Physics: Newton's First Law
Understanding inertia, balanced forces, and motion
Physics: Newton's First Law
Understanding inertia, balanced forces, and motion
Physics - Grade 6-8
- 1
State Newton's First Law in your own words.
Think about what happens to motion when no outside push or pull changes it.
Newton's First Law says that an object at rest stays at rest and an object in motion keeps moving at the same speed and in the same direction unless an unbalanced force acts on it. - 2
A soccer ball is sitting still on the grass. According to Newton's First Law, what will happen to the ball if no one kicks it and the wind does not move it?
The soccer ball will stay at rest because no unbalanced force is acting on it to make it move. - 3
A skateboarder is moving forward on a flat sidewalk. If there were no friction or air resistance, what would Newton's First Law predict about the skateboarder's motion?
Ignore the forces that usually slow moving objects down.
Newton's First Law predicts that the skateboarder would keep moving forward at the same speed and in the same direction because no unbalanced force would slow them down or change their direction. - 4
A book is resting on a desk. Gravity pulls the book downward, and the desk pushes upward on the book with an equal force. Is the book's motion changing? Explain why or why not.
Balanced forces do not change an object's motion.
The book's motion is not changing because the downward force of gravity and the upward force from the desk are balanced. Since the net force is zero, the book remains at rest. - 5
A passenger in a car seems to lurch forward when the driver suddenly brakes. Use Newton's First Law to explain why this happens.
The passenger's body keeps moving forward because of inertia. The car slows down quickly, but the passenger's body tends to keep moving at its previous speed until the seat belt or another force stops it. - 6
What is inertia?
Inertia is related to how much an object resists starting, stopping, or turning.
Inertia is the tendency of an object to resist a change in its motion. An object at rest tends to stay at rest, and an object in motion tends to keep moving unless a force changes its motion. - 7
Which object has more inertia: an empty shopping cart or a full shopping cart? Explain your answer.
A full shopping cart has more inertia because it has more mass. It is harder to start moving, stop moving, or turn than an empty cart. - 8
A hockey puck slides across ice and slowly comes to a stop. Does this disprove Newton's First Law? Explain.
Look for forces that act opposite the puck's motion.
This does not disprove Newton's First Law because friction and air resistance are unbalanced forces that slow the puck down. Without those forces, the puck would keep sliding at the same speed and in the same direction. - 9
A box is being pushed to the right with a force of 20 newtons. Friction pushes left with a force of 20 newtons. What is the net force on the box, and what does Newton's First Law say about its motion?
The net force is 0 newtons because the forces are equal and opposite. Newton's First Law says the box's motion will not change, so it will stay at rest if it was resting or keep moving at a constant velocity if it was already moving. - 10
A box is being pushed to the right with a force of 30 newtons. Friction pushes left with a force of 10 newtons. What is the net force, and will the box's motion change?
Subtract the smaller force from the larger force when the forces act in opposite directions.
The net force is 20 newtons to the right. The box's motion will change because there is an unbalanced force acting to the right. - 11
A spacecraft is moving through deep space far from planets and stars. Its engines turn off, and there is almost no friction. According to Newton's First Law, what happens to the spacecraft's motion?
The spacecraft will continue moving at the same speed and in the same direction because there is no significant unbalanced force acting on it. - 12
A student places a coin on an index card that is resting on top of a cup. The student flicks the card quickly sideways, and the coin drops into the cup. Explain this using inertia.
The card moves, but the coin resists a change in motion.
The coin tends to stay at rest because of inertia. When the card is flicked away quickly, the coin does not move sideways much, so gravity pulls it down into the cup. - 13
Is a constant speed in a straight line evidence of balanced forces or unbalanced forces? Explain your answer.
A constant speed in a straight line is evidence of balanced forces because the object's motion is not changing. The net force is zero if the object continues at the same velocity. - 14
A bus turns sharply to the left, and a backpack on the seat slides to the right side of the bus. Use Newton's First Law to explain what is happening.
The backpack does not want to change direction unless a force makes it turn.
The backpack tends to keep moving in its original direction because of inertia. When the bus turns left, the backpack resists the change in direction and appears to slide toward the right side of the bus. - 15
Write one real-life safety rule or safety device that is based on Newton's First Law, and explain how it helps people.
A seat belt is based on Newton's First Law because it applies a force to stop a passenger from continuing forward when a car suddenly stops. It helps prevent the passenger from hitting the dashboard or windshield.