1. Reviewing Main Ideas Forces A force is a push or pull.
The net force acting on an object is the combination of all the forces acting on the object.
The forces on an object are balanced if the net force is zero.

2. Reviewing Main Ideas Forces
Friction is a force opposing the sliding motion of two surfaces in contact. Friction is caused by microwelds that form where the surfaces are in contact.
Gravity is an attractive force between any two objects with mass. The gravitational force depends on the masses of the objects and the distance between them.

3. Reviewing Main Ideas Newton’s Law of Motion
Inertia is the resistance of an object to a change in motion.
According to Newton’s first law of motion, the motion of an object does not change unless an unbalanced net force acts on the object.
Newton’s second law of motion states that a net force causes an object to accelerate in the direction of the net force and that the acceleration is given by

4. Reviewing Main Ideas Newton’s Laws of Motion
Friction is a force opposing the sliding motion of two surfaces in contact. Friction is caused by microwelds that form where the surfaces are in contact.
Air resistance opposes the motion of objects moving through the air.
Newton’s third law of motion states that for every action there is an equal and opposite reaction.

5. Reviewing Main Ideas Using Newton’s Laws
The centripetal force is the net force on an object in circular motion and is directed toward the center of the circular path.
When two objects collide, momentum can be conserved. Some of the momentum from one object is transferred to the other.

6. Chapter Review Question 1
Friction is the force that opposes __________ of two surfaces that are touching each other.
Answer
Friction opposes the sliding motion of two surfaces that are touching each other.

7. Chapter Review Question 2 Air resistance is similar to __________.
A. acceleration
B. friction
C. gravity
D. net force

8. Chapter Review
Answer
The answer is B. Air resistance is a friction like force that opposes the motion of objects that move through the air.

9. Chapter Review Question 3
You are moving a dresser that has a mass of 36 kg; its acceleration is 0.5 m/s2. What is the force being applied?
A. 8 N
B. 18 N
C N
D. 72 N

10. Chapter Review
Answer
The answer is B. According to Newton's second law of motion, acceleration is calculated by dividing the force exerted on an object by the mass of the object. Rearrange the equation and find force by multiplying the mass of the object by its acceleration.

11. Chapter Review Question 4
What is the gravitational force exerted on an object called?
Answer
The gravitational force exerted on an object is called the object’s weight

12. Standardized Test Practice
Question 1
You are moving an object that has a mass of 25 kg; its acceleration is 0.25 m/s2. What is the force being applied?
A N
B N
C N
D N

13. Standardized Test Practice
Answer
The answer is A. Acceleration is calculated by dividing the force exerted on an object by the mass of the object. Rearrange the equation and find force by multiplying the mass of the object by its acceleration.

14. Standardized Test Practice
Question 2
What is the gravitational force on a skydiver with a mass of 75 kg?
A. 8 N
B N
C N
D N

15. Standardized Test Practice
Answer
The answer is D. Multiply the mass by the strength of gravity, 9.8 N/kg.

16. Standardized Test Practice
Question 3
On which planet is the acceleration due to gravity approximately 25 m/s2?

17. Standardized Test Practice
A. Mars
B. Jupiter
C. Saturn
D. Venus

18. Standardized Test Practice
Answer
The answer is B. Find the mass of an object by dividing its weight on Earth by the strength of gravity on Earth. Then use the object’s mass and the values in the table to determine the strength of gravity on each planet.

19. Standardized Test Practice
Question 4
On Earth, what is the weight of a car that has a mass of 4,000 kg?
A. 39, 200 N
B. 17, 818 N
C. 3, 920 N
D N

20. Standardized Test Practice
Answer
The answer is A. The weight of an object on Earth is equal to its mass multiplied by gravitational strength.

21. Standardized Test Practice
Question 5
Two balls roll toward each other and collide. Ball A has a momentum of 11 kg • m/s east and Ball B has a momentum of 88 kg • m/s west. If there are no external forces acting on the balls, what is the total momentum after the collision?
A. 39, 200 N
B. 17, 818 N
C. 3, 920 N
D N

22. Standardized Test Practice
Answer
The answer is C. Momentum of the system is the same before and after the collision. Before the collision, the total momentum is p = 11 kg • m/s east and 88 kg • m/s west = 77 kg • m/s west. Momentum is conserved, so the final momentum of the system is 77 kg • m/s west.