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Newton’s Laws of Motion

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Newton’s Lawsof Motion

Forces

- What happens to a ball rolling along the ground?
- Why does it keep moving?
- What slows it down?
- What happens to a ball when it is thrown in the air?
- Why does a ball that is at rest stay where it is?

- Objects at rest tend to stay at rest, and objects in motion tend to stay in motion with constant speed and direction, unless acted upon by an outside force.
- Objects maintain constant velocity unless acted upon by an outside force.
- Also called the Law of Inertia

- Newton’s first law is also referred to as the law of inertia.
- Inertia: resistance of a body to a change in motion
- Mass is a measure of inertia, which remains constant for that object no matter where it is located.

- Why does a ball thrown in the sky come back down?
- Why does a ball rolling on a table come to a stop?
- There is a net force acting on the ball.
- Net force: sum of all the forces acting on an object
- Free body diagram: picture of all forces acting on an object

1.5 N

1 N

2 N

1.5 N

- Draw a free body diagram for an object with the following forces:
- 3 N up
- 4 N right
- 1 N down
- 2 N left

3N

2N

4N

1N

- Apply a net force to an object. What happens to the object?
- As a net force is applied, the object will accelerate.
- The greater the force, the greater the acceleration.
- Apply the same net force to two differently massed objects. What happens to the objects?
- One will accelerate faster than the other.

- The acceleration of an object is directly proportional to the force applied and inversely proportional to the object’s mass.
- F=ma or a=F/m

- While seated in a car, it begins to accelerate. Which direction is the force? Which direction is the acceleration?
- While seated in the car, the driver slams on the brakes. Which direction is the force? Which direction is the acceleration?
- While seated in the car, the driver sharply turns the wheel. Which direction is the force? Which direction is the acceleration?

- Weight and mass are not the same thing.
- Your mass is a constant; mass is a measure of inertia.
- Your weight changes based on your location.
- Weight is due to the force of gravity.
- Force = mass x acceleration (F=ma)
- Weight = mass x gravity (W=mg)

- Push on a wall. Does it push back on you? With how much force?
- What about when you push on a ball? Does it push back on you?
- Newton’s Third Law of Motion: For every action there is a reaction of equal magnitude in the opposite direction.

- A horse is attached to a cart, and the owner tells the horse to start moving. The horse, being knowledgeable in Newton’s laws, says that because the cart pulls on the horse with the same force the horse pulls on the cart, they will not be set in motion.
- What do you think?

- Draw a free body diagram
- Look at all the action-reaction force pairs
- Determine net force based on free body diagram

Let’s look at the horse and the cart in greater detail, with a free body diagram.

Fhorse=Fcart

Force of Cart on Ground?

Force of Horse on Ground?

Force of horse on ground = force of ground on horse

Force of cart on ground = force of ground on cart

- The cart will be set in motion because the force pair between the horse and the ground is greater than the force pair between the cart and the ground (in this case, friction in the wheels). Because the horse pushes on the ground greater than the ground (or friction) pushes on the wheels, the cart can move.

- What happens when you throw a ball in outer space?
- What happens to the ball?
- What happens to you?

- The ball will move forward after given acceleration a due to force F.
- You will move backward after given acceleration a due to force F.
- The forces are equal, in opposite direction.
- Will you have more acceleration than the ball?
- No. Let the equation guide your thinking.

- Newton’s three laws can be summed up in one word: momentum.
- Momentum is the resistance to a change in velocity, very much like inertia. However, momentum is dependent upon an object’s velocity as well as its mass.
- r=mv

- The momentum of a closed system will remain constant.
- Remember throwing a ball in outer space.
- Your momentum with the ball is 0 (no motion).
- The ball has momentum mv.
- You have momentum –mv.
- The momentum remains 0.

- When objects collide, momentum is also conserved.
- A larger object in motion collides with a smaller object at rest. What happens?
- A smaller object in motion collides with a larger object at rest. What happens?
- Two objects with equal mass collide, one moving, the other at rest. What happens?

- Newton’s First Law: Objects maintain constant velocity unless acted upon by an outside force. AKA Law of Inertia
- Newton’s Second Law: F=ma
- Newton’s Third Law: For every action there is an equal but opposite reaction
- Momentum: r=mv. Inertia in motion
- Momentum is conserved in closed systems