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Newton's First Law
What is...
An object at rest will stay at rest and an object in motion will stay in motion on a straight path unless an unbalanced and external force acts upon the object.
Newton's Second Law
What is...
The acceleration of an object is dependent upon two variables: (1) the net force acting upon an object and (2) the mass of the object.
F=ma
Newton's Third Law
What is...
For every action there is an equal and opposite reaction.
Define momentum and state the formula
What is...
Momentum can be defined as “mass in motion”
Vector quantity
Standard units: kg • m/s
p = mv
Define a collision
What is...
A collision is an interaction between two objects that have made contact (usually) with each other
As in any interaction, a collision results in a force being applied to the two objects colliding
Forces are equal in magnitude and opposite in direction; however, accelerations are not necessarily equal in magnitude
Define inertia and give an example of Newton's First Law
What is...
If an object is not moving, it will never move.
If an object is moving, it will continue to move in that same direction forever until it is interrupted by a force.
Examples may vary
Define force, mass, and acceleration and give an example of Newton's Second Law
What is...
A force is a push or a pull
Mass is the amount of matter in an object
Acceleration is velocity over time
Examples may vary
Define an action-reaction pair and give an example
What is...
Third Law of Motion:
For every action, there is an equal and opposite reaction.
This means that when one object pushes or pulls on another, the second object pushes or pulls back with the same amount of force but in the opposite direction.
If you push against a wall with your hand (action), the wall pushes back against your hand with an equal force in the opposite direction (reaction).
The Law of Conservation of Momentum states (and the formula)
What is..
The law of momentum conservation states that for a collision occurring between object 1 and object 2 in an isolated system, the total momentum of the two objects before the collision is equal to the total momentum of the two objects after the collision
initial final
m1v1 + m2v2 = m1v1 + m2v2
Difference between the two types of collisions
What are...
Elastic collision occurs when two objects have made contact and the momentum is transferred between the two objects
Inelastic collision occurs when two objects have made contact and the momentum of the two objects is combined
List and define all of the contact forces
What are...
Normal Force - upward force exerted upon an object
Friction Force (rolling, sliding, static) - force exerted by a surface as an object moves across it
Tension - force that is transmitted through a string, rope, cable or wire when it is pulled tight by forces acting from opposite ends
Spring - force exerted by a compressed spring upon any object that is attached to it
DOUBLE JEOPARDY
Describe the relationship between unbalanced forces and acceleration
What is...
Acceleration is the result of unbalanced forces acting on an object.
When the total forces acting on an object are unbalanced, then FNET ≠ 0 (the net force on an object is not equal to zero)
The acceleration of an object is directly proportional to the net force
If all the individual forces acting upon an object are known, then the FNET can be determined (remember: free body diagrams)
A person has a mass of 60 kg on Earth.
What is their weight on Earth? (Use g=9.8 m/s2)
Jupiter’s gravity is much stronger, about 24.8 m/s². What would this person’s weight be on Jupiter?
Does the person’s mass change when they go to Jupiter? Explain.
What are...
Weight on Earth:
W=m×g; W=60×9.8=588 N
Weight on Jupiter:
W=m×g; W=60×24.8=1488 N
The person’s mass does not change because mass is the amount of matter in the body. But their weight changes because weight depends on the force of gravity, which is stronger on Jupiter.
A soccer ball with a mass of 0.5 kg is rolling at a speed of 4 m/s.
What is the momentum of the soccer ball?
What is...
p = mv
p = (0.5kg) (4m/s)
p = 2 kg m/s
A 2 kg toy car is moving at 3 m/s to the right. It collides and sticks to a 3 kg toy truck that is at rest.
What is the velocity of the combined toys right after the collision?
What is...
initial final
m1v1 + m2v2 = m1v1 + m2v2
(2kg) (3 m/s) + (3 kg) (0 m/s) = (3 kg + 2kg) v
v = 1.2 m/s to the right
Find each net force
Consult Google Doc
Solve 2 f=ma problems
1)Two students are playing tug-of-war with a rope. One student pulls with a force of 60 N to the right, and the other pulls with 45 N to the left. The rope is attached to a cart that has a mass of 5 kg and can roll freely. What is the net force acting on the cart? What is the acceleration of the cart?
2) A box is being pushed across a rough floor with a force of 50 N. The box accelerates at 2 m/s². The friction force opposing the motion is 10 N. What is the mass of the box?
What are...
1) Net force: 60 N−45 N=15 N; 60N−45N=15N (to the right)
Acceleration: a=Fnet/m; a=15/5=3 m/s^2
2) Fnet = Fapp - Ff; 50N - 10N = 40N
Mass: m=Fnet/a; m=40/20= 20kg
DOUBLE JEOPARDY
A swimmer pushes backward against the water with her hands to move forward. Describe the action-reaction forces involved, explain why the swimmer moves forward, and discuss what would happen if the water did not push back.
What is...
The swimmer’s hands push the water backward (action), and the water pushes the swimmer forward with an equal and opposite force (reaction). This reaction force propels the swimmer forward. If the water did not push back, the swimmer would not be able to move forward because there would be no force to propel her.
A ball is rolling at 5 m/s and has a momentum of 15 kg·m/s.
What is the mass of the ball?
What is...
p = mv
15 kg m/s = m (5 m/s)
m = 3 kg
Two billiard balls collide on a frictionless pool table.
Ball A has a mass of 0.2 kg and is moving right at 3 m/s.
Ball B has a mass of 0.3 kg and is at rest.
After the collision, Ball A moves left at 1 m/s.
What is the velocity of Ball B after the collision?
What is...
Initial
pinitial = m1v1 + m2v2
pinitial = (0.2 kg)(3 m/s) + (0.3 kg) (0 m/s)
pinitial = 0.6 kg m/s
Final
pfinal = m1v1 + m2v2
pfinal = (0.2 kg) (-1 m/s) + (0.3 kg) (v2)
pfinal = -0.2 kg +(0.3 kg) (v2)
Set Equal
0.6 kg m/s = -0.2 kg +(0.3 kg) (v2)
v2 = 1.33 m/s
A 5 kg box is sitting on a flat, horizontal floor. You push the box to the right with a force of 20 N. The floor is rough, so there is friction opposing the motion. The friction force is 5 N.
Questions:
Draw a free body diagram of the box. Label all the forces acting on it.
What is the net force acting on the box?
What is...
Forces to include on the diagram:
Gravity (Weight): Fg=m⋅g=5⋅9.8=49; Fg=m⋅g=5⋅9.8=49N downward
Normal force: 49 N upward (balancing gravity)
Applied force: 20 N to the right
Friction: 5 N to the left
Net force:
Fnet=20 N (right)−5 N (left)=15 N; Fnet=20N (right)−5N (left)=15N (right)Two astronauts are on different planets. Astronaut A is on Planet X, where the gravity is stronger than Earth’s, and Astronaut B is on Planet Y, where the gravity is weaker than Earth’s.
Both astronauts drop identical balls from the same height on their planets at the same time.
Which ball will hit the ground first? Explain why, using your understanding of gravity and acceleration.
What is...
The ball on Planet X will hit the ground first because stronger gravity means a greater acceleration. The ball falls faster, covering the same distance in less time.
The ball on Planet Y will take longer to hit the ground because weaker gravity means less acceleration, so it falls more slowly.
Two Part Question:
1) A helicopter’s blades push air downwards to stay in the air. Explain the action-reaction forces at work and why the helicopter doesn’t just push the air away without moving.
2) When two ice skaters push off each other on frictionless ice, they move apart. Explain how Newton’s Third Law applies even if one skater is much heavier than the other.
What are...
1)When the helicopter blades spin, they push the air downward — this is the action force. According to Newton’s Third Law, the air pushes back upward on the blades with an equal and opposite force — this is the reaction force.
This upward reaction force on the blades creates lift, which pushes the helicopter up and keeps it in the air. The helicopter doesn’t just push air away because the reaction force from the air pushes back on the blades, allowing the helicopter to rise.
Without this reaction force, pushing the air down wouldn’t help the helicopter move up — it would just push air away with no lift.
2) According to Newton’s Third Law, when Skater A pushes on Skater B, Skater B pushes back on Skater A with an equal and opposite force. These two forces are an action-reaction pair.
Even if one skater is much heavier, the forces they exert on each other are equal in size but opposite in direction. However, because the heavier skater has more mass, they accelerate less, and the lighter skater accelerates more.
So, both skaters move apart, but the lighter one moves faster because acceleration depends on mass (Newton’s Second Law: a=Fma = \frac{F}{m}a=mF).
If an object has a momentum of 20 kg·m/s and a mass of 4 kg, what is its velocity?
What is...
p = mv
20 kg m/s = (4 kg) v
v = 5 m/s
A 4 kg box is sliding to the right at 6 m/s on a frictionless surface. It collides and sticks to a 6 kg box that is sliding to the left at 2 m/s.
What is the velocity of the combined boxes immediately after the collision?
Is momentum conserved in this collision? Explain why.
What is...
Initial
pinitial = m1v1 + m2v2
pinitial=(4kg)(6 m/s)+(6kg)(−2 m/s)=24−12=12kg m/s
Combined Mass After Collision
pcombinedmass = 4 kg + 6 kg = 10 kg
Final
pfinal = mfinal x velocity
12 kg m/s = 10 kg x v
v = 1.2 m/s
Yes! Momentum is conserved because the total momentum before the collision (12 kg·m/s) equals the total momentum after the collision (10 kg × 1.2 m/s = 12 kg·m/s). The collision is in a closed system with no external forces, so momentum must be conserved.