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Forces and Interactions Practice

Unit: Forces and Interactions - Practice (Lessons 2, 3, 4)

            📚 Science
                            🎓 Grade 7
                                        ⏱️ 25 minutes
                    

Learning Objectives

Interpret position-time graphs to determine speed, identify rest and acceleration, and compare the motion of objects
Apply Newton's three laws of motion to predict and explain the behavior of objects under various force conditions
Describe how magnetism and electricity are connected and explain how electromagnets work and are used in technology

            Progress
            6 sections
        

1
Section 1: Graphing Motion Review
                            ~2 minutes                        

Section 1: Graphing Motion Review

A position-time graph plots an object's distance from a starting point (y-axis) against time (x-axis). The shape of the line tells you everything about how the object is moving. A straight diagonal line means constant speed. A flat horizontal line means the object is at rest. A steeper line means faster speed, and a gentler line means slower speed. A curved line that gets steeper indicates the object is accelerating.

The key relationship is that the slope of a position-time graph equals the object's speed. You calculate slope the same way you do in math: rise over run, which on this graph is the change in position divided by the change in time. When two objects are plotted on the same graph, the one with the steeper line is moving faster. Where the lines cross, the two objects are at the same position at the same time.

                📖
                Key Vocabulary: Graphing Motion            

Position-time graph: a graph showing an object's position on the y-axis and time on the x-axis. Slope: the steepness of a line, calculated as rise over run. On a position-time graph, slope equals speed. Constant speed: equal distances covered in equal time intervals (straight line). At rest: no change in position over time (horizontal line). Acceleration: a change in speed over time (curved line on a position-time graph).

2
Graphing Motion Questions

Question 1

A dog runs through a park. On its position-time graph, the first segment is a gentle straight line going up, the second segment is a steep straight line going up, and the third segment is a horizontal line. Which description matches this graph?

The dog ran fast, then slow, then stopped
The dog ran slow, then fast, then stopped
The dog stopped, then ran slow, then ran fast
The dog ran at a constant speed the entire time

Question 2

A jogger's position-time graph passes through (0 s, 0 m) and (20 s, 60 m) in a straight line. What is the jogger's speed?

1,200 m/s
0.33 m/s
3 m/s
40 m/s

Question 3

On a position-time graph, a line that slopes downward means the object is moving back toward its starting point.

Question 4

On a position-time graph, a steeper line means a ______ speed.

Question 5

Two cars are plotted on the same position-time graph. Car X has a line that rises steeply, and Car Y has a line that rises gently. At 5 seconds, their lines cross. What is true at that moment?

Car X is moving slower than Car Y
Both cars have stopped
Both cars are at the same position at the same time
The cars have switched speeds

3
Section 2: Forces and Newton's Laws Review
                            ~2 minutes                        

Section 2: Forces and Newton's Laws Review

A force is a push or pull on an object, measured in newtons (N). Forces have both size and direction, making them vector quantities. When multiple forces act on an object, you combine them to find the net force. Forces in the same direction are added together. Forces in opposite directions are subtracted. If the net force is zero, the forces are balanced and the object's motion does not change. If the net force is not zero, the forces are unbalanced and the object will accelerate.

Newton's First Law (Inertia) states that an object at rest stays at rest, and an object in motion stays in motion at the same speed and direction, unless an unbalanced force acts on it. Newton's Second Law gives the mathematical relationship: F = m x a, where force equals mass times acceleration. More force produces more acceleration, and more mass means less acceleration for the same force. Newton's Third Law states that for every action force, there is an equal and opposite reaction force. These paired forces always act on different objects, which is why they do not cancel each other out.

                📖
                Key Vocabulary: Forces and Newton's Laws            

Force: a push or pull measured in newtons (N). Net force: the combined effect of all forces on an object. Balanced forces: forces that cancel out (net force = 0), producing no change in motion. Unbalanced forces: forces that do not cancel (net force is not 0), causing acceleration. Inertia: an object's resistance to changes in its motion; depends on mass. Newton's Second Law: F = m x a (force = mass times acceleration). Action-reaction pair: two equal and opposite forces acting on different objects (Newton's Third Law).

4
Forces and Newton's Laws Questions

Question 6

A grocery cart is sitting still in a parking lot. No one is touching it, and there is no wind. According to Newton's First Law, what will the cart do?

It will slowly start rolling on its own
It will remain at rest until an unbalanced force acts on it
It will vibrate in place due to gravity
It will move toward the nearest heavy object

Question 7

A net force of 24 N acts on a 6 kg object. What is the object's acceleration?

144 m/s squared
4 m/s squared
30 m/s squared
0.25 m/s squared

Question 8

An object moving at a constant speed in a straight line has a net force of zero acting on it.

Question 9

Which of the following are examples of Newton's Third Law (action-reaction pairs)? (Select all that apply)

A rocket pushes exhaust gases downward, and the gases push the rocket upward
A book sits on a table and does not move because the forces are balanced
A swimmer pushes water backward with her hands, and the water pushes her forward
A ball rolling on grass slows down because of friction
Your feet push backward on the ground when you walk, and the ground pushes your feet forward

Select all that apply.

Question 10

A 2 kg ball, a 5 kg backpack, and a 15 kg suitcase each have the same 30 N force applied to them. Arrange the objects from GREATEST acceleration to LEAST acceleration.

                        ⋮⋮
                        5 kg backpack (a = 30/5 = 6 m/s squared)
                    
                        ⋮⋮
                        15 kg suitcase (a = 30/15 = 2 m/s squared)
                    
                        ⋮⋮
                        2 kg ball (a = 30/2 = 15 m/s squared)

Drag items to reorder, then confirm

5
Section 3: Magnetism and Electricity Review
                            ~2 minutes                        

Section 3: Magnetism and Electricity Review

A magnet produces a magnetic field and attracts iron, nickel, and cobalt. Every magnet has a north pole and a south pole. Opposite poles attract and like poles repel. If you break a magnet in half, each piece becomes a complete magnet with both poles. Magnetic field lines run from the north pole to the south pole outside the magnet. The field is strongest at the poles, where the lines are closest together.

In 1820, Hans Christian Oersted discovered that electric current creates a magnetic field. This connection is called electromagnetism. An electromagnet is made by wrapping wire coils around an iron core and running current through the wire. Unlike a permanent magnet, an electromagnet can be turned on and off, its strength can be adjusted by changing the current, and its poles can be reversed. You can also make it stronger by adding more wire coils or using a larger iron core. Electromagnets power electric motors, generators, speakers, MRI machines, and many other devices.

                📖
                Key Vocabulary: Magnetism and Electricity            

Magnetic field: the invisible region around a magnet where magnetic force acts. Poles: the north (N) and south (S) ends of a magnet; opposite poles attract, like poles repel. Electromagnetism: the connection between electricity and magnetism. Electromagnet: a magnet created by running electric current through wire coils around an iron core; can be turned on/off and adjusted. Permanent magnet: a magnet that retains its magnetism without electricity. Generator: a device that converts motion into electricity using magnets and coils.

6
Magnetism and Electricity Questions

Question 11

A compass needle always points toward geographic north because it is responding to which of the following?

Gravity pulling the needle downward
Earth's magnetic field
Electric current flowing through the compass
The Sun's gravitational pull

Question 12

An electromagnet keeps its magnetism even after the electric current is turned off.

Question 13

Which of the following devices rely on the connection between electricity and magnetism? (Select all that apply)

Electric motor
Magnifying glass
Generator at a power plant
Headphones and speakers
A wooden ruler

Select all that apply.

Question 14

The scientist who first discovered that electric current creates a magnetic field was ______.

Question 15

Match each description to the correct type of magnet.

Always magnetic, cannot be turned off

Magnetic only when current flows through coils of wire

Becomes magnetic only when near another magnet, loses magnetism when removed

Your Score