Quiz · Medium
Practice problems on motion with constant acceleration, free fall, projectiles, forces, friction, work, energy, power and momentum. Uses g = 9.8 m/s² and shows the working for every answer.
For a projectile with no air resistance, what happens to its horizontal velocity during flight?
It decreases steadily
It increases steadily
It is zero at the highest point
It stays constant
No horizontal force acts, so there is no horizontal acceleration. Only the vertical velocity changes. At the peak, the vertical velocity is zero but the horizontal velocity is not.
A car starts from rest and accelerates at 3.0 m/s² for 5.0 s. What is its final speed?
15 m/s
7.5 m/s
37.5 m/s
0.6 m/s
. 37.5 is the distance traveled in meters, not the speed.
How far does that same car (from rest, 3.0 m/s² for 5.0 s) travel?
75 m
15 m
7.5 m
37.5 m
. 75 m forgets the one-half.
A ball is dropped from rest. Ignoring air resistance, how far does it fall in 2.0 s?
39.2 m
19.6 m
9.8 m
4.9 m
. Its speed at that moment is also 19.6 m/s (), which is a coincidence of these numbers.
A ball is thrown straight up at 14.7 m/s. What maximum height does it reach?
About 22.1 m
1.5 m
About 7.4 m
About 11.0 m
At the top , so and . 1.5 is the time to the top in seconds.
A ball thrown straight up is momentarily at rest at its highest point. What is its acceleration there?
Zero
9.8 m/s² upward
9.8 m/s² downward
It changes from up to down
Gravity acts the whole time, so acceleration stays 9.8 m/s² downward even when the velocity is zero. If acceleration were zero at the top, the ball would hover there.
A ball rolls off a 4.9 m high cliff with a horizontal speed of 3.0 m/s. How far from the base of the cliff does it land?
3.0 m
1.5 m
6.0 m
14.7 m
Vertical motion sets the time: , so . Horizontal distance = . The horizontal speed does not change the fall time.
A car moving at 20 m/s brakes with a constant deceleration of 5.0 m/s². What is its stopping distance?
4.0 m
80 m
40 m
100 m
: , so . Because distance depends on , doubling speed quadruples braking distance.
When a bus brakes suddenly, standing passengers lurch forward. Why?
A forward force pushes them
Their bodies keep moving forward because of inertia
Gravity increases during braking
The bus pulls them forward
Newton's first law: objects keep their state of motion unless a net force acts. The bus slows, but nothing slows the passengers' bodies at first. There is no forward force on them.
A net force of 10 N acts on a 2.0 kg object. What is its acceleration?
20 m/s²
0.20 m/s²
5.0 m/s²
12 m/s²
. Multiplying (20) or dividing the wrong way (0.20) are the usual slips.
A 4.0 kg cart is pulled right with 12 N and left with 4.0 N. What is its acceleration?
4.0 m/s² to the right
2.0 m/s² to the right
3.0 m/s² to the right
1.0 m/s² to the left
Net force = 12 − 4.0 = 8.0 N to the right. to the right. Adding the forces (16 N) gives 4.0, which ignores direction.
A truck hits a mosquito. How does the force on the mosquito compare with the force on the truck?
The force on the mosquito is much larger
The force on the truck is much larger
They are equal in size and opposite in direction
Only the mosquito feels a force
Newton's third law: forces come in equal and opposite pairs. The effects differ because of mass: the same force gives the tiny mosquito a huge acceleration (a = F/m) and the truck almost none.
What is the weight of a 60 kg student on Earth?
60 N
60 kg
About 6.1 N
About 588 N
. Mass (kg) is the amount of matter; weight (N) is the gravitational force on it. 6.1 N comes from dividing by g.
A 70 kg person stands on a scale in an elevator that accelerates upward at 2.0 m/s². What does the scale read (normal force)?
686 N
546 N
826 N
140 N
, so . You feel heavier when accelerating up. 546 N is the reading for accelerating downward at 2.0 m/s².
A 10 kg box on a level floor is pushed with 50 N. The coefficient of kinetic friction is 0.30. What is its acceleration?
About 2.1 m/s²
5.0 m/s²
About 7.9 m/s²
0 m/s²
Friction = . Net force = 50 − 29.4 = 20.6 N, so . 5.0 m/s² ignores friction.
A block slides down a frictionless ramp inclined at 30°. What is its acceleration?
9.8 m/s²
4.9 m/s²
8.5 m/s²
0 m/s²
Only the component of gravity along the slope accelerates it: . 8.5 m/s² uses cosine, which is the component into the ramp.
What is the kinetic energy of a 1,000 kg car moving at 20 m/s?
200 kJ
20 kJ
400 kJ
10 kJ
. 400 kJ forgets the one-half, and 10 kJ forgets to square the speed.
If a car's speed doubles, its kinetic energy:
Doubles
Stays the same
Becomes 4 times as large
Becomes 8 times as large
KE depends on , so . This is why crashes at 60 mph are far more destructive than at 30 mph.
How much gravitational potential energy does a 2.0 kg book gain when lifted 5.0 m?
10 J
49 J
196 J
98 J
. 10 J leaves out g.
A rock is dropped from 20 m. Ignoring air resistance, about how fast is it moving when it hits the ground?
About 392 m/s
About 19.8 m/s
9.8 m/s
About 14.0 m/s
Energy is conserved: , so . Mass cancels out. 392 forgets the square root.
A 50 N force pulls a sled 10 m along level ground. The rope makes a 60° angle with the ground. How much work does the force do?
250 J
500 J
433 J
0 J
. Only the component along the motion does work. 433 J uses sine instead of cosine.
A motor does 600 J of work in 3.0 s. What is its power output?
1,800 W
600 W
200 W
0.005 W
. 1,800 multiplies instead of divides.
A 2.0 kg cart moving at 3.0 m/s collides with a 1.0 kg cart at rest, and they stick together. What is their speed afterward?
3.0 m/s
2.0 m/s
1.5 m/s
6.0 m/s
Momentum is conserved: , so . Kinetic energy is not conserved in this perfectly inelastic collision, but momentum is.
