Newton’s Laws of Motion

•In 1686, Sir Isaac Newton explained the relationship between force and the motion of objects with three laws of motion.

1st Law of Motion• Newton’s 1st Law of Motion

states that an object at rest remains at rest and an object in motion stays in motion until acted on by an unbalanced force.

• Two parts: Objects at rest and objects in motion.

• Part 1: A golf ball will remain on the tee until the golf club (unbalanced force) comes into contact with it.

1st Law continued• Part 2: Bumper cars remain

moving at the same speed and in the same direction until an unbalanced force acts on it.

• Often, this force is another car, which transfers force and causes the other car (and yourself since you’re strapped in) to change direction.

• Friction is also an unbalanced force acting in the opposite direction of a moving object.– Demonstration

Inertia• Newton’s 1st Law is often

called the law of inertia.• Inertia is the tendency of all

objects to resist any change in motion.

• All objects (resting or in motion) have inertia.

• Objects with a smaller mass have less inertia than one with a larger mass. It’s easier to start and stop a less massive object.– Mass is a measure of inertia.

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Newton’s 2nd Law of Motion

• Newton’s 2nd law states that the acceleration of an object depends on the mass of the object and the amount of force applied.

• Part 1: Acceleration depends on mass.

– An empty shopping cart accelerates faster than a full cart. Acceleration increases as mass decreases and acceleration decreases as mass increases. (inverse relationship)

Newton’s 2nd Law continued

• Part 2: Acceleration depends on force.– The same cart will move faster when a larger

amount of force is used. Acceleration increases as force increases. (direct relationship)

• Mathematically, Newton’s 2nd law states:– a = F/m or F = ma– Acceleration equals the force applied divided by

the object’s mass. When rearranged, force equals the object’s mass multiplied by acceleration.

• This law explains why objects fall to Earth at the same rate regardless of mass.

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Newton’s 3rd Law of Motion• Newton’s 3rd law states that whenever

an object exerts a force on a second object, the second object exerts an equal and opposite force on the first object.

• The law states that forces act in pairs, but do not act on the same object.

• There are action and reaction forces (acting in pairs).

• Example: A swimmer’s arms act on the water. The water reacts by pushing on the arms, hand, and feet which moves the swimmer forward.

• Effects of a Reaction force can be difficult to see.– When a ball falls to Earth, gravity pulls the

ball (action force). The reaction force is gravity pulling Earth towards the ball (reaction force). The Earth is so massive, its acceleration is much smaller than the ball, so it’s impossible to see.

3rd Law Continued3rd Law Continued

For every action there is an For every action there is an equal & opposite reaction.equal & opposite reaction.

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Momentum• Momentum is a property of

moving objects that depends on the object’s mass and velocity.

• If a small car and truck are traveling with the same velocity, the truck has greater momentum due to its larger mass and will be harder to stop.

• Similarly, if two cars are traveling with different velocities, the car with a greater velocity has greater momentum than the smaller car.

Conservation of Momentum• When a moving object hits

another object, some or all of the momentum from the first object is transferred to the other object.

• For example: Billiards and Bowling

• Law of Conservation of Momentum: Any time two or more objects interact, they may exchange momentum, but the total amount of momentum stays the same.