Physics 7 homework #4

Due 4/28/09

1. Mary decides to push a sled on a frozen lake. Once it is moving, and assuming there is no friction, what force does she need to apply to keep it moving at constant speed and direction?

a. A force along the direction of motion

b. A force opposite to the direction of motion (then the reaction to that force would move the sled in the direction she wants)

c. The force of friction

d. No force at all

2. Newton asserted that gravity keeps the Moon going around the Earth, and that gravity also makes apples fall. If both things are true, why doesn't the Moon fall?

a. Because there is no gravity in space

b. Because the Moon also has a sideways motion; as it falls towards Earth it also moves to the side and this results in its orbiting the Earth

c. Because the Moon is pulled away from Earth by the Sun and this balances Earth’s attraction

3. A Sumo wrestler and a baby jump from the same height, while they are falling

a. Their velocities are constant

b. Their accelerations are identical

c. The forces on both is identical

d. The velocity of the Sumo wrestler is always bigger than the baby’s

4. Two inertial observers measure the speed of the same beam of light and they obtain the same value. This would upset Newton because

a. According to him the speed of anything, including light, should depend on the frame of reference

b. He expected the speed of light to be infinite

c. In inertial reference frames he believed light to be stationary

d. He believed light of different colors to move at different speeds, so the observers should see a range of velocities

5. According to Newton’s laws of motion if two objects are subject to identical forces

a. They will also have identical accelerations

b. They will also have identical velocities

c. The heavier one will acquire a larger acceleration

d. The lighter one will acquire a larger acceleration

6. One of Newton's laws of motion states that "to every action there is an equal and opposite reaction". Consider now a football player kicking a ball, the player certainly acts on the ball by kicking it. Does the ball “kick the player back”? If so, why does he/she not fall backwards?

a. No, the ball does not kick back; this is an exception to Newton’s laws

b. The ball does kick the player but he does not move at all because he is very heavy

c. The ball does kick back, but he does not move because of the counter-push he receives form the air around him

d. The ball does kick back, but the player does not move backward because of the friction between his shoes and the grass

7. Electric charges

a. Always attract

b. Always repel

c. Attract or repel depending on their separation

d. Attract or repel depending on their relative signs

8. If two bar magnets are put in a line with the N pole of one touching the S pole of the other, the resulting object

a. Acts like a magnet with N and S poles

b. Acts like an electric positive charge

c. Acts like a magnet with only a N pole

d. Acts like a magnet with only a S pole

9. An electric motor works because

a. Of the gravitational interaction between its parts

b. The flowing electric current creates an electromagnet and its effect on a permanent magnet makes the motor turn

c. It charges two parts with a lot of positive charges these then repel

d. It charges two parts with a lot of negative charges these then repel

10. An electric generator works by having

a. Electricity move a permanent magnet

b. Electricity move an electromagnet

c. A gas, hydroelectric, geothermal, etc. motor move a magnet back and forth thorough a coil, which generates an electric current

d. Positive and negative charges separated from one another