Motion & Forces Unit 2

What does force have to do with motion? Objects don’t just get up and move on their own. A force must be applied to the object to create the motion. Force- a push or pull Some forces aren’t noticeable like the force the floor exerts on your feet or the atmosphere pushing against you or gravity pulling on you.

Force doesn’t always= motion If two forces are acting on an object in opposite directions they combine to form the net force. Forces on an object that are equal in size and opposite in direction are called balanced forces and the net force is zero.

Unbalanced forces Forces of different sizes can combine and result in motion. Let’s watch some videos about unbalanced forces. Sumo Wrestler and Chimp http://www.youtube.com/watch?v=YFMpWm6ECgQ Hockey Collisions http://www.youtube.com/watch?v=qaK5o6j42zc Smart Car and Regular Car Head On Collision http://www.youtube.com/watch?v=OZs1QuHerTU Cycling Crashes http://www.youtube.com/watch?v=qBE98Raljwo

Inertia Inertia- the tendency of an object to resist any change in its motion. Example: Dirt bike sliding straight even though the driver is turning the wheel If an object is moving it will have uniform motion and continue to move at the same speed and in the same direction unless an unbalanced force acts on it.

Newton’s 1st Law of Motion Object in motion stays in motion/Object at rest stays at rest unless unbalanced force acts on it AKA- Law of Inertia When one billiard ball strikes another and causes it to move what type of force is involved? Unbalanced Forces

Jump In: Using what you learned about Newton’s 1st Law… Why are people required to wear safety belts in vehicles? Because an object in motion will stay in motion. If there’s a crash, the unbelted person will continue to travel in the original direction at the same speed the car was moving just before impact. A belted person will slow down as the car slows down. Safety belts loosen a little as they restrain the person increasing the time it takes to slow them down, but this decreases the force exerted on the person.

Newton’s 2nd Law of Motion Acceleration is produced when a net force acts on a mass The acceleration will be in the same direction as the net force applied on the object Acceleration (m/s2)= net force (Newtons) mass (kg) OR a = FNet m

Calculating Net Force Use the same equation, just rearrange the variables to solve for “F” F= m X a (Force= mass X acceleration)

Jump In: What do these pictures have to do with Newton’s 2nd Law?

What happens when you push a skateboard? Friction Friction- the force that opposes the sliding motion of two surfaces that are touching each other. The amount of friction depends on the types of surfaces in contact and how much force is pushing them together What happens when you push a skateboard?

What causes friction? When two surfaces are in contact and are not completely smooth. There can be pits or bumps on the surface which rub against one another and slow the Force Two surfaces

Static Friction Static friction- the frictional force that prevents two surfaces from sliding past one another. Happens between objects before they move No movement, static friction between box and floor

Sliding Friction Sliding (Kinetic) Friction- the force that opposes the motion of the two surfaces sliding past each other. Happens between objects that are already moving

Air Resistance Air resistance- a friction-like force that opposes the motion of the objects that move through the air. Air resistance acts in the opposite direction to the motion of the object in the air. So if an object is falling, air resistance is pushing upward. Depends on the object’s speed, size and shape

Gravitational Constant (G): Gravity Gravity- attractive force between any 2 objects that depends on the masses of the objects and the distance between them. One of the 4 basic forces Gravitational Constant (G): 6.67 X 10 -11

Force of Earth’s Gravity Force of gravity (N)= mass (kg) X accel. of gravity (m/s2) F= mg Weight- the gravitational force exerted on an object W= mg

Centripetal Acceleration/Force Centripetal Acceleration- acceleration toward the center of a curved or circular path Centripetal Force- the net force exerted toward the center of a cured path

Newton’s 3rd Law of Motion For every action there is an equal (in strength) and opposite reaction (in direction).

Momentum Momentum- how much force is needed to change an object’s motion Momentum (kg m/s)= mass (kg) X velocity (m/s) p= mv