First Law Frenzy
A soccer ball is rolling across a grassy field. Even though nobody touches it, the ball gradually slows down and stops.
Which force causes the ball to stop, and how does this situation relate to Newton’s First Law?
Friction causes the ball to slow down. Newton’s First Law states that an object in motion continues moving at a constant velocity unless acted upon by an unbalanced external force.
Two students push identical carts. Student A pushes with 20 N of net force, while Student B pushes with 40 N of net force.
Which cart will accelerate faster, and why?
Student B’s cart will accelerate twice as fast because the net force is twice as large while the mass remains the same.
A swimmer pushes water backward with their arms.
According to Newton’s Third Law, what force causes the swimmer to move forward?
The water pushes the swimmer forward with an equal force in the opposite direction.
TRIPLE POINTS!!!
A box is being pushed across the floor.
Applied force: 40 N right
Friction: 40 N left
Are the forces balanced or unbalanced?
What is the net force, and what can you conclude about the box’s acceleration?
Balanced forces
Net force = 0 N
The box has no acceleration. It could remain at rest or continue moving at a constant velocity.
Two students are playing tug-of-war.
Student A pulls east with 120 N of force.
Student B pulls west with 85 N of force.
Calculate the net force and identify its direction.
120 − 85 = 35 N
Net force = 35 N east
A student is riding in a school bus when the driver suddenly applies the brakes. The student’s body moves forward.
Explain why the student’s body continues moving forward even though the bus is stopping.
Inertia. The student’s body tends to continue moving forward at its original velocity until another force, such as the seat belt or seat, stops it.
DOUBLE POINTS!!
A student pushes a 10 kg cart with a net force of 50 N.
The force then increases to 70N and the mass doubles.
Calculate the acceleration of the cart.
3.5 m/s2
A basketball hits the floor with a downward force of 75 N.
What force does the floor exert on the basketball?
The floor exerts an upward force of 75 N on the basketball.
A car is traveling east at a constant speed of 35 mph on a straight road.
Are the forces acting on the car balanced or unbalanced?
Explain how you know.
Balanced. Since the car is moving at a constant speed in a straight line, its velocity is not changing. Therefore, its net force is zero.
Identify the Law:
Two football players collide after running into each other at the same speed. The heavier player knocks over the lighter player.
2nd Law
Two shopping carts are sitting still. One is empty, and the other is filled with heavy groceries.
A student tries to push both carts using the same amount of force.
Which cart has greater inertia, and how will that affect its motion?
The full shopping cart has greater inertia because it has more mass. It will be harder to accelerate than the empty cart.
Two objects are pushed with the same net force of 60 N.
Object A has a mass of 5 kg.
Object B has a mass of 15 kg.
Calculate the acceleration of EACH object. Which accelerates faster?
Object A: 60 ÷ 5 = 12 m/s²
Object B: 60 ÷ 15 = 4 m/s²
Object A accelerates three times faster because it has less mass.
A student pushes against a wall with a force of 150 N, but the wall does not move.
The student concludes that the wall must be pushing back with less than 150 N.
Is the student correct? Explain.
No. The wall pushes back on the student with an equal force of 150 N in the opposite direction. The wall does not move because the forces acting on it are balanced by other forces, including forces from its supports.
A box is moving to the right across a floor.
Suddenly, the force of friction becomes greater than the applied force pushing the box forward.
Are the forces balanced or unbalanced?
What happens to the box’s motion?
Unbalanced. The net force acts to the left, opposite the box’s motion. The box slows down while it continues moving right.
A fireman turns on a water hose and is pushed backwards
3rd Law
A spacecraft is traveling through space at a constant velocity. Its engines are turned off, and there are no significant external forces acting on it.
What will happen to the spacecraft’s motion? Explain why.
The spacecraft will continue traveling at the same speed and in the same direction because there is no unbalanced force changing its motion.
A 12 kg wagon accelerates at 3 m/s².
A student then adds more objects to the wagon, increasing its total mass to 18 kg.
If the net force remains the same, what will the wagon’s new acceleration be?
Original force:
F = 12 × 3 = 36 N
New acceleration:
a = 36 ÷ 18
a = 2 m/s²
A small car collides with a large truck.
During the collision, the car experiences significant damage, while the truck experiences very little damage.
A student claims that the truck must have exerted a greater force on the car.
Is this correct? Explain using Newton’s Third Law and Newton’s Second Law.
No. Both vehicles exert equal and opposite forces on each other. However, the smaller car generally experiences greater acceleration because it has less mass. The damage can differ because of differences in vehicle structure and how the collision energy is absorbed
A student pushes a heavy desk with 200 N of force to the right.
Friction pushes against the desk with 200 N to the left.
The desk is already moving to the right at a constant velocity.
What will happen to the desk if these forces remain unchanged?
A. It will immediately stop.
B. It will accelerate to the right.
C. It will continue moving to the right at a constant velocity.
D. It will begin moving to the left.
The forces are balanced, so the desk’s velocity remains unchanged.
A magician pulls a tablecloth out from under dishes and glasses on a table without disturbing them.
what law desctribes this and what physical property explains WHY this happens?
1st Law- Inertia
A student claims:
“If an object is moving, there must always be a force pushing it forward. Otherwise, it would immediately stop.”
Is the student correct or incorrect? Use Newton’s First Law and an example to support your answer.
Incorrect. An object can continue moving at a constant velocity without a forward net force. For example, a hockey puck can glide across ice with very little friction. A force is necessary to change its velocity, not to maintain constant velocity.
A 20 kg cart accelerates at 4 m/s².
The cart’s mass is then doubled, but the net force applied to it is increased by 50%.
Calculate the cart’s new acceleration.
Original net force:
F = 20 × 4 = 80 N
New mass = 40 kg
New net force = 120 N
New acceleration:
a = 120 ÷ 40
a = 3 m/s²
A rocket launches into space by pushing hot gases downward.
A student argues:
“Newton’s Third Law cannot work in space because there is no ground or air for the rocket to push against.”
Explain why this statement is incorrect.
The rocket pushes exhaust gases backward, and the gases push the rocket forward with an equal and opposite force. The rocket does not need air or the ground to move.
A sled is moving to the right.
Three forces act on the sled horizontally:
50 N to the right
30 N to the left
40 N to the left
Calculate the net force and explain what happens to the sled’s motion.
Right = 50 N
Left = 30 + 40 = 70 N
Net force = 20 N left
The sled is experiencing an unbalanced force to the left. Since it is initially moving right, it will slow down. If the leftward net force continues, it will eventually stop and begin accelerating left.