Chp. 5: Laws of Motion Study Guide

Force A push or pull on an object. Represented in Newtons (N)

Balanced Force Forces acting on an object that combine and form a net force of zero. Usually object doesn’t move because forces on each side are equal to each other.

Examples of Balanced Forces A book sitting on a desk A car parked in a garage Sitting on a chair

Unbalanced Force Forces acting on an object that combine and form a net force that is not zero. Object is moving because forces on each side are not equal to each other.

Examples of Unbalanced Forces Kicking a football through the uprights Rocket taking off Rock rolling down a hill

Net Force The combination of all forces acting on an object. If two forces on an object are moving in the same direction then the net force is the sum of the two forces. Ex. Two people are pushing a car with forces of 200N and 100N to the right, the net force is 300N (200N+100N=300N) -In this case this is an unbalanced force because the net force is not zero.

Net Force continued Ex. Two people are pulling on one side of rope with a force of 10N and two other people are pulling on the rope with a force of 12N, the net force is 2 N to the right (12N-10N=2 N) -In this case this is a unbalanced force because the net force is zero, and also a direction is given (right) because more force is being pulled to one side.

Net Force continued If two forces on an object are moving in opposite directions then the net force is the difference between the two forces. Ex. Two people are pushing on each other’s hands with forces of 200N and 200N against each other, the net force is 0 N (200N-200N=0 N) -In this case this is a balanced force because the net force is zero

Newton’s First Law of Motion An object at rest will remain at rest, and an object in motion will remain in motion unless acted upon by an unbalanced force. This is also called the law of inertia because inertia means that an object will resist a change in motion. Ex. A car crashes into a wall and the car crash dummy’s body will continue to move forward resisting the change in motion of the car moving to all of a sudden stopping after hitting the wall.

Newton’s First Law of Motion Examples A soccer ball does not move until someone kicks. A skateboarder hits a rock with his board. The board stops but the boy flies off. A tennis ball is rolled on the floor.

Newton’s Second Law of Motion The size of a force and the mass of an object affect its acceleration. The greater the force, the greater the acceleration; the greater the mass, the greater the force needed to move the object. The relationship among the force, mass and acceleration can be written in one equation: Force = Mass x Acceleration.

Newton’s Second Law of Motion Examples A rock with more mass will accelerate less unless more force it put on it. You have a box of books and a box of tissue boxes. You push the box of books with more force to get it to move. One boy throws a football farther than another boy because he used more force.

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. For every action, there is an equal and opposite reaction.

Newton’s Third Law of Motion Examples The force of the cannon ball being pushed out of the cannon makes the cannon move backwards while the cannon ball moves forward If air is let out of a balloon quickly, air pushes down & balloon goes up. A fireman turns on his hose & is knocked backwards.

Momentum The measure of hard it is to stop a moving object. Formula: SI Unit = kg m/s Momentum(p)=mass(kg) x velocity(m/s) Example: What is the momentum of a 200-kg motorcycle traveling at 10 m/s? p=m x v 200 kg x 10 m/s = 200 kg m/s

Law of Conservation of Momentum The total momentum of a group of objects stays the same unless outside forces act on the objects. Ex. If two pool balls collide with each other their momentum stays the same as the original momentum but they move in a different direction.