# Science chapter studyguide: The Laws of Motion

Chapter 2 study guide: The laws of motion

Know all vocabulary

Force: a push or pull

Contact force: a force that one object exerts on on another object while touching it

Noncontact force: a force one object can exert on another without touching it

Gravity: an attractive force between two objects. The amount depends of objects mass+distance

Mass: matter in an object

Weight: the gravitational force exerted on an object

Friction: a type of force that opposes the sliding of two surfaces touching each other

Net force: the sum of all forces working on one object

Balanced force: when the net force acting on an object does not cause a change in its motion

Unbalanced force: when the net force acting on an object causes the motion of an object to change

Newton's 1st law of motion: an object will move at a constant velocity until unbalanced force acts on it.

Inertia: The tendency of an object to resist a change in its motion

Newton's 2nd law of motion: f=ma force = mass x acceleration

Circular motion: any motion in which an object moves in a curved path

Centripetal force: a force that is exerted toward the center of a curved path

Newton's 3rd law of motion: if one object exerts a force on another the second object exerts an equal yet opposite force on the first object

Force pair: force two objects apply to each other

Momentum: a measure of how hard it is to stop an moving object

p=mv momentum=mass x velocity

Law of conservation of Momentum: the total momentum of a group of objects stays the same unless outside forces act on the objects.

Types of forces

A push or pull on an object is called force. Some forces are applied only when objects touch. Other forces are applied even when objects don't touch. A contact force is a push or pull on one object by another that is touching it. You can have a weak or strong contact force such as pressing your finger on a keyboard to large rocks moving underground to start earthquakes. A force that one object applies to another object without touching it is a noncontact force. The magnetic force which attracts certain metals to magnets is an example. Forces have both strength and direction. Arrows can be used to show forces. The length of this arrow shows the strength of the force and the direction the arrow points shows the direction of the force applied. The SI unit for force is (N) newtons.

What is gravity?

Gravity is an attractive force that exists between all objects that have mass(the amount of matter in an object, measured in kilograms.) Sir Issac Newton developed the law of universal gravitation which states all objects are attracted to each other by a gravitational force. The strength of the force depends on the mass of each object and the distance between them. When the mass of one or more objects increases the gravitational force between them also increases. The attraction between objects decreases as the distance between the objects increases. Weight is the gravitational force exerted on an object; because weight is force it is measured in newtons.

An object's weight is proportional to its mass. Ex) if one object has twice the mass of another object, it also has twice the weight.

In order to find the weight of an object, you multiply its mass by 9.8 m/s2 (the acceleration downward due to gravity)

What is friction?

Friction is a force that resists the motion of two surfaces that are touching. The cause of friction between surfaces is because of dips and bumps that cover the surfaces so when the surfaces slide past each other the dips on one surface catch the dips on the other surface. An example to reduce friction is using soap as it acts as a lubricant and decreases friction.

Rolling

Sliding

Static

Fluid

One object rolls on another

Opposes the motion of surfaces sliding past each other

Prevents surfaces from sliding past each other

Friction between any fluid

Combining forces

The combination of all forces acting on an object is the net force. The way in which forces combine depends on the directions of the forces applied to an object. When the forces applied to an object act in the same direction the net force is the sum of the individual forces.

You can combine forces in opposite directions too and when equal forces act on an

object the net force is zero. Forces that combine and form a net force of zero are

balanced forces. Unbalanced forces are forces acting on an object that combine

and form a net force that is not zero.

Newton's first law of motion

According to Newton's 1st law of motion if the net force on an object is zero, the motion of the object does not change. As a result, balanced and unbalanced forces have different results when they act on an object. According to the 1st law balanced forces cause no change in an object's velocity. Newton’s 1st law only applies to balanced forces acting on an object. When unbalanced forces act on an object at rest, the object starts moving. When unbalanced forces act on an already moving object, the object's speed, direction or motion or both will change.According to Newton's first law the motion of an object will not change if balanced forces act on it. The tendency of an object to resist a change in its motion is called inertia.

Why do objects stop moving?

Friction and inertia cause objects to stop moving.

How do forces change motion?

Forces change an object's motion by changing its speed, its direction,or both speed and direction. Velocity is speed in a certain direction. Only unbalanced forces change an object's velocity. When unbalanced forces act on an object at rest, the object begins moving in the direction of the net force. Unbalanced forces change the velocity of a moving object by changing the speed such as slowing down or speeding up.Another way unbalanced forces can change an object’s velocity is to change it direction or change the velocity by changing it speed, direction or both.

Newton's second law of motion

According to Newton’s second law of motion, the acceleration of an object is equal to the net force acting on the object divided by the object’s mass. The direction of the acceleration is the same as the direction of the net force.

Newton's second law equation: Force (Newtons) = Mass (kg) * Acceleration (m/s2)

• If you increase the mass of an object, it will accelerate less

• If you increase the force exerted on a object, it will accelerate more

• An object will accelerate in the direction of the (unbalanced) net force

Circular motion

Newton's second law describes the relationship between an object’s change in velocity over time or acceleration, and unbalanced forces acting on the object. Circular motion is any motion in which an object is moving along a curved path. In circular motion,a force that acts perpendicular to the direction of motion, towards the center of the curve is centripetal force. A satellite receives centripetal force.

Opposite Forces

When an object applies a force on another object , the second object applies a force of the same strength on the first object, but the force is in the opposite direction.

Newton’s third law of motion

According to Newton’s third law of motion, when one object exerts a force on a second object, the second object exerts an equal force in the opposite direction of the first object. The forces described by Newton's third law depend on each other. A force pair is the force two objects apply to each other. In a force pair, one force is called the action force and the other force is called the reaction force. An action force is when you push against an object, the force you apply is called the action force. The object then pushes back against you that force is called reaction force.

FORCE PAIRS DO NOT CANCEL EACH OTHER OUT BECAUSE THEY ACT ON TWO DIFFERENT OBJECTS!!!!!!!!!!!!!!

Momentum + Conservation of Momentum

Momentum is a measure of how hard it is to stop a moving object. It is a product of an object’s mass and velocity. An object's momentum is in the same direction as its velocity.

Momentum equation:

Momentum (in kg x m/s) = mass (in kg) x velocity (in m/s) p=m x v

According to the law of conservation of momentum, the total momentum of a group of objects stays the same unless outside forces act on the objects. Outside forces include friction.

Types of collisions

Elastic collision: when colliding objects bounce off each other

Inelastic collision: when objects collide and stick together