<< Chapter < Page | Chapter >> Page > |
Calculate the force the professor exerts on the cart in [link] using data from the previous example if needed.
Strategy
If we now define the system of interest to be the cart plus equipment (System 2 in [link] ), then the net external force on System 2 is the force the professor exerts on the cart minus friction. The force she exerts on the cart, , is an external force acting on System 2. was internal to System 1, but it is external to System 2 and will enter Newton’s second law for System 2.
Solution
Newton’s second law can be used to find . Starting with
and noting that the magnitude of the net external force on System 2 is
we solve for , the desired quantity:
The value of is given, so we must calculate net . That can be done since both the acceleration and mass of System 2 are known. Using Newton’s second law we see that
where the mass of System 2 is 19.0 kg ( = 12.0 kg + 7.0 kg) and its acceleration was found to be in the previous example. Thus,
Now we can find the desired force:
Discussion
It is interesting that this force is significantly less than the 150-N force the professor exerted backward on the floor. Not all of that 150-N force is transmitted to the cart; some of it accelerates the professor.
The choice of a system is an important analytical step both in solving problems and in thoroughly understanding the physics of the situation (which is not necessarily the same thing).
Visualize the gravitational force that two objects exert on each other. Change properties of the objects in order to see how it changes the gravity force.
What object or objects commonly exert forces on the following objects in motion? (a) a soccer ball being kicked, (b) a dolphin jumping, (c) a parachutist drifting to Earth.
(a) A soccer player, gravity, air, and friction commonly exert forces on a soccer ball being kicked.
(b) Gravity and the surrounding water commonly exert forces on a dolphin jumping. (The dolphin moves its muscles to exert a force on the water. The water exerts an equal force on the dolphin, resulting in the dolphin’s motion.)
(c) Gravity and air exert forces on a parachutist drifting to Earth.
A ball with a mass of 0.25 kg hits a gym ceiling with a force of 78.0 N. What happens next?
Which of the following is true?
(c)
Notification Switch
Would you like to follow the 'College physics for ap® courses' conversation and receive update notifications?