Physics: Newton's Three Laws of Motion
Exploring inertia, force, acceleration, and action-reaction pairs
Physics: Newton's Three Laws of Motion
Exploring inertia, force, acceleration, and action-reaction pairs
Physics - Grade 6-8
- 1
A soccer ball sitting on the grass stays still until a player kicks it. Which of Newton's laws best explains why the ball stays still at first, and why it starts moving after the kick?
Think about inertia and what is needed to change motion.
Newton's first law explains this situation. The ball stays at rest because objects at rest stay at rest unless acted on by an outside force, and the kick provides the outside force that changes the ball's motion. - 2
A student pushes a 10 kg cart with a force of 30 N. What is the cart's acceleration if there is no friction?
Use the formula F = ma and solve for acceleration.
The cart's acceleration is 3 m/s². Using Newton's second law, F = ma, so a = F ÷ m = 30 N ÷ 10 kg = 3 m/s². - 3
When a swimmer pushes water backward with their hands, the swimmer moves forward. Which of Newton's laws is shown, and what is the action-reaction pair?
Newton's third law is shown. The swimmer pushes the water backward, and the water pushes the swimmer forward with an equal and opposite force. - 4
A book is resting on a table. Gravity pulls the book downward, and the table pushes upward on the book with the same amount of 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 forces are balanced. The upward force from the table equals the downward force of gravity, so the net force is 0 N. - 5
A 2 kg toy car accelerates at 4 m/s². What net force is acting on the toy car?
Multiply mass by acceleration.
The net force is 8 N. Using Newton's second law, F = ma, so F = 2 kg × 4 m/s² = 8 N. - 6
During a sudden stop in a car, a passenger's body seems to keep moving forward. Which of Newton's laws explains this, and how does a seat belt help?
Newton's first law explains this because the passenger's body tends to keep moving forward due to inertia. A seat belt applies a force that slows the passenger safely with the car. - 7
Two students push a box in opposite directions. One student pushes right with 40 N, and the other pushes left with 25 N. What is the net force on the box, and in which direction will it accelerate?
Subtract opposite forces and keep the direction of the larger force.
The net force is 15 N to the right. The forces are opposite, so 40 N - 25 N = 15 N, and the box accelerates in the direction of the larger force. - 8
A rocket launches upward when hot gases are pushed downward out of the engine. Identify the action force and the reaction force.
Action and reaction forces act on different objects.
The action force is the rocket pushing hot gases downward. The reaction force is the gases pushing the rocket upward with an equal and opposite force. - 9
A bowling ball and a basketball are pushed with the same force across a smooth floor. Which ball will have the greater acceleration, and why?
The basketball will have the greater acceleration because it has less mass. According to Newton's second law, the same force causes a larger acceleration when the mass is smaller. - 10
A hockey puck slides across ice and eventually slows down. Newton's first law says moving objects keep moving unless acted on by an outside force. What outside force slows the puck?
Look for a force that acts against motion.
Friction slows the hockey puck. Even on ice, friction between the puck and the ice acts opposite the puck's motion and reduces its speed. - 11
A 5 kg object has a net force of 20 N acting on it. What is its acceleration?
The acceleration is 4 m/s². Using a = F ÷ m, the acceleration is 20 N ÷ 5 kg = 4 m/s². - 12
A skateboarder pushes backward on the ground with one foot and moves forward. Explain how Newton's third law applies.
Identify the two objects pushing on each other.
Newton's third law applies because the skateboarder pushes backward on the ground, and the ground pushes forward on the skateboarder. These forces are equal in size and opposite in direction. - 13
A box is pulled to the right with 50 N and pushed to the right with 20 N. Friction acts to the left with 15 N. What is the net force on the box?
Add forces in the same direction, then subtract forces in the opposite direction.
The net force is 55 N to the right. The rightward forces add to 70 N, and then the 15 N friction force is subtracted, so 70 N - 15 N = 55 N to the right. - 14
A teacher places a coin on top of an index card that rests on a cup. When the card is flicked quickly sideways, the coin drops into the cup. Which law explains the coin's motion?
Newton's first law explains the coin's motion. The coin tends to stay at rest because of inertia, so when the card moves away quickly, gravity pulls the coin straight down into the cup. - 15
A student says, "If a person pushes on a wall, the wall pushes back with the same force, so the person should move backward every time." Explain why the person may not move.
Newton's third law describes force pairs, but motion depends on the net force on one object.
The wall does push back with an equal and opposite force, but the person may not move because friction between the person's shoes and the floor can balance the wall's push. If the forces on the person are balanced, the person's motion does not change.