10.3 Friction

What is friction? Friction is a Force that opposes motion. Friction happens when 2 substances rub together. Why you stop swing on a swing, why a ball stops rolling. The strength of friction depends on: a) type of surfaces rubbing, and b) how hard they are pushed together.

Friction can be useful or harmful depending on the situation Some times we want to increase frictional forces – when using the brakes on a bike or a car, and wearing shoes with rough soles. Other times we want to reduce frictional forces – when polishing surfaces, lubricating with oil, or using roller instead of sliding.

3 Types of Friction Sliding friction: ex. skis on snow; walking on the ground. Rolling friction: ex. tires on road. Fluid friction: ex. lubricated parts of a machine. More force is needed to overcome Sliding friction than Rolling or Fluid friction.

Gravity Gravity is the force that pulls objects toward Earth. Objects are in free fall when only gravity force is involved. Objects that are thrown vertically or horizontally will land at the same time.

Air Resistance Objects are falling through air experience air resistance (type of fluid friction). The greater the surface area of the object, the greater the air resistance. As objects start falling they accelerate, but they reach a point where air resistance is high enough to stop the acceleration. Then they continue to fall at their greatest velocity, or terminal velocity.

Universal Gravitation Every object exerts a gravitational force on every other object. This is known as Universal Gravitation. When an apple breaks from its stem, the apple falls down because the gravitational force between the apple and the earth is greater than the gravitational force between the apple and the tree. The force of gravity acts even when the objects don't touch

Mass & Distance The force of gravity between two objects depends on their masses and on the distances between the two objects. The greater the mass the greater the gravitational force it exerts on other objects. The force of gravity changes as the distance changes. Gravity is weaker than other types of forces, even though it holds the planets, stars, and galaxies together. Because their HUGE! Even though they don’t touch

Acceleration Due to Gravity If we ignore air resistance, a freely falling body picks up speed (accelerates) at the rate of about 9.8 m/s 2 due to gravity pull. 9.8 m/s 2 is a constant called acceleration due to gravity or “g”.

Newton’s Third Law of Motion It states, “If one object exerts a force on another object, then the second object exerts a force of equal strength in the opposite direction on the first object.” The first force is called “action”. The second opposite force is called “reaction”. Another way to state the 3 rd law is, “For every action there is an equal and opposite reaction.” All forces come in pairs.

Examples You walk by pushing the ground (action), the ground pushes you back (reaction). Squids shoot water from its body back ward (action), the water surrounding it pushes it forward (reaction).

Momentum Newton used the terms “velocity” and “quantity of motion” to describe his 3 laws. We call “quantity of motion” momentum. Momentum = mass × velocity Momentum unit is kg. m/s The ↑ the mass the ↑ the momentum. The ↑ the velocity the ↑ the momentum.

p = mv m p v p:momentum (kg ·m/s) m:mass (kg) v:velocity (m/s)

Find the momentum of a bumper car if it has a total mass of 280 kg and a velocity of 3.2 m/s. GIVEN: p = ? m = 280 kg v = 3.2 m/s WORK: p = mv p = (280 kg)(3.2 m/s) p = 896 kg·m/s m p v

The momentum of a second bumper car is 675 kg·m/s. What is its velocity if its total mass is 300 kg? GIVEN: p = 675 kg·m/s m = 300 kg v = ? WORK: v = p ÷ m v = (675 kg·m/s)÷(300 kg) v = 2.25 m/s

Conservation of Momentum The law of conservation of momentum states that the total momentum of the objects that interact does not change. Check Figure 19,a,b,&c on page p before = p after

A 5-kg cart traveling at 4.2 m/s strikes a stationary 2-kg cart and they connect. Find their speed after the collision. BEFORE Cart 1: m = 5 kg v = 4.2 m/s Cart 2 : m = 2 kg v = 0 m/s AFTER Cart 1 + 2: m = 7 kg v = ? v = p ÷ m v = (21 kg·m/s) ÷ (7 kg) v = 3 m/s p before = 21 kg·m/s p after = 21 kg·m/s

A 50-kg clown is shot out of a 250-kg cannon at a speed of 20 m/s. What is the recoil speed of the cannon? BEFORE Clown: m = 50 kg v = 0 m/s Cannon: m = 250 kg v = 0 m/s AFTER Clown: m = 50 kg v = 20 m/s Cannon: m = 250 kg v = ? m/s p = 0 p before = 0 p = 1000 kg·m/s p after = 0 p = kg·m/s

So…now we can solve for velocity. GIVEN: p = kg·m/s m = 250 kg v = ? WORK : v = p ÷ m v = (-1000 kg·m/s)÷(250 kg) v = - 4 m/s (4 m/s backwards) m p v

Momentum Problems 2 train cars with the mass of 10,000 kg each and both traveling in the same direction. The front car is traveling with a velocity of 20 m/s, while the car in the back is traveling with a velocity of 24 m/s. If the back car caught up with the front car and bumped it, what is the new velocity of each car? What is the momentum before and after collision?

A car of mass 600 kg traveling at 20 m/s collides with a stationary truck of mass 1400 kg. The 2 vehicles are locked together after collision; what is their combined velocity?