Coach book Lesson 7: Forces, Motion and Inertia

Forces, Motion and Inertia

Motion is a change of position.

Any time an object begins to move, some force is acting on it. Any time the motion of an object changes, such as speeding up, slowing down, turning, or stopping, a force is responsible for that change.

A force is a push or pull. A force has magnitude and direction. The magnitude of a force is how strong it is.

• Produce a net force that is not zero.

• The object acted on moves in the direction of the greater force.

Unbalanced Forces

• Have a net force of zero.

• No change in motion.

Balanced Forces

Balanced and Unbalanced Forces

Newton's First Law

The law of inertia.
An object at rest says at rest, and an object in motion stays in motion at the same speed and in the same direction, unless acted upon by an unbalanced force.

An object's motion remains the same unless a net force acts on the object.

Remember that wen balanced forces are acting on an object at rest, the object does not move. When balanced forces are acting on an object in motion, the object keeps moving at the same velocity (at the same speed and in the same direction).

The tendency of an object to resist a change in its motion.
An object's inertia depends on its mass. The greater an object's mass, the greater it's inertia. The greater an object's inertia, the more difficult it is to change motion.

Inertia

Multiple Choice

Why does Newton's First Law refer only to an unbalanced force and not to a balanced force?

• If an object was acted on by a balanced force, these forces cancel out resulting in a net force of 0 newtons, which means they are stationary.

• Because it's first

• Because of the speed

Multiple Choice

Consider a car being acted on by balanced forces. Can you conclude whether the car is moving or at rest? Explain your response.

• Yes, it would be in motion if the forces are balanced.

• Yes, it would be stopped if the forces are balanced.

• No, a car in motion would remain at the same velocity if acted upon by balanced forces and a car at rest would remain at rest if acted upon by balanced forces.

Newton's Second Law

The net force acting on an object is equal to the object's mass multiplied by its acceleration.

F = ma

Acceleration is the rate of change in velocity.

Multiple Choice

A full shopping cart has more mass than an empty cart. Suppose that an empty cart has a mass of 20kg. You want to accelerate the cart at a rate of .2 m/s^2. How much force would you need to apply?

• 6 kgxm/s^2

• 4 kgxm/s^2

• 100 kg.m/s^2

• A lot

Newton's third law

For every action there is an equal and opposite reaction.

This law means that forces act in pairs.

Think of a fish swimming in a pond. The fish swims by moving its fins. The fins push the water back, and the water pushes the fish forward with an equal force.

Multiple Choice

A rocket is traveling through the atmosphere at a constant velocity. Which of the following is true?

• The rocket's engine is producing an unbalanced force that

• 
  The rocket exerts a greater force on its exhaust gases than the gases exerted on the rocket.

• All the forces acting on the rocket are balanced.

• The rocket is moving, so some net force must be acting on it.

Multiple Choice

How much fore is needed to accelerate a mass of 160 kg by 2 m/s²?

• 
  40 N

• 
  80 N

• 640 N

• 
  320 N

Multiple Choice

Student A and Student B are each standing on a skateboard. Student A pushes gently on Student B to start her moving. Student B starts moving away from Student A. What will happen to Student A and why?

• Student A will move away from Student B, because an equal and opposite force pushes on her.

• Student A will move toward Student B, because the same force affects both of them.

• 
  Student A will not move, because no forces are acting on her.

• 
  Student A will immediately stop moving because of friction.

Multiple Choice

Which of Newton's laws states that the net force acting on an object is equal to the object's mass multiplied by its acceleration?

• Newton's 1st Law of Motion

• Newton's 3rd Law of Motion

• 
  Newton's 2nd Law of Motion

Multiple Choice

Which of Newton's laws states that for every action there is an equal and opposite reaction?

• Newton's 1st Law of Motion

• Newton's 3rd Law of Motion

• Newton's 2nd Law of Motion

Multiple Choice

Which of Newton's laws states that an object at rest will remain at rest and an object in motion will remain in motion?

• 
  Newton's 1st Law of Motion

• 
  Newton's 2nd Law of Motion

• 
  Newton's 3rd Law of Motion