more friction?

Static Friction - Friction that acts on objects that are not moving 4 Types of Friction Static Friction - Friction that acts on objects that are not moving Example: moving a piece of furniture across room. Must use extra force to start friction of stationary objects. Sliding Friction - Friction where two solid surfaces slide over each other. When an object is pulled at a constant velocity across a level surface, the pulling force equals the sliding friction. Rolling friction - Friction where an object rolls across a surface. Rolling friction is less than sliding friction. Fluid friction - Friction where a solid object moves through a fluid. Less than sliding friction. Parts of machines bathed in oil.

Gravity Gravity is a force that pulls objects toward each other. Newton realized gravity acts everywhere in universe. Law of Universal Gravitation – force of gravity acts between all objects in the universe. Any two objects in the universe, without exception, attract each other

Mass and Distance Force of gravity between object increases with greater mass and decreases with greater distance. Further from Earth – less gravitational force Mass – measure of the amount of matter in an object Kg

Gravity & Weight Weight is gravitational force exerted on a person or object at the surface of a planet. Stepping on scale – gravitational force Earth exerting on you Objects with greater mass have greater weight Weight = Mass x Acceleration due to gravity

Gravity & Weight Weight – Newtons (N) Mass – Kilograms (KG) Acceleration due to gravity = 9.8 m/s² (at Earth’s surface)

Gravity & Weight Weight varies with strength of gravitational force – mass doesn’t. Strength of gravitational force exerted on an object or person by the moon is 1/6 of the force exerted by Earth.

Gravity and Motion Free fall – when the only force acting on an object is gravity. In free fall, the force of gravity alone causes an object to accelerate in the downward direction. All objects in free fall accelerate at the same rate.

Air Resistance Air resistance is a type of fluid friction – an upward force exerted on falling objects. Objects with greater surface area experience more air resistance as they fall. Flat piece of paper vs. crumpled piece of paper

Projectile Motion Projectile – an object that is thrown. Ball thrown horizontally falls at the same rate as ball dropped.

Elastic Forces Matter is considered elastic if it returns to its original shape after is squeezed or stretched. Compression is an elastic force that squeezes or pushes matter together. Tension an elastic force that stretches or pulls matter