IB Physics SLGOHS
Chapter 4.1-4.4 Worksheet: Laws of Motion
Quick Quizzes
4.1)True or false? (a) It’s possible to have motion in the absence of a force. (b) If an object isn’t moving, no external force acts on it.
4.2) True or false? (a) If a single force acts on an object, the object accelerates. (b) If an object is accelerating, a force is acting on it. (c) If an object is not accelerating, no external force is acting on it.
4.3) True or false? If the net force acting on an object is in the positive x-direction, the object moves only in the positive x-direction.
Conceptual Questions
2. If a car is traveling westward with a constant speed of 20 m/s, what is the resultant force acting on it?
4. A rubber ball is dropped onto the floor. What force causes the ball to bounce?
6. If gold were sold by weight, would you rather buy it in Denver or in Death Valley? If it were sold by mass, in which of the two locations would you prefer to buy it? Why?
10. Analyze the motion of a rock dropped in water in terms of its speed and acceleration as it falls. Assume that a resistive force is acting on the rock that increases as the velocity of the rock increases.
Problems
1. A 6.0-kg object undergoes an acceleration of 2.0 m/s2. (a) What is the magnitude of the resultant force acting on
it? (b) If this same force is applied to a 4.0-kg object, what acceleration is produced?
2. A football punter accelerates a football from rest to a speed of 10 m/s during the time in which his toe is in contact with the ball (about 0.20 s). If the football has a mass of 0.50 kg, what average force does the punter exert on the ball? (Hint: kinematic equations)
6. A freight train has a mass of 1.5 _ 107 kg. If the locomotive can exert a constant pull of 7.5 _ 105 N, how long does it take to increase the speed of the train from rest to 80 km/h?
7. The air exerts a forward force of 10 N on the propeller of a 0.20-kg model airplane. If the plane accelerates forward at 2.0 m/s2, what is the magnitude of the resistive force exerted by the air on the airplane?
8. A 5.0-g bullet leaves the muzzle of a rifle with a speed of 320 m/s. What force (assumed constant) is exerted on the bullet while it is traveling down the 0.82-m-long barrel of the rifle? (Hint: kinematic equations)
11. A boat moves through the water with two forces acting on it. One is a 2 000-N forward push by the water on the propellor, and the other is a 1 800-N resistive force due to the water around the bow. (a) What is the acceleration of the 1000-kg boat? (b) If it starts from rest, how far will the boat move in 10.0 s? (c) What will its velocity be at the end of that time? (Hint: kinematic equations)
12. Two forces are applied to a car in an effort to move it, as shown in Figure P4.12. (a) What is the resultant of these two forces? (b) If the car has a mass of 3 000 kg, what acceleration does it have? Ignore friction.
ANSWERS
Quick Quizzes
1.(a)True. Motion requires no force. Newton’s first law says an object in motion continues to move by itself in the absence of external forces.
(b)False. It is possible for forces to act on an object with no resulting motion if the forces are balanced.
2.(a)True. If a single force acts on an object, it must accelerate. From Newton's second law, , and a single force must represent a non-zero net force.
(b)True. If an object accelerates, at least one force must act on it.
(c)False. If an object has no acceleration, you cannot conclude that no forces act on it. In this case, you can only say that the net force on the object is zero.
3.False. If the object begins at rest or is moving with a velocity with only an x component, the net force in the x direction causes the object to move in the x direction. In any other case, however, the motion of the object involves velocity components in directions other than x. Thus, the direction of the velocity vector is not generally along the x axis. What we can say with confidence is that a net force in the x direction causes the object to accelerate in the x direction.
Concept Questions
2.If the car is traveling at constant velocity, it has zero acceleration. Hence, the resultant force acting on it is zero.
4.The force causing the ball to rebound upward is the normal force exerted on the ball by the floor.
6.w = mg and g decreases with altitude. Thus, to get a good buy, purchase it in Denver. If gold was sold by mass, it would not matter where you bought it.
10.The net force acting on the object decreases as the resistive force increases. Eventually, the resistive force becomes equal to the weight of the object, and the net force goes to zero. In this condition, the object stops accelerating, and the velocity stays constant. The rock has reached its terminal velocity.
Problems
4.1(a)
(b)
4.2From , the acceleration given to the football is
Then, from Newton’s law, we find
4.6, and
gives
4.7Summing the forces on the plane shown gives
From which,
4.8The acceleration of the bullet is given by
Then,
4.11(a)From the second law, the acceleration of the
boat is
(b)The distance moved is
(c)The final velocity is
4.12(a)Choose the positive y-axis in the forward direction. We
resolve the forces into their components as
400 N / 200 N / 346 N
450 N / –78.1 N / 443 N
Resultant / /
The magnitude and direction of the resultant force is
, to right of y-axis.
Thus,
(b)The acceleration is in the same direction as and has magnitude