Physics 122 Force Worksheet Answers

1. a) Kinematics describes the motion of objects as they move. b) Dynamics provides explanations, i.e., the causes of the object's motion.

2. a) Kinematics would answer questions about how fast an object moves.
b) Dynamics answers questions about why an object speeds up.

3. A force is simply a push or a pull.

4. Sometimes contact is involved with a force, e.g., when you reach out to grab hold of a glass of pop and pull it toward you. Other times there is no contact, e.g., we feel the tug of gravity even though there is nothing touching us.

5. Forces can cause objects to accelerate or decelerate, change their direction, flex or alter their shape.

6. Although forces are acting in the world all around us and inside us all the time, they can be classified into one of four basic types.

7. a) The strong nuclear force binds together the matter that forms the nucleus. b) The electromagnetic force occurs between charged particles. c) The gravitational force (the force Isaac Newton investigated) exists between all matter. d) The weak nuclear force (a type of electromagnetic force), is involved in the process of radioactive decay. e) The gravitational force is the weakest of the forces. f) The strong nuclear is by far the strongest force. g) The gravitational force's range is enormous, reaching out across the universe.

8. Isaac Newton grouped all instances of motion into just three laws, a summary of movement.

9. a) Acceleration depends on the force applied to the object versus the mass of the object; 2^nd b) Your applied force is resisted by an opposing force; 3^rd c) Your applied force is opposed by a resisting force; 3^rd d) An object with no net force on it (minimal gravity in deep space) does what it has been doing; 1^st e) An object with no net force on it (the floor supports the box, nothing pulling on it) does what it has been doing 1^st f) Acceleration depends on the force applied to the object versus the mass of the object; 2^nd g) An applied force is resisted by an opposing force; 3^rd h) The astronaut's applied force against the satellite is resisted by an opposing force the satellite applies to her. Since neither astronaut nor satellite is anchored, both move in the direction of the forces (pushes) and both move apart; 3^rd i) Acceleration depends on the force applied to the object versus the mass of the object. Since the mass is less with only one person onboard, the performance is increased; 2^nd.

10. a) Mass is a measure of the amount of matter in some substance. b) Weight is a measure of how strongly gravity pull on the substance.

11. The amount of matter in a substance can remain constant on Earth or in space so the mass will be constant. But, gravity is reduced in space and so the weight will be reduced.

12. We often do not notice the constant downward tug of gravity on us (creating our weight) because we are used to the sensation of our muscles supporting ourselves. But, what about after a long day on our tired feet? Or slipping and falling? And then limping around on a sprained ankle? In these circumstances, we become very aware of the tug of gravity and of the sensation of weight. We can "see" our weight when we walk across wet sand or snow. The depth of our footprints is a rough visual indicator of our weight, of the force of gravity pulling down on us.

13. Does the constant force of gravity accelerate a skydiver faster and faster? Up to a point but then, because she picks up speed, the drag force due to air friction becomes larger and larger until it prevents a further increase in speed. The terminal velocity of an object depends on its mass and how smoothly its aerodynamic shape allows it to slip through the air.

14. Inertia is a property of matter that can be described as "resistance to change". An object's inertia tends to keep it doing what it's doing: keeping it moving in the same path and at the same speed if its in motion, or keeping it motionless if it's at rest.

15. a) Inertia makes a heavy crate hard to move because inertia is related to mass. The more mass, the greater the inertia. So, a heavy object has a large amount of inertia which tries to keep it sitting just where it is. b) A heavy football player running down the field develops a large inertia that tries to keep him moving in just one direction. If the player attempts to suddenly change direction, his knee and ankle muscles are put under enormous strain trying to overcome his inertia. Sometimes, the strain is too much for the muscles and they tear.

16. Inertia has interesting effects in space. Imagine an astronaut trying to manoeuver a satellite out of the space shuttle's cargo bay. Although both the person and satellite experience weightlessness because of the path in which the shuttle travels around the Earth, their mass is unchanged. So, if the astronaut pushes on the satellite, its large inertia tends to keep it from moving and it pushes back. Unless she and the satellite are anchored to the shuttle, she and it will drift apart. And, since her mass is so much less than the satellite's, she will move the most. Astronaut's can find it difficult to get massive satellites moving (and to get them stopped before things get squished, bent or damaged.)

17. A small mass has a lower inertia (less tendency to stay put) and so an applied force can move it easier and faster.

18. a) The assumed role of inertia is that it keeps the coin in its original spot so that when the card zips away, there is no more support and the coin falls into the glass. b) The bit of misdirection here is that the card is so smooth that when it starts to slide out from underneath the coin, it applies no force to it and so of course the coin will not move. With no force trying to move the coin, the role of inertia is a nonissue. Now, if the coin were attached to the card, then snapping the card would transfer force to the coin and inertia would be a useful thing to discuss. In fact, with a heavy coin stuck to the card, the mass might be more than the finger could move. A bent card and bruised finger might result.