Describing Motion Force and Newton’s Laws

First we need to define the word FORCE: The cause of motion (what causes objects to move) Force always has magnitude and direction. Two types of forces –Push –Pull

Forces are measured in Newtons 2SI unit of force, 1 N = 1 kg·m·/ s 2kgm Symbol: N Measured by using a spring scale

Net Force Net force is the total/cumulative force on an object If two or more forces are going in the same direction, add the forces together. If two or more forces are going in the opposite direction, subtract the forces from each other.

Forces may be balanced or unbalanced Balanced forces – all forces acting on an object are in equal magnitude and in opposite directions. Net force is 0 There is NO change in MOTION There can be either No Movement or No change in velocity.

Unbalanced Forces Unbalanced forces – one or more forces acting on an object are stronger than others. These forces cause an object to stop moving, start moving, or change direction. This results in a net force and causes a change in the object’s velocity. –There is a change in MOTION There is A NET FORCE

Why are these examples of unbalanced forces? What is the Net Force for the green speed racer?  What is the Net Force for the red airplane?

BRAIN BREAK! With your partner, discuss how balanced and unbalanced forces can apply to an arm wrestling match.

Newton’s Laws First Law – Inertia The resistance of an object to change its motion. Second Law – F= m x a Force =Mass x Acceleration, Third Law – Action-Reaction For every action there is an equal and opposite reaction

First Law Inertia –An object at rest [not moving] remains at rest unless acted on by a force [push or pull] –An object in motion remains in motion unless acted on by a force [push or pull]

First Law Inertia & Mass –Mass is the amount of matter in an object –The more MASS an object has, the more INERTIA the object has. –Bigger objects are harder to start & stop

Greater Mass = Greater Inertia therefore…Greater the force required to change its motion

Is the Force strong enough to move the car?

Second Law F=M x a Acceleration & Mass Definitions –Acceleration is a change in velocity [speed or direction] –Mass is the amount of matter in an object

Newton’s 2 nd Law of Motion Acceleration of an object depends on the Net Force acting on the object and the object’s mass. Acceleration = Net Force Mass Or Net Force = Mass x Acceleration It is this equation that explains why objects fall at the same acceleration during FREE FALL despite mass.

Second Law Acceleration & Force –The more force placed on an object, the more it will accelerate [change its motion] Acceleration & Mass –The more mass [or inertia] an object has, the more force it takes to accelerate the object

Greater Mass means greater force pulling down… Acceleration = Force/Mass

Elephant & Feather fall at SAME rate during Free Fall!!

TRUE or FALSE: The elephant and the feather each have the same force of gravity. The elephant has more mass, yet both elephant and feather experience the same force of gravity. The elephant experiences a greater force of gravity, yet both the elephant and the feather have the same mass. On earth, all objects (whether an elephant or a feather) have the same force of gravity. The elephant weighs more than the feather, yet they each have the same mass. The elephant clearly has more mass than the feather, yet they each weigh the same. The elephant clearly has more mass than the feather, yet the amount of gravity (force) is the same for each. The elephant has the greatest acceleration, yet the amount of gravity is the same for each.

What about when there is Air Resistance?

Newton’s 3 rd Law of Motion Forces are always produced in pairs.If one object exerts a force on another object, then the second object exerts a force of equal strength in the opposite direction. What does that mean? For every ACTION…there is an equal and opposite REACTION!

Action-Reaction Pairs

When the gymnast does a flip, she pushes down on the vaulting horse (action force). The reaction force of the vaulting horse pushes her up to complete the flip.

Do Action-Reaction Forces cancel each other out? NO! The Action-Reaction Forces act on different objects.

The truck is in motion. What is the force that causes it to stop? The push of the stopped car. The car is at rest. What is the force that causes it to move? The push of the truck.

What about the ladder on top of the truck? The ladder is in motion because the truck is in motion. When the truck stops, the ladder stays in motion. The truck is stopped by the force of the car, but the ladder is not. What force stops the ladder? Gravity.

The truck is in motion, the car is at rest. How do each of these vehicles accelerate? The truck stops moving. The car starts moving. Which one will be the hardest to accelerate? The truck because it has the most mass.

Why does the car move [accelerate] when it is hit by the truck? The heavy and moving truck has more force than the small, at rest car. Why does the truck stop moving when it hits the car? The force of the car pushing back on the truck, plus the force of friction between the massive truck and the road slow down, the stop the truck.

The truck hits the car. An action force stops the truck. What is the equal and opposite reaction force? The force that pushes the car forward.

Momentum! Quantity of Motion Momentum = Mass x Velocity Kg x m/s Momentum is a vector quantity Momentum of an object is in the same direction as its velocity

If a bicycle and a fire engine are both moving at a speed of 5 km/hr, which would be easier to stop? Why? The Bicycle! It has less mass, and therefore, has less momentum. Momentum = Mass x Velocity When a car slows down, its velocity decreases and its mass stays the same. How does slowing down affect the car’s momentum? The Momentum Decreases! Momentum= Mass x Velocity!

Let’s Review What happens to passengers when a car stops quickly? They keep moving until the seatbelt stops them. How does Newton’s 1 st Law explain what happens to passengers when a car stops quickly? An object’s motion does not change unless acted upon by an unbalanced force…resistance to change in motion is inertia. The seatbelt exerts the unbalanced force.