A bike is hanging from a hook in a garage. You need to consider the following forces.
(a) The force of the Earth pulling down on the bike.
(b) The force of the bike pulling up on the Earth.
(c) The force of the hook pulling up on the bike.
(d) The force of the hook pulling down on the ceiling.
Which two forces are equal and opposite because of Newton's third law?
Which two forces are equal and opposite because of Newton's first law?
I would like to say A and B for the first question and A and C for the second one but I'm not sure
Yes a and b for the first one, the force the bike exerts on earth is the same magnitude as the force earth exerts on the bike.
Yes c and d for the second one. The net force on the hook is zero so its momentum does not change.
You're on the right track with your answers, but let me explain in more detail.
According to Newton's third law of motion, for every action, there is an equal and opposite reaction. This means that if there is a force exerted in one direction, there will be another force of equal magnitude but in the opposite direction.
Now let's analyze the forces in this scenario:
(a) The force of the Earth pulling down on the bike: This force is the gravitational force acting on the bike due to the mass of the Earth. It pulls the bike downwards.
(b) The force of the bike pulling up on the Earth: This is Newton's third law pair for the force in (a). It is the reaction force to the downward force of the Earth, and it pulls the Earth upwards.
Therefore, the forces (a) and (b) are equal in magnitude and opposite in direction due to Newton's third law.
Moving on to the second question:
Newton's first law of motion, also known as the law of inertia, states that an object at rest tends to stay at rest, and an object in motion tends to stay in motion unless acted upon by an external force.
In this case, since the bike is hanging from the hook and not moving, we can consider it to be at rest. According to Newton's first law, an object at rest experiences balanced forces.
(c) The force of the hook pulling up on the bike: This is the force exerted by the hook in order to support the weight of the bike. It counteracts the downward force of the Earth.
(d) The force of the hook pulling down on the ceiling: This force is acting in the opposite direction to keep the bike hanging. It is the reaction force to the force exerted by the hook on the bike.
Therefore, the forces (c) and (d) are equal in magnitude and opposite in direction because they balance each other out to keep the bike at rest.
To summarize, the forces (a) and (b) are equal and opposite due to Newton's third law, while the forces (c) and (d) are equal and opposite because of Newton's first law.