Newton's 1st Law
What is Newton’s 1st Law also called?
Law of Intertia
What is the formula for Newton’s 2nd Law?
F = M x A
Complete the sentence: For every action force, there is an equal and ______ reaction force.
Opposite
What does acceleration describe? (What is it?)
A change in velocity over time.
An astronaut is floating through space far away from planets and stars. She throws a wrench forward. She then stops pushing it and does nothing else.
What will happen to the wrench, and why?
It will fly forward at the same velocity in the same direction until another unbalanced force acts upon it, causing a change of motion
What is inertia?
An object’s tendency to resist a change in its motion.
If the force on an object increases while its mass stays the same, what happens to its acceleration?
Its acceleration increases.
When you jump off the ground, what force pushes you upward?
The ground pushes upward on you.
What is the formula for acceleration?
a = (final velocity − initial velocity) ÷ time
Two students pull on opposite ends of a rope with exactly 100 N of force each. The rope does not move.
A student says, “Newton’s 3rd Law explains why the rope doesn't move because the forces are equal and opposite.”
Is the student correct? Explain why or why not.
No. The forces the students exert on the rope are opposite, but the important idea is that the net force on the rope is zero. Newton's 3rd Law pairs act on different objects; they don't cancel each other on the same object.
A soccer ball is sitting still on the ground. What will happen if no unbalanced force acts on it?
It will remain at rest.
A 5 kg object is pushed with a net force of 20 N. What is its acceleration?
4 m/s²
20/5 = 4 m/s squared
A swimmer pushes water backward with their arms. What does the water do?
The water pushes the swimmer forward.
A car’s velocity changes from 10 m/s to 30 m/s in 5 seconds. What is its acceleration?
4 m/s²
You push an empty shopping cart and a completely full shopping cart with exactly the same force.
The empty cart accelerates much faster.
A student says, “The full cart must have more inertia, but inertia isn't a force, so it shouldn't affect acceleration.” Is the student correct? Explain.
Answer: The student is incorrect. Inertia is not a force, but greater mass means greater resistance to changes in motion. According to Newton's 2nd Law, with the same force, the object with greater mass has less acceleration.
When a car suddenly stops, passengers continue moving forward. Explain why
answers can vary slightly, but should explain that due to inertia, the passengers are traveling at the same speed as the car. When the car suddenly stops, the objects have inertia and don't want to change their motion, causing them to fly forward.
A 10 kg object accelerates at 3 m/s². What net force is acting on it?
30 N
A rocket pushes gases downward. What happens to the rocket?
The gases push the rocket upward.
A skateboarder slows from 12 m/s to 4 m/s in 2 seconds. What is the acceleration?
−4 m/s²
A student drops a bowling ball and a tennis ball at the same time.
The student says:
“Gravity pulls harder on the bowling ball because it has more mass. Therefore, the bowling ball should have a much greater acceleration.”
What is wrong with this reasoning?
Answer: The bowling ball does experience a greater gravitational force, but it also has greater mass. The increased force and increased mass balance in the calculation F = ma, so both objects have approximately the same acceleration due to gravity when air resistance is ignored.
A book is sitting on a table. Gravity pulls it downward, but it does not fall. Why?
The table provides an upward force that balances gravity, so there is no unbalanced force causing the book to accelerate.
Two objects are pushed with the same force. Object A has a mass of 5 kg, while Object B has a mass of 10 kg. Which object accelerates more, and why?
Object A. It has less mass, so the same force produces a greater acceleration.
A student says, “If the action and reaction forces are equal, they should cancel each other out.” Why is this incorrect?
The forces act on different objects, so they do not cancel each other out.
True or False: An object can be accelerating even if its speed stays the same. Explain.
True. It can change direction
A small car and a large truck collide head-on.
A student says:
“The truck exerts more force on the car because the truck is heavier.”
Another student says:
“The car exerts more force because it gets damaged more.”
Both students are wrong. Explain why.
The car and truck exert equal and opposite forces on each other. That's Newton's 3rd Law. The reason the car may experience more damage or a greater acceleration is that its smaller mass makes the same force produce a greater acceleration.