Lesson 19 All motion is due to forces acting on objects often, more than one force acts on an object at the same time When more than one force acts on an object, scientists can determine the net force, or total combination of these forces

Balanced and Unbalanced Forces A balanced force is one in which the net forces equal zero There is no motion

Balanced and Unbalanced Forces During a tug of war, if the teams are equal in strength the forces pulling on both ends of the rope are equal and there is no movement The forces are balanced, with a net force of zero

Balanced and Unbalanced Forces An unbalanced force has a net force greater than zero The object acted on shows movement in one direction Only an unbalanced force can change the motion of an object

Balanced and Unbalanced Forces An unbalanced force acting on a motionless object can cause the object to move An unbalanced force that acts on an object that is already in motion can change the peed or direction of the object

Balanced and Unbalanced Forces Unbalanced forces can act in the same direction Sometimes unbalanced forces act in opposite directions

Balanced and Unbalanced Forces Only an unbalanced force can change the motion of an object An unbalanced force acting on a motionless object can cause the object to move

Balanced and Unbalanced Forces An unbalanced force that acts on an object that is already in motion can change the speed or direction of the object * Unbalanced forces can act in the same direction

Balanced and Unbalanced Forces Sometimes unbalanced forces act in opposite directions

Mass and Inertia Objects have a tendency to resist any change in motion If an object is at rest, the object will stay at rest until an unbalanced force causes it to move

Mass and Inertia If an object is in motion, the object will keep moving in the same direction and at the same speed until an unbalanced force acts on it to change its direction or speed

Mass and Inertia Inertia is the tendency of objects to resist a change in motion Inertia is why you wear seat belts

Mass and Inertia If a car is going 50 kilometers per hour and it comes to a sudden stop, the people inside continue moving 50 kilometers per hour unless a force prevents their forward motion through the windshield

Mass and Inertia Mass affects inertia An object with a large mass has more inertia than an object with a small mass

Mass and Inertia Changing the motion of an object with a large mass is harder than changing the motion of an object with small mass

Mass and Inertia Pulling a wagon full of rocks is harder than pulling an empty wagon because the full wagon has more mass

Mass and Inertia A train takes longer to accelerate to 60 kilometers per hour than a car because the train has more mass

Mass and Inertia Stopping a train that is moving at 60 kilometers per hour also takes longer than stopping a car moving at the same speed, this is because the train has much more mass

Friction and Gravity Two forces that can always affect the motion of an object are Friction Gravity

Mass and Inertia Friction is a force that opposes the motion of an object This contact force acts whenever an object in motion rubs against a surface Friction opposes motion when two surfaces tough

Mass and Inertia The contact reduces the speed of the moving object and releases heat The quantity of friction depends on the push on the object, the bumpiness of the surfaces, and the weight of the object being pushed

Gravity Gravity is a force of attraction between all objects Newton stated that the force of gravity between two objects depends on their masses and the distance between them

Gravity Since Earth is so large, all objects are attracted to it Gravity acts on the motion of objects that are not touching the planet and causes objects to stop their forward movement in the atmosphere and fall to the ground

Gravity Gravity acts at a distance A simple example of the gravitational force on motion is a person throwing a ball * The ball is pulled to Earth’s surface by gravity unless it is stopped by an outside force, such as someone catching it