Science: Newtons Laws of Motion
Understanding inertia, force, mass, and action-reaction pairs
Science: Newtons Laws of Motion
Understanding inertia, force, mass, and action-reaction pairs
Science - Grade 9-12
- 1
A soccer ball is resting on the field. According to Newtons First Law, what will happen to the ball if no unbalanced force acts on it?
Think about what inertia means for an object that is not moving.
According to Newtons First Law, the soccer ball will remain at rest because an object at rest stays at rest unless acted on by an unbalanced force. - 2
A hockey puck slides across smooth ice. Explain why it keeps moving for a long distance before stopping.
The hockey puck keeps moving because of inertia. It would continue moving at a constant velocity unless an unbalanced force, such as friction, acts on it. Because the ice has little friction, the puck travels a long distance before stopping. - 3
A net force of 20 N acts on a 5 kg cart. What is the cart's acceleration?
Use Newtons Second Law: F = ma.
The cart's acceleration is 4 m/s^2 because acceleration equals force divided by mass, so 20 N divided by 5 kg equals 4 m/s^2. - 4
A 2 kg object accelerates at 3 m/s^2. What net force is acting on it?
The net force acting on the object is 6 N because force equals mass times acceleration, so 2 kg times 3 m/s^2 equals 6 N. - 5
Two shopping carts are pushed with the same force. One cart is empty and one cart is full. Which cart will have the greater acceleration, and why?
Compare how mass affects acceleration when force stays the same.
The empty cart will have the greater acceleration because with the same force, the object with less mass accelerates more according to Newtons Second Law. - 6
A 10 kg box accelerates at 2 m/s^2 to the right. What is the net force on the box, including direction?
The net force on the box is 20 N to the right because force equals mass times acceleration, so 10 kg times 2 m/s^2 equals 20 N. - 7
A student pushes on a wall, but the wall does not move. According to Newtons Third Law, what force acts in response to the student's push?
Every action force has a matching reaction force.
According to Newtons Third Law, the wall pushes back on the student with an equal force in the opposite direction. - 8
Identify the action-reaction force pair when a swimmer pushes water backward and moves forward.
The swimmer pushes the water backward, and the water pushes the swimmer forward with an equal force in the opposite direction. These two forces form the action-reaction pair. - 9
A 60 kg runner starts moving from rest and reaches a speed of 4 m/s in 2 seconds. What is the runner's acceleration?
Use a = change in velocity divided by time.
The runner's acceleration is 2 m/s^2 because acceleration equals change in velocity divided by time, and 4 m/s divided by 2 s equals 2 m/s^2. - 10
Using the previous problem, what net force acted on the 60 kg runner?
The net force acting on the runner is 120 N because force equals mass times acceleration, and 60 kg times 2 m/s^2 equals 120 N. - 11
Explain why passengers lurch forward when a car stops suddenly.
Focus on Newtons First Law and motion continuing unless a force changes it.
Passengers lurch forward because of inertia. Their bodies were moving with the car, and when the car stops suddenly, their bodies tend to keep moving forward until another force, such as a seat belt, acts on them. - 12
A rocket pushes exhaust gases downward. Explain how this allows the rocket to rise upward.
The rocket pushes exhaust gases downward, and the gases push the rocket upward with an equal force in the opposite direction. This is an example of Newtons Third Law. - 13
A 15 N net force acts on an object and causes it to accelerate at 5 m/s^2. What is the object's mass?
Rearrange Newtons Second Law to solve for mass.
The object's mass is 3 kg because mass equals force divided by acceleration, so 15 N divided by 5 m/s^2 equals 3 kg. - 14
A book rests on a table. Is the book experiencing forces? Explain why it does not accelerate.
Yes, the book is experiencing forces. Gravity pulls it downward, and the table pushes upward with an equal support force. Because these forces are balanced, the net force is zero and the book does not accelerate. - 15
A car and a truck collide. The truck is much more massive than the car. According to Newtons Third Law, how do the forces on the car and truck compare during the collision?
Equal force does not always mean equal motion.
The car and the truck exert equal forces on each other in opposite directions during the collision. Newtons Third Law says the interaction forces are equal even though the vehicles may have different masses and different accelerations.